+ mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions.patch added to mm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions
has been added to the -mm mm-unstable branch.  Its filename is
     mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions.patch

This patch will later appear in the mm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: David Hildenbrand <david@redhat.com>
Subject: mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions
Date: Thu, 2 May 2024 10:52:59 +0200

Let's document why hugetlb still uses folio_mapcount() and is prone to
leaking memory between processes, for example using vmsplice() that still
uses FOLL_GET.

More details can be found in [1], especially around how hugetlb pages
cannot really be overcommitted, and why we don't particularly care about
these vmsplice() leaks for hugetlb -- in contrast to ordinary memory.

[1] https://lore.kernel.org/all/8b42a24d-caf0-46ef-9e15-0f88d47d2f21@redhat.com/

Link: https://lkml.kernel.org/r/20240502085259.103784-3-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Suggested-by: Peter Xu <peterx@redhat.com>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 mm/hugetlb.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/mm/hugetlb.c~mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions
+++ a/mm/hugetlb.c
@@ -5963,6 +5963,13 @@ retry_avoidcopy:
 	/*
 	 * If no-one else is actually using this page, we're the exclusive
 	 * owner and can reuse this page.
+	 *
+	 * Note that we don't rely on the (safer) folio refcount here, because
+	 * copying the hugetlb folio when there are unexpected (temporary)
+	 * folio references could harm simple fork()+exit() users when
+	 * we run out of free hugetlb folios: we would have to kill processes
+	 * in scenarios that used to work. As a side effect, there can still
+	 * be leaks between processes, for example, with FOLL_GET users.
 	 */
 	if (folio_mapcount(old_folio) == 1 && folio_test_anon(old_folio)) {
 		if (!PageAnonExclusive(&old_folio->page)) {
_

Patches currently in -mm which might be from david@redhat.com are

drivers-virt-acrn-fix-pfnmap-pte-checks-in-acrn_vm_ram_map.patch
mm-pass-vma-instead-of-mm-to-follow_pte.patch
mm-follow_pte-improvements.patch
mm-allow-for-detecting-underflows-with-page_mapcount-again.patch
mm-allow-for-detecting-underflows-with-page_mapcount-again-fix.patch
mm-rmap-always-inline-anon-file-rmap-duplication-of-a-single-pte.patch
mm-rmap-add-fast-path-for-small-folios-when-adding-removing-duplicating.patch
mm-track-mapcount-of-large-folios-in-single-value.patch
mm-improve-folio_likely_mapped_shared-using-the-mapcount-of-large-folios.patch
mm-make-folio_mapcount-return-0-for-small-typed-folios.patch
mm-make-folio_mapcount-return-0-for-small-typed-folios-fix.patch
mm-memory-use-folio_mapcount-in-zap_present_folio_ptes.patch
mm-huge_memory-use-folio_mapcount-in-zap_huge_pmd-sanity-check.patch
mm-memory-failure-use-folio_mapcount-in-hwpoison_user_mappings.patch
mm-page_alloc-use-folio_mapped-in-__alloc_contig_migrate_range.patch
mm-migrate-use-folio_likely_mapped_shared-in-add_page_for_migration.patch
sh-mm-cache-use-folio_mapped-in-copy_from_user_page.patch
mm-filemap-use-folio_mapcount-in-filemap_unaccount_folio.patch
mm-migrate_device-use-folio_mapcount-in-migrate_vma_check_page.patch
trace-events-page_ref-trace-the-raw-page-mapcount-value.patch
xtensa-mm-convert-check_tlb_entry-to-sanity-check-folios.patch
mm-debug-print-only-page-mapcount-excluding-folio-entire-mapcount-in-__dump_folio.patch
documentation-admin-guide-cgroup-v1-memoryrst-dont-reference-page_mapcount.patch
mm-ksm-rename-get_ksm_page_flags-to-ksm_get_folio_flags.patch
mm-ksm-remove-page_mapcount-usage-in-stable_tree_search.patch
fs-proc-task_mmu-convert-pagemap_hugetlb_range-to-work-on-folios.patch
fs-proc-task_mmu-convert-smaps_hugetlb_range-to-work-on-folios.patch
mm-huge_memory-improve-split_huge_page_to_list_to_order-return-value-documentation.patch
mm-huge_memory-improve-split_huge_page_to_list_to_order-return-value-documentation-v2.patch
mm-khugepaged-replace-page_mapcount-check-by-folio_likely_mapped_shared.patch
selftests-mm-cow-flag-vmsplice-hugetlb-tests-as-xfail.patch
mm-hugetlb-document-why-hugetlb-uses-folio_mapcount-for-cow-reuse-decisions.patch