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Message-ID: <51ffd399-7fa3-b2f2-b6e5-61a8b609e350@redhat.com> Date: Thu, 24 Nov 2022 09:49:44 +0100 From: David Hildenbrand <david@...hat.com> To: Matthew Wilcox <willy@...radead.org>, Alistair Popple <apopple@...dia.com> Cc: Hugh Dickins <hughd@...gle.com>, Gavin Shan <gshan@...hat.com>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, akpm@...ux-foundation.org, william.kucharski@...cle.com, ziy@...dia.com, kirill.shutemov@...ux.intel.com, zhenyzha@...hat.com, shan.gavin@...il.com, riel@...riel.com Subject: Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation 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@...hat.com> 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@...r.kernel.org # v5.8+ >>>>> Reported-by: Zhenyu Zhang <zhenyzha@...hat.com> >>>>> Suggested-by: David Hildenbrand <david@...hat.com> >>>>> Signed-off-by: Gavin Shan <gshan@...hat.com> >>>> 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@...r.kernel.org # 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
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