Messages in this thread | | | Date | Thu, 24 Nov 2022 15:09:32 +0100 | Subject | Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation | From | David Hildenbrand <> |
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On 24.11.22 14:22, David Hildenbrand wrote: > 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. >
After some unsuccessful attempts and many head-scratches, I realized that it's quite simple why we don't have to worry about swapcache pages here:
page_mapping() is != NULL for pages in the swapcache: folio_mapping() makes this rather obvious:
if (unlikely(folio_test_swapcache(folio)) return swap_address_space(folio_swap_entry(folio));
I think the get_page_unless_zero() might also be a fix for the page_mapping() call, smells like something could blow up on concurrent page freeing. (what about concurrent removal from the swapcache? nobody knows :) )
Thanks Gavin!
Acked-by: David Hildenbrand <david@redhat.com>
-- Thanks,
David / dhildenb
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