Commit eb168769 authored by David Hildenbrand's avatar David Hildenbrand Committed by Andrew Morton
Browse files

fs/proc/task_mmu: remove per-page mapcount dependency for PM_MMAP_EXCLUSIVE... 

fs/proc/task_mmu: remove per-page mapcount dependency for PM_MMAP_EXCLUSIVE (CONFIG_NO_PAGE_MAPCOUNT)

Let's implement an alternative when per-page mapcounts in large folios are
no longer maintained -- soon with CONFIG_NO_PAGE_MAPCOUNT.

PM_MMAP_EXCLUSIVE will now be set if folio_likely_mapped_shared() is true
-- when the folio is considered "mapped shared", including when it once
was "mapped shared" but no longer is, as documented.

This might result in and under-indication of "exclusively mapped", which
is considered better than over-indicating it: under-estimating the USS
(Unique Set Size) is better than over-estimating it.

As an alternative, we could simply remove that flag with
CONFIG_NO_PAGE_MAPCOUNT completely, but there might be value to it.  So,
let's keep it like that and document the behavior.

Link: https://lkml.kernel.org/r/20250303163014.1128035-18-david@redhat.com


Signed-off-by: default avatarDavid 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: default avatarAndrew Morton <akpm@linux-foundation.org>
parent ae4192b7

Documentation/admin-guide/mm/pagemap.rst

+11 −0
Original line number Diff line number Diff line
@@ -38,6 +38,17 @@ There are four components to pagemap: 
   precisely which pages are mapped (or in swap) and comparing mapped

   pages between processes.



   Traditionally, bit 56 indicates that a page is mapped exactly once and bit

   56 is clear when a page is mapped multiple times, even when mapped in the

   same process multiple times. In some kernel configurations, the semantics

   for pages part of a larger allocation (e.g., THP) can differ: bit 56 is set

   if all pages part of the corresponding large allocation are *certainly*

   mapped in the same process, even if the page is mapped multiple times in that

   process. Bit 56 is clear when any page page of the larger allocation

   is *maybe* mapped in a different process. In some cases, a large allocation

   might be treated as "maybe mapped by multiple processes" even though this

   is no longer the case.



   Efficient users of this interface will use ``/proc/pid/maps`` to

   determine which areas of memory are actually mapped and llseek to

   skip over unmapped regions.

fs/proc/task_mmu.c

+9 −2
Original line number Diff line number Diff line
@@ -1652,6 +1652,13 @@ static int add_to_pagemap(pagemap_entry_t *pme, struct pagemapread *pm) 
	return 0;

}



static bool __folio_page_mapped_exclusively(struct folio *folio, struct page *page)

{

	if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT))

		return folio_precise_page_mapcount(folio, page) == 1;

	return !folio_maybe_mapped_shared(folio);

}



static int pagemap_pte_hole(unsigned long start, unsigned long end,

			    __always_unused int depth, struct mm_walk *walk)

{
@@ -1742,7 +1749,7 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm, 
		if (!folio_test_anon(folio))

			flags |= PM_FILE;

		if ((flags & PM_PRESENT) &&

		    folio_precise_page_mapcount(folio, page) == 1)

		    __folio_page_mapped_exclusively(folio, page))

			flags |= PM_MMAP_EXCLUSIVE;

	}

	if (vma->vm_flags & VM_SOFTDIRTY)
@@ -1817,7 +1824,7 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end, 
			pagemap_entry_t pme;



			if (folio && (flags & PM_PRESENT) &&

			    folio_precise_page_mapcount(folio, page + idx) == 1)

			    __folio_page_mapped_exclusively(folio, page))

				cur_flags |= PM_MMAP_EXCLUSIVE;



			pme = make_pme(frame, cur_flags);