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.