Messages in this thread | | | Date | Thu, 24 Nov 2022 14:22:06 +0100 | Subject | Re: [PATCH v2] mm: migrate: Fix THP's mapcount on isolation | From | David Hildenbrand <> |
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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
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