On Sun, Oct 08, 2023 at 11:42:26PM -0700, Suren Baghdasaryan wrote: > From: Andrea Arcangeli <[email protected]> > > 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 <[email protected]> > Signed-off-by: Suren Baghdasaryan <[email protected]> > --- > 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)? > + } > + > /* > * If the folio is still mapped, then this anon_vma is still > * its anon_vma, and holding the mutex ensures that it will -- Peter Xu
