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mm: CONFIG_NO_PAGE_MAPCOUNT to prepare for not maintain per-page mapcounts in large folios

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We're close to the finishing line: let's introduce a new
CONFIG_NO_PAGE_MAPCOUNT config option where we will incrementally remove
any dependencies on per-page mapcounts in large folios.  Once that's done,
we'll stop maintaining the per-page mapcounts with this config option
enabled.

CONFIG_NO_PAGE_MAPCOUNT will be EXPERIMENTAL for now, as we'll have to
learn about some of the real world impact of some of the implications.

As writing "!CONFIG_NO_PAGE_MAPCOUNT" is really nasty, let's introduce a
helper config option "CONFIG_PAGE_MAPCOUNT" that expresses the negation.

Link: https://lkml.kernel.org/r/20250303163014.1128035-16-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Andy Lutomirks^H^Hski <luto@kernel.org>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Matthew Wilcow (Oracle) <willy@infradead.org>
Cc: Michal Koutn <mkoutny@suse.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: tejun heo <tj@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Zefan Li <lizefan.x@bytedance.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand
2025-03-03 17:30:08 +01:00
committed by Andrew Morton
parent 003fde4492
commit e63ee43e3e
1 changed files with 17 additions and 0 deletions
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mm/Kconfig
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@@ -887,8 +887,25 @@ config READ_ONLY_THP_FOR_FS
support of file THPs will be developed in the next few release
cycles.
config NO_PAGE_MAPCOUNT
bool "No per-page mapcount (EXPERIMENTAL)"
help
Do not maintain per-page mapcounts for pages part of larger
allocations, such as transparent huge pages.
When this config option is enabled, some interfaces that relied on
this information will rely on less-precise per-allocation information
instead: for example, using the average per-page mapcount in such
a large allocation instead of the per-page mapcount.
EXPERIMENTAL because the impact of some changes is still unclear.
endif # TRANSPARENT_HUGEPAGE
# simple helper to make the code a bit easier to read
config PAGE_MAPCOUNT
def_bool !NO_PAGE_MAPCOUNT
#
# The architecture supports pgtable leaves that is larger than PAGE_SIZE
#