From: David Hildenbrand david@redhat.com
next inclusion from next-20240510 commit ebb34f78d72c2320620ba6d55cb22a52949047a1 category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I9OCYO CVE: NA
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
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Callers of folio_estimated_sharers() only care about "mapped shared vs. mapped exclusively", not the exact estimate of sharers. Let's consolidate and unify the condition users are checking. While at it clarify the semantics and extend the discussion on the fuzziness.
Use the "likely mapped shared" terminology to better express what the (adjusted) function actually checks.
Whether a partially-mappable folio is more likely to not be partially mapped than partially mapped is debatable. In the future, we might be able to improve our estimate for partially-mappable folios, though.
Note that we will now consistently detect "mapped shared" only if the first subpage is actually mapped multiple times. When the first subpage is not mapped, we will consistently detect it as "mapped exclusively". This change should currently only affect the usage in madvise_free_pte_range() and queue_folios_pte_range() for large folios: if the first page was already unmapped, we would have skipped the folio.
[david@redhat.com: folio_likely_mapped_shared() kerneldoc fixup] Link: https://lkml.kernel.org/r/dd0ad9f2-2d7a-45f3-9ba3-979488c7dd27@redhat.com Link: https://lkml.kernel.org/r/20240227201548.857831-1-david@redhat.com Signed-off-by: David Hildenbrand david@redhat.com Reviewed-by: Khalid Aziz khalid.aziz@oracle.com Acked-by: Barry Song v-songbaohua@oppo.com Reviewed-by: Vishal Moola (Oracle) vishal.moola@gmail.com Reviewed-by: Ryan Roberts ryan.roberts@arm.com Reviewed-by: Zi Yan ziy@nvidia.com Signed-off-by: Andrew Morton akpm@linux-foundation.org Conflicts: mm/mempolicy.c [ Context conflicts due to lack of commit 1cb5d11a370f. ] Signed-off-by: Liu Shixin liushixin2@huawei.com --- include/linux/mm.h | 48 ++++++++++++++++++++++++++++++++++++---------- mm/huge_memory.c | 2 +- mm/madvise.c | 6 +++--- mm/memory.c | 2 +- mm/mempolicy.c | 14 ++++++-------- mm/migrate.c | 8 ++++---- 6 files changed, 53 insertions(+), 27 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h index 83519e1cfc95..f86fd573a4a1 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2160,21 +2160,49 @@ static inline size_t folio_size(struct folio *folio) }
/** - * folio_estimated_sharers - Estimate the number of sharers of a folio. + * folio_likely_mapped_shared - Estimate if the folio is mapped into the page + * tables of more than one MM * @folio: The folio. * - * folio_estimated_sharers() aims to serve as a function to efficiently - * estimate the number of processes sharing a folio. This is done by - * looking at the precise mapcount of the first subpage in the folio, and - * assuming the other subpages are the same. This may not be true for large - * folios. If you want exact mapcounts for exact calculations, look at - * page_mapcount() or folio_total_mapcount(). + * This function checks if the folio is currently mapped into more than one + * MM ("mapped shared"), or if the folio is only mapped into a single MM + * ("mapped exclusively"). * - * Return: The estimated number of processes sharing a folio. + * As precise information is not easily available for all folios, this function + * estimates the number of MMs ("sharers") that are currently mapping a folio + * using the number of times the first page of the folio is currently mapped + * into page tables. + * + * For small anonymous folios (except KSM folios) and anonymous hugetlb folios, + * the return value will be exactly correct, because they can only be mapped + * at most once into an MM, and they cannot be partially mapped. + * + * For other folios, the result can be fuzzy: + * #. For partially-mappable large folios (THP), the return value can wrongly + * indicate "mapped exclusively" (false negative) when the folio is + * only partially mapped into at least one MM. + * #. For pagecache folios (including hugetlb), the return value can wrongly + * indicate "mapped shared" (false positive) when two VMAs in the same MM + * cover the same file range. + * #. For (small) KSM folios, the return value can wrongly indicate "mapped + * shared" (false negative), when the folio is mapped multiple times into + * the same MM. + * + * Further, this function only considers current page table mappings that + * are tracked using the folio mapcount(s). + * + * This function does not consider: + * #. If the folio might get mapped in the (near) future (e.g., swapcache, + * pagecache, temporary unmapping for migration). + * #. If the folio is mapped differently (VM_PFNMAP). + * #. If hugetlb page table sharing applies. Callers might want to check + * hugetlb_pmd_shared(). + * + * Return: Whether the folio is estimated to be mapped into more than one MM. */ -static inline int folio_estimated_sharers(struct folio *folio) +static inline bool folio_likely_mapped_shared(struct folio *folio) { - return page_mapcount(folio_page(folio, 0)); + return page_mapcount(folio_page(folio, 0)) > 1; }
#ifndef HAVE_ARCH_MAKE_PAGE_ACCESSIBLE diff --git a/mm/huge_memory.c b/mm/huge_memory.c index fd3ff0016ca4..5e8374aa8b59 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -1833,7 +1833,7 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, * If other processes are mapping this folio, we couldn't discard * the folio unless they all do MADV_FREE so let's skip the folio. */ - if (folio_estimated_sharers(folio) != 1) + if (folio_likely_mapped_shared(folio)) goto out;
if (!folio_trylock(folio)) diff --git a/mm/madvise.c b/mm/madvise.c index 8df46ebffdea..e6570812f723 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -409,7 +409,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, folio = pfn_folio(pmd_pfn(orig_pmd));
/* Do not interfere with other mappings of this folio */ - if (folio_estimated_sharers(folio) != 1) + if (folio_likely_mapped_shared(folio)) goto huge_unlock;
if (pageout_anon_only_filter && !folio_test_anon(folio)) @@ -485,7 +485,7 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd, if (folio_test_large(folio)) { int err;
- if (folio_estimated_sharers(folio) > 1) + if (folio_likely_mapped_shared(folio)) break; if (pageout_anon_only_filter && !folio_test_anon(folio)) break; @@ -704,7 +704,7 @@ static int madvise_free_pte_range(pmd_t *pmd, unsigned long addr, if (folio_test_large(folio)) { int err;
- if (folio_estimated_sharers(folio) != 1) + if (folio_likely_mapped_shared(folio)) break; if (!folio_trylock(folio)) break; diff --git a/mm/memory.c b/mm/memory.c index 1b3309b50357..2a0280384b67 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -5098,7 +5098,7 @@ static vm_fault_t do_numa_page(struct vm_fault *vmf) * Flag if the folio is shared between multiple address spaces. This * is later used when determining whether to group tasks together */ - if (folio_estimated_sharers(folio) > 1 && (vma->vm_flags & VM_SHARED)) + if (folio_likely_mapped_shared(folio) && (vma->vm_flags & VM_SHARED)) flags |= TNF_SHARED;
nid = folio_nid(folio); diff --git a/mm/mempolicy.c b/mm/mempolicy.c index f28f4c277099..a80f99751904 100644 --- a/mm/mempolicy.c +++ b/mm/mempolicy.c @@ -608,12 +608,11 @@ static int queue_folios_hugetlb(pte_t *pte, unsigned long hmask, * With MPOL_MF_MOVE, we try to migrate only unshared folios. If it * is shared it is likely not worth migrating. * - * To check if the folio is shared, ideally we want to make sure - * every page is mapped to the same process. Doing that is very - * expensive, so check the estimated mapcount of the folio instead. + * See folio_likely_mapped_shared() on possible imprecision when we + * cannot easily detect if a folio is shared. */ if (flags & (MPOL_MF_MOVE_ALL) || - (flags & MPOL_MF_MOVE && folio_estimated_sharers(folio) == 1 && + (flags & MPOL_MF_MOVE && !folio_likely_mapped_shared(folio) && !hugetlb_pmd_shared(pte))) { if (!isolate_hugetlb(folio, qp->pagelist) && (flags & MPOL_MF_STRICT)) @@ -1040,11 +1039,10 @@ static int migrate_folio_add(struct folio *folio, struct list_head *foliolist, * We try to migrate only unshared folios. If it is shared it * is likely not worth migrating. * - * To check if the folio is shared, ideally we want to make sure - * every page is mapped to the same process. Doing that is very - * expensive, so check the estimated mapcount of the folio instead. + * See folio_likely_mapped_shared() on possible imprecision when we + * cannot easily detect if a folio is shared. */ - if ((flags & MPOL_MF_MOVE_ALL) || folio_estimated_sharers(folio) == 1) { + if ((flags & MPOL_MF_MOVE_ALL) || !folio_likely_mapped_shared(folio)) { if (folio_isolate_lru(folio)) { list_add_tail(&folio->lru, foliolist); node_stat_mod_folio(folio, diff --git a/mm/migrate.c b/mm/migrate.c index 936ed28f90c8..141509ec9a48 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -2599,11 +2599,11 @@ int migrate_misplaced_folio(struct folio *folio, struct vm_area_struct *vma, /* * Don't migrate file folios that are mapped in multiple processes * with execute permissions as they are probably shared libraries. - * To check if the folio is shared, ideally we want to make sure - * every page is mapped to the same process. Doing that is very - * expensive, so check the estimated mapcount of the folio instead. + * + * See folio_likely_mapped_shared() on possible imprecision when we + * cannot easily detect if a folio is shared. */ - if (folio_estimated_sharers(folio) != 1 && folio_is_file_lru(folio) && + if (folio_likely_mapped_shared(folio) && folio_is_file_lru(folio) && (vma->vm_flags & VM_EXEC)) goto out;