Patch series "zram I/O path cleanups and fixups", v3.
This series cleans up the zram I/O path, and fixes the handling of
synchronous I/O to the underlying device in the writeback_store function
or for > 4K PAGE_SIZE systems.
The fixes are at the end, as I could not fully reason about them being
safe before untangling the callchain.
This patch (of 17):
read_from_bdev_sync is currently only compiled for non-4k PAGE_SIZE, which
means it won't be built with the most common configurations.
Replace the ifdef with a check for the PAGE_SIZE in an if instead. The
check uses an extra symbol and IS_ENABLED to allow the compiler to
eliminate the dead code, leading to the same generated code size:
text data bss dec hex filename
16709 1428 12 18149 46e5 drivers/block/zram/zram_drv.o.old
16709 1428 12 18149 46e5 drivers/block/zram/zram_drv.o.new
Link: https://lkml.kernel.org/r/20230411171459.567614-1-hch@lst.de
Link: https://lkml.kernel.org/r/20230411171459.567614-2-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
smatch reports
mm/backing-dev.c:266:1: warning: symbol
'dev_attr_min_bytes' was not declared. Should it be static?
mm/backing-dev.c:294:1: warning: symbol
'dev_attr_max_bytes' was not declared. Should it be static?
These variables are only used in one file so should be static.
Link: https://lkml.kernel.org/r/20230408141609.2703647-1-trix@redhat.com
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Commit 700d2e9a36 ("mm, page_alloc: reduce page alloc/free sanity
checks") has introduced a new static key check_pages_enabled to control
when struct pages are sanity checked during allocation and freeing. Mel
Gorman suggested that free_tail_pages_check() could use this static key as
well, instead of relying on CONFIG_DEBUG_VM. That makes sense, so do
that. Also rename the function to free_tail_page_prepare() because it
works on a single tail page and has a struct page preparation component as
well as the optional checking component.
Also remove some unnecessary unlikely() within static_branch_unlikely()
statements that Mel pointed out for commit 700d2e9a36.
Link: https://lkml.kernel.org/r/20230405142840.11068-1-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Suggested-by: Mel Gorman <mgorman@techsingularity.net>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Cc: Alexander Halbuer <halbuer@sra.uni-hannover.de>
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
If we end up with a writable migration entry that has the uffd-wp bit set,
we already messed up: the source PTE/PMD was writable, which means we
could have modified the page without notifying uffd first. Setting the
uffd-wp bit always implies converting migration entries to !writable
migration entries.
Commit 8f34f1eac3 ("mm/userfaultfd: fix uffd-wp special cases for
fork()") documents that "3. Forget to carry over uffd-wp bit for a write
migration huge pmd entry", but it doesn't really say why that should be
relevant.
So let's remove that code to avoid hiding an eventual underlying issue (in
the future, we might want to warn when creating writable migration entries
that have the uffd-wp bit set -- or even better when turning a PTE
writable that still has the uffd-wp bit set).
This now matches the handling for hugetlb migration entries in
hugetlb_change_protection().
In copy_huge_pmd()/copy_nonpresent_pte()/copy_hugetlb_page_range(), we
still transfer the uffd-bit also for writable migration entries, but
simply because we have unified handling for "writable" and
"readable-exclusive" migration entries, and we care about transferring the
uffd-wp bit for the latter.
Link: https://lkml.kernel.org/r/20230405160236.587705-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
If a library wants to get information from auxv (for instance,
AT_HWCAP/AT_HWCAP2), it has a few options, none of them perfectly reliable
or ideal:
- Be main or the pre-main startup code, and grub through the stack above
main. Doesn't work for a library.
- Call libc getauxval. Not ideal for libraries that are trying to be
libc-independent and/or don't otherwise require anything from other
libraries.
- Open and read /proc/self/auxv. Doesn't work for libraries that may run
in arbitrarily constrained environments that may not have /proc
mounted (e.g. libraries that might be used by an init program or a
container setup tool).
- Assume you're on the main thread and still on the original stack, and
try to walk the stack upwards, hoping to find auxv. Extremely bad
idea.
- Ask the caller to pass auxv in for you. Not ideal for a user-friendly
library, and then your caller may have the same problem.
Add a prctl that copies current->mm->saved_auxv to a userspace buffer.
Link: https://lkml.kernel.org/r/d81864a7f7f43bca6afa2a09fc2e850e4050ab42.1680611394.git.josh@joshtriplett.org
Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
As noticed by Geert, commit b5c88f2153 ("microblaze/mm: support
__HAVE_ARCH_PTE_SWP_EXCLUSIVE") modified m68k code by accident. While
replacing 0x080 by CF_PAGE_NOCACHE is correct, although it should have
been part of commit ed4154067a ("m68k/mm: support
__HAVE_ARCH_PTE_SWP_EXCLUSIVE"), replacing "bit 7" by "bit 24" in the
comment was wrong.
Let's revert to the previous, correct, comment.
Link: https://lkml.kernel.org/r/20230404085636.121409-1-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
cgroups v1 has a unique way of setting up memory pressure notifications:
the user opens "memory.pressure_level" of the cgroup they want to monitor
for pressure, then open "cgroup.event_control" and write the fd (among
other things) to that file. memory.pressure_level has no other use,
specifically it does not support any read or write operations.
Consequently, no handlers are provided, and cgroup_file_mode() sets the
permissions to 000. However, to actually use the mechanism, the
subscribing user must have read access to the file and open the fd for
reading, see memcg_write_event_control().
This is all fine as long as the subscribing process runs as root and is
otherwise unconfined by further restrictions. However, if you add strict
access controls such as selinux, the permission bits will be enforced, and
opening memory.pressure_level for reading will fail, preventing the
process from subscribing, even as root.
To work around this issue, introduce a dummy read handler. When
memory.pressure_level is created, cgroup_file_mode() will notice the
existence of a handler, and therefore add read permissions to the file.
Link: https://lkml.kernel.org/r/20230404105900.2005-1-flosch@nutanix.com
Signed-off-by: Florian Schmidt <flosch@nutanix.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Make sure that collapse_file doesn't interfere with checking the uptodate
flag in the page cache by only inserting hpage into the page cache after
it has been updated and marked uptodate. This is achieved by simply not
replacing present pages with hpage when iterating over the target range.
The present pages are already locked, so replacing them with the locked
hpage before the collapse is finalized is unnecessary. However, it is
necessary to stop freezing the present pages after validating them, since
leaving long-term frozen pages in the page cache can lead to deadlocks.
Simply checking the reference count is sufficient to ensure that there are
no long-term references hanging around that would the collapse would
break. Similar to hpage, there is no reason that the present pages
actually need to be frozen in addition to being locked.
This fixes a race where folio_seek_hole_data would mistake hpage for an
fallocated but unwritten page. This race is visible to userspace via data
temporarily disappearing from SEEK_DATA/SEEK_HOLE. This also fixes a
similar race where pages could temporarily disappear from mincore.
Link: https://lkml.kernel.org/r/20230404120117.2562166-5-stevensd@google.com
Fixes: f3f0e1d215 ("khugepaged: add support of collapse for tmpfs/shmem pages")
Signed-off-by: David Stevens <stevensd@chromium.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Make sure that collapse_file respects any userfaultfds registered with
MODE_MISSING. If userspace has any such userfaultfds registered, then for
any page which it knows to be missing, it may expect a
UFFD_EVENT_PAGEFAULT. This means collapse_file needs to be careful when
collapsing a shmem range would result in replacing an empty page with a
THP, to avoid breaking userfaultfd.
Synchronization when checking for userfaultfds in collapse_file is tricky
because the mmap locks can't be used to prevent races with the
registration of new userfaultfds. Instead, we provide synchronization by
ensuring that userspace cannot observe the fact that pages are missing
before we check for userfaultfds. Although this allows registration of a
userfaultfd to race with collapse_file, it ensures that userspace cannot
observe any pages transition from missing to present after such a race
occurs. This makes such a race indistinguishable to the collapse
occurring immediately before the userfaultfd registration.
The first step to provide this synchronization is to stop filling gaps
during the loop iterating over the target range, since the page cache lock
can be dropped during that loop. The second step is to fill the gaps with
XA_RETRY_ENTRY after the page cache lock is acquired the final time, to
avoid races with accesses to the page cache that only take the RCU read
lock.
The fact that we don't fill holes during the initial iteration means that
collapse_file now has to handle faults occurring during the collapse.
This is done by re-validating the number of missing pages after acquiring
the page cache lock for the final time.
This fix is targeted at khugepaged, but the change also applies to
MADV_COLLAPSE. MADV_COLLAPSE on a range with a userfaultfd will now
return EBUSY if there are any missing pages (instead of succeeding on
shmem and returning EINVAL on anonymous memory). There is also now a
window during MADV_COLLAPSE where a fault on a missing page will cause the
syscall to fail with EAGAIN.
The fact that intermediate page cache state can no longer be observed
before the rollback of a failed collapse is also technically a
userspace-visible change (via at least SEEK_DATA and SEEK_END), but it is
exceedingly unlikely that anything relies on being able to observe that
transient state.
Link: https://lkml.kernel.org/r/20230404120117.2562166-4-stevensd@google.com
Signed-off-by: David Stevens <stevensd@chromium.org>
Acked-by: Peter Xu <peterx@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Patch series "mm/khugepaged: fixes for khugepaged+shmem", v6.
This series reworks collapse_file so that the intermediate state of the
collapse does not leak out of collapse_file. Although this makes
collapse_file a bit more complicated, it means that the rest of the
kernel doesn't have to deal with the unusual state. This directly fixes
races with both lseek and mincore.
This series also fixes the fact that khugepaged completely breaks
userfaultfd+shmem. The rework of collapse_file provides a convenient
place to check for registered userfaultfds without making the shmem
userfaultfd implementation care about khugepaged.
Finally, this series adds a lru_add_drain after swapping in shmem pages,
which makes the subsequent folio_isolate_lru significantly more likely to
succeed.
This patch (of 4):
Call lru_add_drain after swapping in shmem pages so that isolate_lru_page
is more likely to succeed.
Link: https://lkml.kernel.org/r/20230404120117.2562166-1-stevensd@google.com
Link: https://lkml.kernel.org/r/20230404120117.2562166-2-stevensd@google.com
Signed-off-by: David Stevens <stevensd@chromium.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jiaqi Yan <jiaqiyan@google.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Peter Xu <peterx@redhat.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Make collapse_file roll back when copying pages failed. More concretely:
- extract copying operations into a separate loop
- postpone the updates for nr_none until both scanning and copying
succeeded
- postpone joining small xarray entries until both scanning and copying
succeeded
- postpone the update operations to NR_XXX_THPS until both scanning and
copying succeeded
- for non-SHMEM file, roll back filemap_nr_thps_inc if scan succeeded but
copying failed
Tested manually:
0. Enable khugepaged on system under test. Mount tmpfs at /mnt/ramdisk.
1. Start a two-thread application. Each thread allocates a chunk of
non-huge memory buffer from /mnt/ramdisk.
2. Pick 4 random buffer address (2 in each thread) and inject
uncorrectable memory errors at physical addresses.
3. Signal both threads to make their memory buffer collapsible, i.e.
calling madvise(MADV_HUGEPAGE).
4. Wait and then check kernel log: khugepaged is able to recover from
poisoned pages by skipping them.
5. Signal both threads to inspect their buffer contents and make sure no
data corruption.
Link: https://lkml.kernel.org/r/20230329151121.949896-4-jiaqiyan@google.com
Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
Reviewed-by: Yang Shi <shy828301@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: David Stevens <stevensd@chromium.org>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Tong Tiangen <tongtiangen@huawei.com>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Problem
=======
Memory DIMMs are subject to multi-bit flips, i.e. memory errors. As
memory size and density increase, the chances of and number of memory
errors increase. The increasing size and density of server RAM in the
data center and cloud have shown increased uncorrectable memory errors.
There are already mechanisms in the kernel to recover from uncorrectable
memory errors. This series of patches provides the recovery mechanism for
the particular kernel agent khugepaged when it collapses memory pages.
Impact
======
The main reason we chose to make khugepaged collapsing tolerant of memory
failures was its high possibility of accessing poisoned memory while
performing functionally optional compaction actions. Standard
applications typically don't have strict requirements on the size of its
pages. So they are given 4K pages by the kernel. The kernel is able to
improve application performance by either
1) giving applications 2M pages to begin with, or
2) collapsing 4K pages into 2M pages when possible.
This collapsing operation is done by khugepaged, a kernel agent that is
constantly scanning memory. When collapsing 4K pages into a 2M page, it
must copy the data from the 4K pages into a physically contiguous 2M page.
Therefore, as long as there exists one poisoned cache line in collapsible
4K pages, khugepaged will eventually access it. The current impact to
users is a machine check exception triggered kernel panic. However,
khugepaged’s compaction operations are not functionally required kernel
actions. Therefore making khugepaged tolerant to poisoned memory will
greatly improve user experience.
This patch series is for cases where khugepaged is the first guy that
detects the memory errors on the poisoned pages. IOW, the pages are not
known to have memory errors when khugepaged collapsing gets to them. In
our observation, this happens frequently when the huge page ratio of the
system is relatively low, which is fairly common in virtual machines
running on cloud.
Solution
========
As stated before, it is less desirable to crash the system only because
khugepaged accesses poisoned pages while it is collapsing 4K pages. The
high level idea of this patch series is to skip the group of pages
(usually 512 4K-size pages) once khugepaged finds one of them is poisoned,
as these pages have become ineligible to be collapsed.
We are also careful to unwind operations khuagepaged has performed before
it detects memory failures. For example, before copying and collapsing a
group of anonymous pages into a huge page, the source pages will be
isolated and their page table is unlinked from their PMD. These
operations need to be undone in order to ensure these pages are not
changed/lost from the perspective of other threads (both user and kernel
space). As for file backed memory pages, there already exists a rollback
case. This patch just extends it so that khugepaged also correctly rolls
back when it fails to copy poisoned 4K pages.
This patch (of 3):
Make __collapse_huge_page_copy return whether copying anonymous pages
succeeded, and make collapse_huge_page handle the return status.
Break existing PTE scan loop into two for-loops. The first loop copies
source pages into target huge page, and can fail gracefully when running
into memory errors in source pages. If copying all pages succeeds, the
second loop releases and clears up these normal pages. Otherwise, the
second loop rolls back the page table and page states by:
- re-establishing the original PTEs-to-PMD connection.
- releasing source pages back to their LRU list.
Tested manually:
0. Enable khugepaged on system under test.
1. Start a two-thread application. Each thread allocates a chunk of
non-huge anonymous memory buffer.
2. Pick 4 random buffer locations (2 in each thread) and inject
uncorrectable memory errors at corresponding physical addresses.
3. Signal both threads to make their memory buffer collapsible, i.e.
calling madvise(MADV_HUGEPAGE).
4. Wait and check kernel log: khugepaged is able to recover from poisoned
pages and skips collapsing them.
5. Signal both threads to inspect their buffer contents and make sure no
data corruption.
Link: https://lkml.kernel.org/r/20230329151121.949896-1-jiaqiyan@google.com
Link: https://lkml.kernel.org/r/20230329151121.949896-2-jiaqiyan@google.com
Signed-off-by: Jiaqi Yan <jiaqiyan@google.com>
Cc: David Stevens <stevensd@chromium.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Naoya Horiguchi <naoya.horiguchi@nec.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Tong Tiangen <tongtiangen@huawei.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Currently, all contexts that flush memcg stats do so with sleeping not
allowed. Some of these contexts are perfectly safe to sleep in, such as
reading cgroup files from userspace or the background periodic flusher.
Flushing is an expensive operation that scales with the number of cpus and
the number of cgroups in the system, so avoid doing it atomically where
possible.
Refactor the code to make mem_cgroup_flush_stats() non-atomic (aka
sleepable), and provide a separate atomic version. The atomic version is
used in reclaim, refault, writeback, and in mem_cgroup_usage(). All other
code paths are left to use the non-atomic version. This includes
callbacks for userspace reads and the periodic flusher.
Since refault is the only caller of mem_cgroup_flush_stats_ratelimited(),
change it to mem_cgroup_flush_stats_atomic_ratelimited(). Reclaim and
refault code paths are modified to do non-atomic flushing in separate
later patches -- so it will eventually be changed back to
mem_cgroup_flush_stats_ratelimited().
Link: https://lkml.kernel.org/r/20230330191801.1967435-6-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vasily Averin <vasily.averin@linux.dev>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
As Johannes notes in [1], stats_flush_lock is currently used to:
(a) Protect updated to stats_flush_threshold.
(b) Protect updates to flush_next_time.
(c) Serializes calls to cgroup_rstat_flush() based on those ratelimits.
However:
1. stats_flush_threshold is already an atomic
2. flush_next_time is not atomic. The writer is locked, but the reader
is lockless. If the reader races with a flush, you could see this:
if (time_after(jiffies, flush_next_time))
spin_trylock()
flush_next_time = now + delay
flush()
spin_unlock()
spin_trylock()
flush_next_time = now + delay
flush()
spin_unlock()
which means we already can get flushes at a higher frequency than
FLUSH_TIME during races. But it isn't really a problem.
The reader could also see garbled partial updates if the compiler
decides to split the write, so it needs at least READ_ONCE and
WRITE_ONCE protection.
3. Serializing cgroup_rstat_flush() calls against the ratelimit
factors is currently broken because of the race in 2. But the race
is actually harmless, all we might get is the occasional earlier
flush. If there is no delta, the flush won't do much. And if there
is, the flush is justified.
So the lock can be removed all together. However, the lock also served
the purpose of preventing a thundering herd problem for concurrent
flushers, see [2]. Use an atomic instead to serve the purpose of
unifying concurrent flushers.
[1]https://lore.kernel.org/lkml/20230323172732.GE739026@cmpxchg.org/
[2]https://lore.kernel.org/lkml/20210716212137.1391164-2-shakeelb@google.com/
Link: https://lkml.kernel.org/r/20230330191801.1967435-5-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vasily Averin <vasily.averin@linux.dev>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Patch series "memcg: avoid flushing stats atomically where possible", v3.
rstat flushing is an expensive operation that scales with the number of
cpus and the number of cgroups in the system. The purpose of this series
is to minimize the contexts where we flush stats atomically.
Patches 1 and 2 are cleanups requested during reviews of prior versions of
this series.
Patch 3 makes sure we never try to flush from within an irq context.
Patches 4 to 7 introduce separate variants of mem_cgroup_flush_stats() for
atomic and non-atomic flushing, and make sure we only flush the stats
atomically when necessary.
Patch 8 is a slightly tangential optimization that limits the work done by
rstat flushing in some scenarios.
This patch (of 8):
cgroup_rstat_flush_irqsafe() can be a confusing name. It may read as
"irqs are disabled throughout", which is what the current implementation
does (currently under discussion [1]), but is not the intention. The
intention is that this function is safe to call from atomic contexts.
Name it as such.
Link: https://lkml.kernel.org/r/20230330191801.1967435-1-yosryahmed@google.com
Link: https://lkml.kernel.org/r/20230330191801.1967435-2-yosryahmed@google.com
Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Shakeel Butt <shakeelb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vasily Averin <vasily.averin@linux.dev>
Cc: Zefan Li <lizefan.x@bytedance.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
In __kfence_alloc() and __kfence_free(), we will set and check canary.
Assuming that the size of the object is close to 0, nearly 4k memory
accesses are required because setting and checking canary is executed byte
by byte.
canary is now defined like this:
KFENCE_CANARY_PATTERN(addr) ((u8)0xaa ^ (u8)((unsigned long)(addr) & 0x7))
Observe that canary is only related to the lower three bits of the
address, so every 8 bytes of canary are the same. We can access 8-byte
canary each time instead of byte-by-byte, thereby optimizing nearly 4k
memory accesses to 4k/8 times.
Use the bcc tool funclatency to measure the latency of __kfence_alloc()
and __kfence_free(), the numbers (deleted the distribution of latency) is
posted below. Though different object sizes will have an impact on the
measurement, we ignore it for now and assume the average object size is
roughly equal.
Before patching:
__kfence_alloc:
avg = 5055 nsecs, total: 5515252 nsecs, count: 1091
__kfence_free:
avg = 5319 nsecs, total: 9735130 nsecs, count: 1830
After patching:
__kfence_alloc:
avg = 3597 nsecs, total: 6428491 nsecs, count: 1787
__kfence_free:
avg = 3046 nsecs, total: 3415390 nsecs, count: 1121
The numbers indicate that there is ~30% - ~40% performance improvement.
Link: https://lkml.kernel.org/r/20230403122738.6006-1-zhangpeng.00@bytedance.com
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
A global vmap_blocks-xarray array can be contented under heavy usage of
the vm_map_ram()/vm_unmap_ram() APIs. The lock_stat shows that a
"vmap_blocks.xa_lock" lock is a second in a top-list when it comes to
contentions:
<snip>
----------------------------------------
class name con-bounces contentions ...
----------------------------------------
vmap_area_lock: 2554079 2554276 ...
--------------
vmap_area_lock 1297948 [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
vmap_area_lock 1256330 [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
vmap_area_lock 1 [<00000000c95c05a7>] find_vm_area+0x16/0x70
--------------
vmap_area_lock 1738590 [<00000000dd41cbaa>] alloc_vmap_area+0x1c7/0x910
vmap_area_lock 815688 [<000000009d927bf3>] free_vmap_block+0x4a/0xe0
vmap_area_lock 1 [<00000000c1d619d7>] __get_vm_area_node+0xd2/0x170
vmap_blocks.xa_lock: 862689 862698 ...
-------------------
vmap_blocks.xa_lock 378418 [<00000000625a5626>] vm_map_ram+0x359/0x4a0
vmap_blocks.xa_lock 484280 [<00000000caa2ef03>] xa_erase+0xe/0x30
-------------------
vmap_blocks.xa_lock 576226 [<00000000caa2ef03>] xa_erase+0xe/0x30
vmap_blocks.xa_lock 286472 [<00000000625a5626>] vm_map_ram+0x359/0x4a0
...
<snip>
that is a result of running vm_map_ram()/vm_unmap_ram() in
a loop. The test creates 64(on 64 CPUs system) threads and
each one maps/unmaps 1 page.
After this change the "xa_lock" can be considered as a noise
in the same test condition:
<snip>
...
&xa->xa_lock#1: 10333 10394 ...
--------------
&xa->xa_lock#1 5349 [<00000000bbbc9751>] xa_erase+0xe/0x30
&xa->xa_lock#1 5045 [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
--------------
&xa->xa_lock#1 7326 [<0000000018def45d>] vm_map_ram+0x3a4/0x4f0
&xa->xa_lock#1 3068 [<00000000bbbc9751>] xa_erase+0xe/0x30
...
<snip>
Running the test_vmalloc.sh run_test_mask=1024 nr_threads=64 nr_pages=5
shows around ~8 percent of throughput improvement of vm_map_ram() and
vm_unmap_ram() APIs.
This patch does not fix vmap_area_lock/free_vmap_area_lock and
purge_vmap_area_lock bottle-necks, it is rather a separate rework.
Link: https://lkml.kernel.org/r/20230330190639.431589-1-urezki@gmail.com
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Reviewed-by: Lorenzo Stoakes <lstoakes@gmail.com>
Reviewed-by: Baoquan He <bhe@redhat.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
