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

[David Hildenbrand]

On 24.11.22 01:14, Gavin Shan wrote:
> On 11/23/22 4:56 PM, David Hildenbrand wrote:
> > 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+
>
> Right, commit 1da2f328fa64 looks more accurate in this particular
> case, I will fix it up in next revision.
>
> > > 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.
>
> Yeah, it sounds reasonable to me to grab a page->__refcount in the
> first place. Looking at isolate_migratepages_block(), the page's refcount
> is increased by get_page_unless_zero(), but it's too late. To increase
> the page's refcount at the first place in the function will be conflicting
> with hugetlb page and non-LRU page. I mean there will be a series to refactor
> the code so that the page's refcount can be grabbed in the first place.
>
> So I plan to post a followup series to refactor the code and grab
> the page's refcount in the first place. In this way, the fix can be
> merged as soon as possible. David and Hugh, please let me know if
> it's reasonable plan? :)


Can't you just temporarily grab the refcount and drop it again? I mean, 
it's all racy either way and the code has to be able to cope with such 
races.

--
Thanks,

David / dhildenb