On Wed, Feb 19, 2025 at 10:30 AM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 19.02.25 19:26, Suren Baghdasaryan wrote: > > On Wed, Feb 19, 2025 at 3:25 AM Barry Song <21cnbao@xxxxxxxxx> wrote: > >> > >> From: Barry Song <v-songbaohua@xxxxxxxx> > >> > >> userfaultfd_move() checks whether the PTE entry is present or a > >> swap entry. > >> > >> - If the PTE entry is present, move_present_pte() handles folio > >> migration by setting: > >> > >> src_folio->index = linear_page_index(dst_vma, dst_addr); > >> > >> - If the PTE entry is a swap entry, move_swap_pte() simply copies > >> the PTE to the new dst_addr. > >> > >> This approach is incorrect because even if the PTE is a swap > >> entry, it can still reference a folio that remains in the swap > >> cache. > >> > >> If do_swap_page() is triggered, it may locate the folio in the > >> swap cache. However, during add_rmap operations, a kernel panic > >> can occur due to: > >> page_pgoff(folio, page) != linear_page_index(vma, address) > > > > Thanks for the report and reproducer! > > > >> > >> $./a.out > /dev/null > >> [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c > >> [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0 > >> [ 13.337716] memcg:ffff00000405f000 > >> [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff) > >> [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 > >> [ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 > >> [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 > >> [ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 > >> [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001 > >> [ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000 > >> [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address)) > >> [ 13.340190] ------------[ cut here ]------------ > >> [ 13.340316] kernel BUG at mm/rmap.c:1380! > >> [ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP > >> [ 13.340969] Modules linked in: > >> [ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299 > >> [ 13.341470] Hardware name: linux,dummy-virt (DT) > >> [ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) > >> [ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 > >> [ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 > >> [ 13.342018] sp : ffff80008752bb20 > >> [ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001 > >> [ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001 > >> [ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00 > >> [ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff > >> [ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f > >> [ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0 > >> [ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40 > >> [ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8 > >> [ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000 > >> [ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f > >> [ 13.343876] Call trace: > >> [ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P) > >> [ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320 > >> [ 13.344333] do_swap_page+0x1060/0x1400 > >> [ 13.344417] __handle_mm_fault+0x61c/0xbc8 > >> [ 13.344504] handle_mm_fault+0xd8/0x2e8 > >> [ 13.344586] do_page_fault+0x20c/0x770 > >> [ 13.344673] do_translation_fault+0xb4/0xf0 > >> [ 13.344759] do_mem_abort+0x48/0xa0 > >> [ 13.344842] el0_da+0x58/0x130 > >> [ 13.344914] el0t_64_sync_handler+0xc4/0x138 > >> [ 13.345002] el0t_64_sync+0x1ac/0x1b0 > >> [ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000) > >> [ 13.345504] ---[ end trace 0000000000000000 ]--- > >> [ 13.345715] note: a.out[107] exited with irqs disabled > >> [ 13.345954] note: a.out[107] exited with preempt_count 2 > >> > >> Fully fixing it would be quite complex, requiring similar handling > >> of folios as done in move_present_pte. > > > > How complex would that be? Is it a matter of adding > > folio_maybe_dma_pinned() checks, doing folio_move_anon_rmap() and > > folio->index = linear_page_index like in move_present_pte() or > > something more? > > If the entry is pte_swp_exclusive(), and the folio is order-0, it cannot > be pinned and we may be able to move it I think. > > So all that's required is to check pte_swp_exclusive() and the folio size. > > ... in theory :) Not sure about the swap details. Looking some more into it, I think we would have to perform all the folio and anon_vma locking and pinning that we do for present pages in move_pages_pte(). If that's correct then maybe treating swapcache pages like a present page inside move_pages_pte() would be simpler? > > -- > Cheers, > > David / dhildenb >
