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

[David Hildenbrand]

On 24.11.22 04:33, Matthew Wilcox wrote:
> On Thu, Nov 24, 2022 at 12:06:56PM +1100, Alistair Popple wrote:
> > David Hildenbrand <david@xxxxxxxxxx> writes:
> >
> > > On 23.11.22 06:14, Hugh Dickins wrote:
> > > > On Wed, 23 Nov 2022, 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.
> > > > >
> > > > > Fixes: 3917c80280c9 ("thp: change CoW semantics for anon-THP")
> > > > > Cc: stable@xxxxxxxxxxxxxxx   # v5.8+
> > > > > Reported-by: Zhenyu Zhang <zhenyzha@xxxxxxxxxx>
> > > > > Suggested-by: David Hildenbrand <david@xxxxxxxxxx>
> > > > > Signed-off-by: Gavin Shan <gshan@xxxxxxxxxx>
> > > > Interesting, good catch, looked right to me: except for the Fixes
> > > > line
> > > > and mention of v5.8.  That CoW change may have added a case which easily
> > > > demonstrates the problem, but it would have been the wrong test on a THP
> > > > for long before then - but only in v5.7 were compound pages allowed
> > > > through at all to reach that test, so I think it should be
> > > > Fixes: 1da2f328fa64 ("mm,thp,compaction,cma: allow THP migration for
> > > > CMA allocations")
> > > > Cc: stable@xxxxxxxxxxxxxxx   # v5.7+
> > > > Oh, no, stop: this is not so easy, even in the latest tree.
> > > > Because at the time of that "admittedly racy check", we have no hold
> > > > at all on the page in question: and if it's PageLRU or PageCompound
> > > > at one instant, it may be different the next instant.  Which leaves it
> > > > vulnerable to whatever BUG_ON()s there may be in the total_mapcount()
> > > > path - needs research.  *Perhaps* there are no more BUG_ON()s in the
> > > > total_mapcount() path than in the existing page_mapcount() path.
> > > > I suspect that for this to be safe (before your patch and more so
> > > > after),
> > > > it will be necessary to shift the "admittedly racy check" down after the
> > > > get_page_unless_zero() (and check the sequence of operations when a
> > > > compound page is initialized).
> > >
> > > Grabbing a reference first sounds like the right approach to me.
> >
> > I think you're right. Without a page reference I don't think it is even
> > safe to look at struct page, at least not without synchronisation
> > against memory hot unplug which could remove the struct page. From a
> > quick glance I didn't see anything here that obviously did that though.
>
> Memory hotplug is the offending party here.  It has to make sure that
> everything else is definitely quiescent before removing the struct pages.
> Otherwise you can't even try_get a refcount.

At least alloc_contig_range() and memory offlining are mutually 
exclusive due to MIGRATE_ISOLTAE. I recall that ordinary memory 
compaction similarly deals with isolated pageblocks (or some other 
mechanism I forgot) to not race with memory offlining. Wouldn't worry 
about that for now.

--
Thanks,

David / dhildenb