On 27.02.24 21:15, David Hildenbrand wrote:
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. Cc: Barry Song <v-songbaohua@xxxxxxxx> Cc: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx> Cc: Ryan Roberts <ryan.roberts@xxxxxxx> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> ---
The following fixup on top to make kerneldoc happy: From 3e472636d266e3acba3755ed5712992adbc2151d Mon Sep 17 00:00:00 2001 From: David Hildenbrand <david@xxxxxxxxxx> Date: Mon, 25 Mar 2024 09:23:03 +0100 Subject: [PATCH] folio_likely_mapped_shared() kerneldoc fixup Fixup "mm: convert folio_estimated_sharers() to folio_likely_mapped_shared()". Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> --- include/linux/mm.h | 32 +++++++++++++++++--------------- 1 file changed, 17 insertions(+), 15 deletions(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index afe27ff3fa94..fb3724723448 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2183,23 +2183,25 @@ static inline size_t folio_size(struct folio *folio) * at most once into an MM, and they cannot be partially mapped. * * For other folios, the result can be fuzzy: - * (a) 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. - * (b) 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. - * (c) 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. + * #. 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). It does not consider: - * (1) If the folio might get mapped in the (near) future (e.g., swapcache, - * pagecache, temporary unmapping for migration). - * (2) If the folio is mapped differently (VM_PFNMAP). - * (3) If hugetlb page table sharing applies. Callers might want to check - * hugetlb_pmd_shared(). + * 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. */ -- 2.43.2 -- Cheers, David / dhildenb
