Messages in this thread | | | Date | Thu, 24 Nov 2022 09:46:57 +0100 | Subject | Re: [PATCH] mm: migrate: Fix THP's mapcount on isolation | From | David Hildenbrand <> |
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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@vger.kernel.org # v5.8+ >>>> Reported-by: Zhenyu Zhang <zhenyzha@redhat.com> >>>> Suggested-by: David Hildenbrand <david@redhat.com> >>>> Signed-off-by: Gavin Shan <gshan@redhat.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@vger.kernel.org # 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
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