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.
*/
