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fs/proc/task_mmu: account non-present entries as "maybe shared, but no idea how often"

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We currently rely on mapcount information for pages referenced by
non-present entries to calculate the USS (shared vs.  private) and the
PSS.

However, relying on mapcounts for non-present entries doesn't make any
sense.  We have to treat such entries as "maybe shared, but no idea how
often", implying that they will *not* get accounted towards the USS, and
will get fully accounted to the PSS (no idea how often shared).

There is one exception: device exclusive entries essentially behave like
present entries (e.g., mapcount incremented).

In smaps_pmd_entry(), use is_pfn_swap_entry() instead of
is_migration_entry(), which should not make a real difference but makes
the code look more similar to the PTE variant.

While at it, adjust the comments in smaps_account().

Link: https://lkml.kernel.org/r/20240607122357.115423-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Lance Yang <ioworker0@gmail.com>
Cc: Oscar Salvador <osalvador@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
David Hildenbrand 2024-06-07 14:23:55 +02:00 committed by Andrew Morton
parent 2c1f057e5b
commit 3689c3ebdd
1 changed files with 31 additions and 22 deletions
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53
fs/proc/task_mmu.c
View file

@ -442,7 +442,7 @@ static void smaps_page_accumulate(struct mem_size_stats *mss,
static void smaps_account(struct mem_size_stats *mss, struct page *page,
bool compound, bool young, bool dirty, bool locked,
bool migration)
bool present)
{
struct folio *folio = page_folio(page);
int i, nr = compound ? compound_nr(page) : 1;
@ -471,22 +471,27 @@ static void smaps_account(struct mem_size_stats *mss, struct page *page,
* Then accumulate quantities that may depend on sharing, or that may
* differ page-by-page.
*
* refcount == 1 guarantees the page is mapped exactly once.
* If any subpage of the compound page mapped with PTE it would elevate
* the refcount.
* refcount == 1 for present entries guarantees that the folio is mapped
* exactly once. For large folios this implies that exactly one
* PTE/PMD/... maps (a part of) this folio.
*
* The page_mapcount() is called to get a snapshot of the mapcount.
* Without holding the page lock this snapshot can be slightly wrong as
* we cannot always read the mapcount atomically. It is not safe to
* call page_mapcount() even with PTL held if the page is not mapped,
* especially for migration entries. Treat regular migration entries
* as mapcount == 1.
* Treat all non-present entries (where relying on the mapcount and
* refcount doesn't make sense) as "maybe shared, but not sure how
* often". We treat device private entries as being fake-present.
*
* Note that it would not be safe to read the mapcount especially for
* pages referenced by migration entries, even with the PTL held.
*/
if ((folio_ref_count(folio) == 1) || migration) {
if (folio_ref_count(folio) == 1 || !present) {
smaps_page_accumulate(mss, folio, size, size << PSS_SHIFT,
dirty, locked, true);
dirty, locked, present);
return;
}
/*
* The page_mapcount() is called to get a snapshot of the mapcount.
* Without holding the folio lock this snapshot can be slightly wrong as
* we cannot always read the mapcount atomically.
*/
for (i = 0; i < nr; i++, page++) {
int mapcount = page_mapcount(page);
unsigned long pss = PAGE_SIZE << PSS_SHIFT;
@ -531,13 +536,14 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
struct vm_area_struct *vma = walk->vma;
bool locked = !!(vma->vm_flags & VM_LOCKED);
struct page *page = NULL;
bool migration = false, young = false, dirty = false;
bool present = false, young = false, dirty = false;
pte_t ptent = ptep_get(pte);
if (pte_present(ptent)) {
page = vm_normal_page(vma, addr, ptent);
young = pte_young(ptent);
dirty = pte_dirty(ptent);
present = true;
} else if (is_swap_pte(ptent)) {
swp_entry_t swpent = pte_to_swp_entry(ptent);
@ -555,8 +561,8 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
mss->swap_pss += (u64)PAGE_SIZE << PSS_SHIFT;
}
} else if (is_pfn_swap_entry(swpent)) {
if (is_migration_entry(swpent))
migration = true;
if (is_device_private_entry(swpent))
present = true;
page = pfn_swap_entry_to_page(swpent);
}
} else {
@ -567,7 +573,7 @@ static void smaps_pte_entry(pte_t *pte, unsigned long addr,
if (!page)
return;
smaps_account(mss, page, false, young, dirty, locked, migration);
smaps_account(mss, page, false, young, dirty, locked, present);
}
#ifdef CONFIG_TRANSPARENT_HUGEPAGE
@ -578,18 +584,17 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
struct vm_area_struct *vma = walk->vma;
bool locked = !!(vma->vm_flags & VM_LOCKED);
struct page *page = NULL;
bool present = false;
struct folio *folio;
bool migration = false;
if (pmd_present(*pmd)) {
page = vm_normal_page_pmd(vma, addr, *pmd);
present = true;
} else if (unlikely(thp_migration_supported() && is_swap_pmd(*pmd))) {
swp_entry_t entry = pmd_to_swp_entry(*pmd);
if (is_migration_entry(entry)) {
migration = true;
if (is_pfn_swap_entry(entry))
page = pfn_swap_entry_to_page(entry);
}
}
if (IS_ERR_OR_NULL(page))
return;
@ -604,7 +609,7 @@ static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
mss->file_thp += HPAGE_PMD_SIZE;
smaps_account(mss, page, true, pmd_young(*pmd), pmd_dirty(*pmd),
locked, migration);
locked, present);
}
#else
static void smaps_pmd_entry(pmd_t *pmd, unsigned long addr,
@ -735,17 +740,21 @@ static int smaps_hugetlb_range(pte_t *pte, unsigned long hmask,
struct vm_area_struct *vma = walk->vma;
pte_t ptent = huge_ptep_get(pte);
struct folio *folio = NULL;
bool present = false;
if (pte_present(ptent)) {
folio = page_folio(pte_page(ptent));
present = true;
} else if (is_swap_pte(ptent)) {
swp_entry_t swpent = pte_to_swp_entry(ptent);
if (is_pfn_swap_entry(swpent))
folio = pfn_swap_entry_folio(swpent);
}
if (folio) {
if (folio_likely_mapped_shared(folio) ||
/* We treat non-present entries as "maybe shared". */
if (!present || folio_likely_mapped_shared(folio) ||
hugetlb_pmd_shared(pte))
mss->shared_hugetlb += huge_page_size(hstate_vma(vma));
else