mm: stop maintaining the per-page mapcount of large folios (CONFIG_NO_PAGE_MAPCOUNT)

Everything is in place to stop using the per-page mapcounts in large
folios: the mapcount of tail pages will always be logically 0 (-1 value),
just like it currently is for hugetlb folios already, and the page
mapcount of the head page is either 0 (-1 value) or contains a page type
(e.g., hugetlb).

Maintaining _nr_pages_mapped without per-page mapcounts is impossible, so
that one also has to go with CONFIG_NO_PAGE_MAPCOUNT.

There are two remaining implications:

(1) Per-node, per-cgroup and per-lruvec stats of "NR_ANON_MAPPED"
    ("mapped anonymous memory") and "NR_FILE_MAPPED"
    ("mapped file memory"):

    As soon as any page of the folio is mapped -- folio_mapped() -- we
    now account the complete folio as mapped. Once the last page is
    unmapped -- !folio_mapped() -- we account the complete folio as
    unmapped.

    This implies that ...

    * "AnonPages" and "Mapped" in /proc/meminfo and
      /sys/devices/system/node/*/meminfo
    * cgroup v2: "anon" and "file_mapped" in "memory.stat" and
      "memory.numa_stat"
    * cgroup v1: "rss" and "mapped_file" in "memory.stat" and
      "memory.numa_stat

    ... can now appear higher than before. But note that these folios do
    consume that memory, simply not all pages are actually currently
    mapped.

    It's worth nothing that other accounting in the kernel (esp. cgroup
    charging on allocation) is not affected by this change.

    [why oh why is "anon" called "rss" in cgroup v1]

 (2) Detecting partial mappings

     Detecting whether anon THPs are partially mapped gets a bit more
     unreliable. As long as a single MM maps such a large folio
     ("exclusively mapped"), we can reliably detect it. Especially before
     fork() / after a short-lived child process quit, we will detect
     partial mappings reliably, which is the common case.

     In essence, if the average per-page mapcount in an anon THP is < 1,
     we know for sure that we have a partial mapping.

     However, as soon as multiple MMs are involved, we might miss detecting
     partial mappings: this might be relevant with long-lived child
     processes. If we have a fully-mapped anon folio before fork(), once
     our child processes and our parent all unmap (zap/COW) the same pages
     (but not the complete folio), we might not detect the partial mapping.
     However, once the child processes quit we would detect the partial
     mapping.

     How relevant this case is in practice remains to be seen.
     Swapout/migration will likely mitigate this.

     In the future, RMAP walkers could check for that for that case
     (e.g., when collecting access bits during reclaim) and simply flag
     them for deferred-splitting.

Link: https://lkml.kernel.org/r/20250303163014.1128035-21-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>

2 files changed, 12 insertions, 2 deletions

diff --git a/Documentation/admin-guide/cgroup-v1/memory.rst b/Documentation/admin-guide/cgroup-v1/memory.rst
index 286d16fc22eb..53cf081b22e8 100644
--- a/Documentation/admin-guide/cgroup-v1/memory.rst
+++ b/Documentation/admin-guide/cgroup-v1/memory.rst
@@ -609,6 +609,10 @@ memory.stat file includes following statistics:
 
 	'rss + mapped_file" will give you resident set size of cgroup.
 
+	Note that some kernel configurations might account complete larger
+	allocations (e.g., THP) towards 'rss' and 'mapped_file', even if
+	only some, but not all that memory is mapped.
+
 	(Note: file and shmem may be shared among other cgroups. In that case,
 	mapped_file is accounted only when the memory cgroup is owner of page
 	cache.)
diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index cb1b4e759b7e..f8a894a16307 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1440,7 +1440,10 @@ The following nested keys are defined.
 
 	  anon
 		Amount of memory used in anonymous mappings such as
-		brk(), sbrk(), and mmap(MAP_ANONYMOUS)
+		brk(), sbrk(), and mmap(MAP_ANONYMOUS). Note that
+		some kernel configurations might account complete larger
+		allocations (e.g., THP) if only some, but not all the
+		memory of such an allocation is mapped anymore.
 
 	  file
 		Amount of memory used to cache filesystem data,
@@ -1483,7 +1486,10 @@ The following nested keys are defined.
 		Amount of application memory swapped out to zswap.
 
 	  file_mapped
-		Amount of cached filesystem data mapped with mmap()
+		Amount of cached filesystem data mapped with mmap(). Note
+		that some kernel configurations might account complete
+		larger allocations (e.g., THP) if only some, but not
+		not all the memory of such an allocation is mapped.
 
 	  file_dirty
 		Amount of cached filesystem data that was modified but