Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation

[Gavin Shan]

On 11/24/22 6:09 PM, David Hildenbrand wrote:
> On 24.11.22 10:55, Gavin Shan wrote:
> > The issue is reported when removing memory through virtio_mem device.
> > The transparent huge page, experienced copy-on-write fault, is wrongly
> > regarded as pinned. The transparent huge page is escaped from being
> > isolated in isolate_migratepages_block(). The transparent huge page
> > can't be migrated and the corresponding memory block can't be put
> > into offline state.
> >
> > Fix it by replacing page_mapcount() with total_mapcount(). With this,
> > the transparent huge page can be isolated and migrated, and the memory
> > block can be put into offline state. Besides, The page's refcount is
> > increased a bit earlier to avoid the page is released when the check
> > is executed.
>
> Did you look into handling pages that are in the swapcache case as well?
>
> See is_refcount_suitable() in mm/khugepaged.c.
>
> Should be easy to reproduce, let me know if you need inspiration.

Nope, I didn't look into the case. Please elaborate the details so that
I can reproduce it firstly.

> > Fixes: 1da2f328fa64 ("mm,thp,compaction,cma: allow THP migration for CMA allocations")
> > Cc: stable@xxxxxxxxxxxxxxx   # v5.7+
> > Reported-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
> > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > ---
> > v2: Corrected fix tag and increase page's refcount before the check
> > ---
> >   mm/compaction.c | 22 +++++++++++-----------
> >   1 file changed, 11 insertions(+), 11 deletions(-)
> >
> > diff --git a/mm/compaction.c b/mm/compaction.c
> > index c51f7f545afe..1f6da31dd9a5 100644
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -984,29 +984,29 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >               goto isolate_fail;
> >           }
> > +        /*
> > +         * Be careful not to clear PageLRU until after we're
> > +         * sure the page is not being freed elsewhere -- the
> > +         * page release code relies on it.
> > +         */
> > +        if (unlikely(!get_page_unless_zero(page)))
> > +            goto isolate_fail;
> > +
> >           /*
> >            * Migration will fail if an anonymous page is pinned in memory,
> >            * so avoid taking lru_lock and isolating it unnecessarily in an
> >            * admittedly racy check.
> >            */
> >           mapping = page_mapping(page);
> > -        if (!mapping && page_count(page) > page_mapcount(page))
> > -            goto isolate_fail;
> > +        if (!mapping && (page_count(page) - 1) > total_mapcount(page))
> > +            goto isolate_fail_put;
> >           /*
> >            * Only allow to migrate anonymous pages in GFP_NOFS context
> >            * because those do not depend on fs locks.
> >            */
> >           if (!(cc->gfp_mask & __GFP_FS) && mapping)
> > -            goto isolate_fail;
> > -
> > -        /*
> > -         * Be careful not to clear PageLRU until after we're
> > -         * sure the page is not being freed elsewhere -- the
> > -         * page release code relies on it.
> > -         */
> > -        if (unlikely(!get_page_unless_zero(page)))
> > -            goto isolate_fail;
> > +            goto isolate_fail_put;
> >           /* Only take pages on LRU: a check now makes later tests safe */
> >           if (!PageLRU(page))

Thanks,
Gavin