Messages in this thread | | | Date | Fri, 13 Oct 2023 10:04:12 +0200 | Subject | Re: [PATCH v3 1/3] mm/rmap: support move to different root anon_vma in folio_move_anon_rmap() | From | David Hildenbrand <> |
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On 13.10.23 00:01, Peter Xu wrote: > On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote: >> From: Andrea Arcangeli <aarcange@redhat.com> >> >> For now, folio_move_anon_rmap() was only used to move a folio to a >> different anon_vma after fork(), whereby the root anon_vma stayed >> unchanged. For that, it was sufficient to hold the folio lock when >> calling folio_move_anon_rmap(). >> >> However, we want to make use of folio_move_anon_rmap() to move folios >> between VMAs that have a different root anon_vma. As folio_referenced() >> performs an RMAP walk without holding the folio lock but only holding the >> anon_vma in read mode, holding the folio lock is insufficient. >> >> When moving to an anon_vma with a different root anon_vma, we'll have to >> hold both, the folio lock and the anon_vma lock in write mode. >> Consequently, whenever we succeeded in folio_lock_anon_vma_read() to >> read-lock the anon_vma, we have to re-check if the mapping was changed >> in the meantime. If that was the case, we have to retry. >> >> Note that folio_move_anon_rmap() must only be called if the anon page is >> exclusive to a process, and must not be called on KSM folios. >> >> This is a preparation for UFFDIO_MOVE, which will hold the folio lock, >> the anon_vma lock in write mode, and the mmap_lock in read mode. >> >> Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> >> Signed-off-by: Suren Baghdasaryan <surenb@google.com> >> --- >> mm/rmap.c | 24 ++++++++++++++++++++++++ >> 1 file changed, 24 insertions(+) >> >> diff --git a/mm/rmap.c b/mm/rmap.c >> index c1f11c9dbe61..f9ddc50269d2 100644 >> --- a/mm/rmap.c >> +++ b/mm/rmap.c >> @@ -542,7 +542,9 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, >> struct anon_vma *root_anon_vma; >> unsigned long anon_mapping; >> >> +retry: >> rcu_read_lock(); >> +retry_under_rcu: >> anon_mapping = (unsigned long)READ_ONCE(folio->mapping); >> if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) >> goto out; >> @@ -552,6 +554,16 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, >> anon_vma = (struct anon_vma *) (anon_mapping - PAGE_MAPPING_ANON); >> root_anon_vma = READ_ONCE(anon_vma->root); >> if (down_read_trylock(&root_anon_vma->rwsem)) { >> + /* >> + * folio_move_anon_rmap() might have changed the anon_vma as we >> + * might not hold the folio lock here. >> + */ >> + if (unlikely((unsigned long)READ_ONCE(folio->mapping) != >> + anon_mapping)) { >> + up_read(&root_anon_vma->rwsem); >> + goto retry_under_rcu; > > Is adding this specific label worthwhile? How about rcu unlock and goto > retry (then it'll also be clear that we won't hold rcu read lock for > unpredictable time)?
+1, sounds good to me
-- Cheers,
David / dhildenb
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