On Tue, Jul 25, 2023 at 01:51:55PM +0100, Matthew Wilcox wrote: > On Tue, Jul 25, 2023 at 01:24:03PM +0300, Kirill A . Shutemov wrote: > > On Tue, Jul 18, 2023 at 04:44:53PM -0700, Sean Christopherson wrote: > > > diff --git a/mm/compaction.c b/mm/compaction.c > > > index dbc9f86b1934..a3d2b132df52 100644 > > > --- a/mm/compaction.c > > > +++ b/mm/compaction.c > > > @@ -1047,6 +1047,10 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn, > > > if (!mapping && (folio_ref_count(folio) - 1) > folio_mapcount(folio)) > > > goto isolate_fail_put; > > > > > > + /* The mapping truly isn't movable. */ > > > + if (mapping && mapping_unmovable(mapping)) > > > + goto isolate_fail_put; > > > + > > > > I doubt that it is safe to dereference mapping here. I believe the folio > > can be truncated from under us and the mapping freed with the inode. > > > > The folio has to be locked to dereference mapping safely (given that the > > mapping is still tied to the folio). > > There's even a comment to that effect later on in the function: > > /* > * Only pages without mappings or that have a > * ->migrate_folio callback are possible to migrate > * without blocking. However, we can be racing with > * truncation so it's necessary to lock the page > * to stabilise the mapping as truncation holds > * the page lock until after the page is removed > * from the page cache. > */ > > (that could be reworded to make it clear how dangerous dereferencing > ->mapping is without the lock ... and it does need to be changed to say > "folio lock" instead of "page lock", so ...) > > How does this look? > > /* > * Only folios without mappings or that have > * a ->migrate_folio callback are possible to > * migrate without blocking. However, we can > * be racing with truncation, which can free > * the mapping. Truncation holds the folio lock > * until after the folio is removed from the page > * cache so holding it ourselves is sufficient. > */ > Looks good to me. -- Kiryl Shutsemau / Kirill A. Shutemov
