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