On 24 Nov 2022, at 5:43, 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 Added the original THP swapout code author, Ying. At this step, the THP will be split, right? https://elixir.bootlin.com/linux/latest/source/mm/vmscan.c#L1786 Even if a THP has PMD mapping, IIRC, it is split in the add_to_swap() then swapped out. But I cannot find that split code now. > > 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, > > David / dhildenb -- Best Regards, Yan Zi
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