Commit c0d0381ade ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
synchronization") added code to take i_mmap_rwsem in read mode for the
duration of fault processing. However, this has been shown to cause
performance/scaling issues. Revert the code and go back to only taking
the semaphore in huge_pmd_share during the fault path.
Keep the code that takes i_mmap_rwsem in write mode before calling
try_to_unmap as this is required if huge_pmd_unshare is called.
NOTE: Reverting this code does expose the following race condition.
Faulting thread Unsharing thread
... ...
ptep = huge_pte_offset()
or
ptep = huge_pte_alloc()
...
i_mmap_lock_write
lock page table
ptep invalid <------------------------ huge_pmd_unshare()
Could be in a previously unlock_page_table
sharing process or worse i_mmap_unlock_write
...
ptl = huge_pte_lock(ptep)
get/update pte
set_pte_at(pte, ptep)
It is unknown if the above race was ever experienced by a user. It was
discovered via code inspection when initially addressed.
In subsequent patches, a new synchronization mechanism will be added to
coordinate pmd sharing and eliminate this race.
Link: https://lkml.kernel.org/r/20220914221810.95771-3-mike.kravetz@oracle.com
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: James Houghton <jthoughton@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Prakash Sangappa <prakash.sangappa@oracle.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Patch series "hugetlb: Use new vma lock for huge pmd sharing
synchronization", v2.
hugetlb fault scalability regressions have recently been reported [1].
This is not the first such report, as regressions were also noted when
commit c0d0381ade ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
synchronization") was added [2] in v5.7. At that time, a proposal to
address the regression was suggested [3] but went nowhere.
The regression and benefit of this patch series is not evident when
using the vm_scalability benchmark reported in [2] on a recent kernel.
Results from running,
"./usemem -n 48 --prealloc --prefault -O -U 3448054972"
48 sample Avg
next-20220913 next-20220913 next-20220913
unmodified revert i_mmap_sema locking vma sema locking, this series
-----------------------------------------------------------------------------
498150 KB/s 501934 KB/s 504793 KB/s
The recent regression report [1] notes page fault and fork latency of
shared hugetlb mappings. To measure this, I created two simple programs:
1) map a shared hugetlb area, write fault all pages, unmap area
Do this in a continuous loop to measure faults per second
2) map a shared hugetlb area, write fault a few pages, fork and exit
Do this in a continuous loop to measure forks per second
These programs were run on a 48 CPU VM with 320GB memory. The shared
mapping size was 250GB. For comparison, a single instance of the program
was run. Then, multiple instances were run in parallel to introduce
lock contention. Changing the locking scheme results in a significant
performance benefit.
test instances unmodified revert vma
--------------------------------------------------------------------------
faults per sec 1 393043 395680 389932
faults per sec 24 71405 81191 79048
forks per sec 1 2802 2747 2725
forks per sec 24 439 536 500
Combined faults 24 1621 68070 53662
Combined forks 24 358 67 142
Combined test is when running both faulting program and forking program
simultaneously.
Patches 1 and 2 of this series revert c0d0381ade and 87bf91d39b which
depends on c0d0381ade. Acquisition of i_mmap_rwsem is still required in
the fault path to establish pmd sharing, so this is moved back to
huge_pmd_share. With c0d0381ade reverted, this race is exposed:
Faulting thread Unsharing thread
... ...
ptep = huge_pte_offset()
or
ptep = huge_pte_alloc()
...
i_mmap_lock_write
lock page table
ptep invalid <------------------------ huge_pmd_unshare()
Could be in a previously unlock_page_table
sharing process or worse i_mmap_unlock_write
...
ptl = huge_pte_lock(ptep)
get/update pte
set_pte_at(pte, ptep)
Reverting 87bf91d39b exposes races in page fault/file truncation. When
the new vma lock is put to use in patch 8, this will handle the fault/file
truncation races. This is explained in patch 9 where code associated with
these races is cleaned up.
Patches 3 - 5 restructure existing code in preparation for using the new
vma lock (rw semaphore) for pmd sharing synchronization. The idea is that
this semaphore will be held in read mode for the duration of fault
processing, and held in write mode for unmap operations which may call
huge_pmd_unshare. Acquiring i_mmap_rwsem is also still required to
synchronize huge pmd sharing. However it is only required in the fault
path when setting up sharing, and will be acquired in huge_pmd_share().
Patch 6 adds the new vma lock and all supporting routines, but does not
actually change code to use the new lock.
Patch 7 refactors code in preparation for using the new lock. And, patch
8 finally adds code to make use of this new vma lock. Unfortunately, the
fault code and truncate/hole punch code would naturally take locks in the
opposite order which could lead to deadlock. Since the performance of
page faults is more important, the truncation/hole punch code is modified
to back out and take locks in the correct order if necessary.
[1] https://lore.kernel.org/linux-mm/43faf292-245b-5db5-cce9-369d8fb6bd21@infradead.org/
[2] https://lore.kernel.org/lkml/20200622005551.GK5535@shao2-debian/
[3] https://lore.kernel.org/linux-mm/20200706202615.32111-1-mike.kravetz@oracle.com/
This patch (of 9):
Commit c0d0381ade ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
synchronization") added code to take i_mmap_rwsem in read mode for the
duration of fault processing. The use of i_mmap_rwsem to prevent
fault/truncate races depends on this. However, this has been shown to
cause performance/scaling issues. As a result, that code will be
reverted. Since the use i_mmap_rwsem to address page fault/truncate races
depends on this, it must also be reverted.
In a subsequent patch, code will be added to detect the fault/truncate
race and back out operations as required.
Link: https://lkml.kernel.org/r/20220914221810.95771-1-mike.kravetz@oracle.com
Link: https://lkml.kernel.org/r/20220914221810.95771-2-mike.kravetz@oracle.com
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: James Houghton <jthoughton@google.com>
Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mina Almasry <almasrymina@google.com>
Cc: Muchun Song <songmuchun@bytedance.com>
Cc: Naoya Horiguchi <naoya.horiguchi@linux.dev>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Prakash Sangappa <prakash.sangappa@oracle.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
The name "check_free_page()" provides no information regarding its return
value when the page is indeed found to be bad.
Renaming it to "free_page_is_bad()" makes it clear that a `true' return
value means the page was bad.
And make it return a bool, not an int.
[akpm@linux-foundation.org: don't use bool as int]
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: ke.wang <ke.wang@unisoc.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Zhaoyang Huang <huangzhaoyang@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
When the 'kdamond_wait_activation()' function or 'after_sampling()' or
'after_aggregation()' DAMON callbacks return an error, it is unnecessary
to use bool 'done' to check if kdamond should be finished. This commit
simplifies the kdamond stop mechanism by removing 'done' and break the
while loop directly in the cases.
Link: https://lkml.kernel.org/r/1663060287-30201-4-git-send-email-kaixuxia@tencent.com
Signed-off-by: Kaixu Xia <kaixuxia@tencent.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
DAMON_LRU_SORT receives monitoring attributes by parameters one by one to
separate variables, and then combines those into 'struct damon_attrs'.
This commit makes the module directly stores the parameter values to a
static 'struct damon_attrs' variable and use it to simplify the code.
Link: https://lkml.kernel.org/r/20220913174449.50645-9-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
DAMON_RECLAIM receives monitoring attributes by parameters one by one to
separate variables, and then combine those into 'struct damon_attrs'.
This commit makes the module directly stores the parameter values to a
static 'struct damon_attrs' variable and use it to simplify the code.
Link: https://lkml.kernel.org/r/20220913174449.50645-8-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
DAMON monitoring attributes are directly defined as fields of 'struct
damon_ctx'. This makes 'struct damon_ctx' a little long and complicated.
This commit defines and uses a struct, 'struct damon_attrs', which is
dedicated for only the monitoring attributes to make the purpose of the
five values clearer and simplify 'struct damon_ctx'.
Link: https://lkml.kernel.org/r/20220913174449.50645-6-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
The 'struct damos' creation function, 'damon_new_scheme()', does
initialization of private fileds of 'struct damos_quota' in it. As its
verbose and makes the function unnecessarily long, this commit factors it
out to separate function.
Link: https://lkml.kernel.org/r/20220913174449.50645-5-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
The function for new 'struct damos' creation, 'damon_new_scheme()', copies
each field of the struct one by one, though it could simply copied via
struct to struct. This commit replaces the unnecessarily verbose
field-to-field copies with struct-to-struct copies to make code simple and
short.
Link: https://lkml.kernel.org/r/20220913174449.50645-4-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Patch series "mm/damon: cleanup code".
DAMON code was not so clean from the beginning, but it has been too much
nowadays, especially due to the duplicates in DAMON_RECLAIM and
DAMON_LRU_SORT. This patchset cleans some of the mess.
This patch (of 22):
The 'switch-case' statement in 'damon_va_apply_scheme()' function provides
a 'case' for every supported DAMOS action while all not-yet-supported
DAMOS actions fall through the 'default' case, and comment it so that
people can easily know which actions are supported. Its counterpart in
'paddr', 'damon_pa_apply_scheme()', however, doesn't. This commit makes
the 'paddr' side function follows the pattern of 'vaddr' for better
readability and consistency.
Link: https://lkml.kernel.org/r/20220913174449.50645-1-sj@kernel.org
Link: https://lkml.kernel.org/r/20220913174449.50645-2-sj@kernel.org
Signed-off-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
To handle the discontiguous case, mem_map_next() has a parameter named
`offset`. As a function caller, one would be confused why "get next
entry" needs a parameter named "offset". The other drawback of
mem_map_next() is that the callers must take care of the map between
parameter "iter" and "offset", otherwise we may get an hole or duplication
during iteration. So we use nth_page instead of mem_map_next.
And replace mem_map_offset with nth_page() per Matthew's comments.
Link: https://lkml.kernel.org/r/1662708669-9395-1-git-send-email-lic121@chinatelecom.cn
Signed-off-by: Cheng Li <lic121@chinatelecom.cn>
Fixes: 69d177c2fc ("hugetlbfs: handle pages higher order than MAX_ORDER")
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
In lru_sort.c and reclaim.c, they are all defining get_monitoring_region()
function, there is no need to define it separately.
As 'get_monitoring_region()' is not a 'static' function anymore, we try to
use a prefix to distinguish with other functions, so there rename it to
'damon_find_biggest_system_ram'.
Link: https://lkml.kernel.org/r/20220909213606.136221-1-sj@kernel.org
Signed-off-by: Xin Hao <xhao@linux.alibaba.com>
Signed-off-by: SeongJae Park <sj@kernel.org>
Suggested-by: SeongJae Park <sj@kernel.org>
Reviewed-by: SeongJae Park <sj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
