| Message ID | 20231028003819.652322-2-surenb@google.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | userfaultfd move option | expand |
On Fri, Oct 27, 2023 at 05:38:11PM -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> Acked-by: Peter Xu <peterx@redhat.com>
diff --git a/mm/rmap.c b/mm/rmap.c index 7a27a2b41802..525c5bc0b0b3 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -542,6 +542,7 @@ 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(); anon_mapping = (unsigned long)READ_ONCE(folio->mapping); if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON) @@ -552,6 +553,17 @@ 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); + rcu_read_unlock(); + goto retry; + } + /* * If the folio is still mapped, then this anon_vma is still * its anon_vma, and holding the mutex ensures that it will @@ -586,6 +598,18 @@ struct anon_vma *folio_lock_anon_vma_read(struct folio *folio, rcu_read_unlock(); anon_vma_lock_read(anon_vma); + /* + * 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)) { + anon_vma_unlock_read(anon_vma); + put_anon_vma(anon_vma); + anon_vma = NULL; + goto retry; + } + if (atomic_dec_and_test(&anon_vma->refcount)) { /* * Oops, we held the last refcount, release the lock