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| author | David Hildenbrand <david@redhat.com> | 2024-05-02 10:52:59 +0200 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2024-05-07 19:36:58 +0200 |
| commit | b8a2528835b31718286e7436529917e1f521bf6f (patch) | |
| tree | a34bf516f8b77c9e5cfa8a498ef440a15c21bdb6 /mm | |
| parent | selftests: mm: cow: flag vmsplice() hugetlb tests as XFAIL (diff) | |
| download | linux-b8a2528835b31718286e7436529917e1f521bf6f.tar.xz linux-b8a2528835b31718286e7436529917e1f521bf6f.zip | |
mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions
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>
Diffstat (limited to 'mm')
| -rw-r--r-- | mm/hugetlb.c | 7 |
1 files changed, 7 insertions, 0 deletions
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 417fc5cdb6ee..a7efb350f5d0 100644 --- a/mm/hugetlb.c +++ b/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)) { |