On 24.11.22 13:55, Gavin Shan wrote:
On 11/24/22 6:43 PM, David Hildenbrand wrote:
On 24.11.22 11:21, Gavin Shan wrote:
On 11/24/22 6:09 PM, David Hildenbrand wrote:
On 24.11.22 10:55, 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. Besides, The page's refcount is
increased a bit earlier to avoid the page is released when the check
is executed.
Did you look into handling pages that are in the swapcache case as well?
See is_refcount_suitable() in mm/khugepaged.c.
Should be easy to reproduce, let me know if you need inspiration.
Nope, I didn't look into the case. Please elaborate the details so that
I can reproduce it firstly.
A simple reproducer would be (on a system with ordinary swap (not zram))
1) mmap a region (MAP_ANON|MAP_PRIVATE) that can hold a THP
2) Enable THP for that region (MADV_HUGEPAGE)
3) Populate a THP (e.g., write access)
4) PTE-map the THP, for example, using MADV_FREE on the last subpage
5) Trigger swapout of the THP, for example, using MADV_PAGEOUT
6) Read-access to some subpages to fault them in from the swapcache
Now you'd have a THP, which
1) Is partially PTE-mapped into the page table
2) Is in the swapcache (each subpage should have one reference from the swapache)
Now we could test, if alloc_contig_range() will still succeed (e.g., using virtio-mem).
Thanks for the details. Step (4) and (5) can be actually combined. To swap part of
the THP (e.g. one sub-page) will force the THP to be split.
I followed your steps in the attached program, there is no issue to do memory hot-remove
through virtio-mem with or without this patch.
Interesting. But I don't really see how we could pass this check with a
page that's in the swapcache, maybe I'm missing something else.
I'll try to see if I can reproduce it.
--
Thanks,
David / dhildenb
