[RFC PATCH v11 10/29] mm: Add AS_UNMOVABLE to mark mapping as completely unmovable

[Matthew Wilcox]

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.
                         */