On Fri, Oct 13, 2023 at 1:04 AM David Hildenbrand david@redhat.com wrote:
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
Sorry for the delay, I was travelling for a week.
I was hesitant about RCU unlocking and then immediately re-locking but your point about holding it for unpredictable time makes sense. Will change. Thanks!
-- Cheers,
David / dhildenb