[PATCH v1 4/6] fs/proc/task_mmu: account non-present entries as "maybe shared, but no idea how often"

David Hildenbrand
2024-06-07T12:24:12Z

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().

Signed-off-by: David Hildenbrand <david@redhat.com>
---
fs/proc/task_mmu.c | 53 +++++++++++++++++++++++++++-------------------
1 file changed, 31 insertions(+), 22 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index f427176ce2c34..67d9b406c7586 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -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,
@@ -732,17 +737,21 @@ 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
--
2.45.2