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AJvYcCVe97Lpc10sfRkSRMm+R1hk2+ed2asllSESj1qQLrilq+fTcps3FWsUm07Td/Ejk23ToSKwLoThkA==@kvack.org X-Gm-Message-State: AOJu0YyN1S9QtzSYJG3fmowmUrxA24H5nGaWn0gCVgIpIR8eo3wItMgT TYQ+x8DmDefn/uYlBmuI/48J5W05Dymm8J3A7os+0aWbYwXUJOfNeMH6yEE+Yw7f8JUS7LpRqTa QVYVcWeoU3OYoX+xVH8PZ1ss9nJQ= X-Gm-Gg: ASbGncurkjH+VoyDu3TVJfWXMlHrXjyW2frHuhJ9ONfAeC6McQ3sgPzsQ15urh5lngC meM5SMNQZa8rnSGHol2R7VHjRQDEVXGOQxfjbsJ92h3AUHyDd26NwZ+8kXdeXpyRWFRIDCJ6U X-Google-Smtp-Source: AGHT+IE+DeW4FAWGYKbe62H0bYUYypDQeyaLnwZL1hvomSwU7SMvCmXWFbRxG542EwfT9Kwr+nHOnYKfdRwj4zDHqpc= X-Received: by 2002:a05:6102:3592:b0:4bb:d7f0:6e7d with SMTP id ada2fe7eead31-4be85ce5084mr3878917137.25.1740012575247; Wed, 19 Feb 2025 16:49:35 -0800 (PST) MIME-Version: 1.0 References: <20250219112519.92853-1-21cnbao@gmail.com> In-Reply-To: From: Barry Song <21cnbao@gmail.com> Date: Thu, 20 Feb 2025 13:49:24 +1300 X-Gm-Features: AWEUYZlSNZ_rn1r4QH5a9UT9GHos2aaViRar0322BOr_WPgv_gXhJqRVPBFQnek Message-ID: Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache To: Lokesh Gidra Cc: Peter Xu , Suren Baghdasaryan , linux-mm@kvack.org, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, zhengtangquan@oppo.com, Barry Song , Andrea Arcangeli , Al Viro , Axel Rasmussen , Brian Geffon , Christian Brauner , David Hildenbrand , Hugh Dickins , Jann Horn , Kalesh Singh , "Liam R . Howlett" , Matthew Wilcox , Michal Hocko , Mike Rapoport , Nicolas Geoffray , Ryan Roberts , Shuah Khan , ZhangPeng , Yu Zhao Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Queue-Id: 69F75180006 X-Rspamd-Server: rspam07 X-Stat-Signature: 5wnz6wm7fnxqsca54tzndngupwuxa7ti X-HE-Tag: 1740012576-370890 X-HE-Meta: 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 Ag+0g0VV nrujrfGbOtj2+L2u3GJ1AOsCajfiv2U14GAzIYIfi7q+YSSeO/pBnCSeUixdJEP+VcayCcqtJxqikTuqRE27XNJO/oV3Xyq+NPRB6RMYrrU+d3YM9aj+02/l+na9yoiloOqFym4LQ2EYGe7P5+4ntdRJBqgHtKaOocA3u4SZ5JG9PuxTgcnVW1OUS0dJ78Bx8FE4rRDK+/nQhco24TxLhB6/l6oEliZXOaxD5MYNElpa5kpvAIlh9Of+9qSeuaQnLt4zdiQmTJl5m+LIw44udKEeW3Q0lYYu+3wSRgNfpLsBsVlfObgN4aSrdkqdeKScS/Wg2teAWN7gdVSIfBwJbSJMUC0aEFHgAqSokB2HaeBHTq32EFfhwoQ51L6oGJtj81SK+EWjWe1wI71Ag58J8GEuZjYIXqCnuJkUa4LUYwFfR/5Iz+euohFAc3GiGpyB8RanF4mywQD0HnZA= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Feb 20, 2025 at 12:19=E2=80=AFPM Lokesh Gidra wrote: > > On Wed, Feb 19, 2025 at 3:04=E2=80=AFPM Barry Song <21cnbao@gmail.com> wr= ote: > > > > On Thu, Feb 20, 2025 at 11:15=E2=80=AFAM Peter Xu w= rote: > > > > > > On Thu, Feb 20, 2025 at 09:37:50AM +1300, Barry Song wrote: > > > > On Thu, Feb 20, 2025 at 7:27=E2=80=AFAM Suren Baghdasaryan wrote: > > > > > > > > > > On Wed, Feb 19, 2025 at 3:25=E2=80=AFAM Barry Song <21cnbao@gmail= .com> wrote: > > > > > > > > > > > > From: Barry Song > > > > > > > > > > > > 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 =3D linear_page_index(dst_vma, dst_addr); > > > > > > > > > > > > - If the PTE entry is a swap entry, move_swap_pte() simply copi= es > > > > > > 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) !=3D linear_page_index(vma, address) > > > > > > > > > > Thanks for the report and reproducer! > > > > > > > > > > > > > > > > > $./a.out > /dev/null > > > > > > [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43d= b19c index:0xffffaf150 pfn:0x4667c > > > > > > [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pa= ges_mapped:1 pincount:0 > > > > > > [ 13.337716] memcg:ffff00000405f000 > > > > > > [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|di= rty|owner_priv_1|head|swapbacked|node=3D0|zone=3D0|lastcpupid=3D0xffff) > > > > > > [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008= 507b538 ffff000006260361 > > > > > > [ 13.338831] raw: 0000000ffffaf150 0000000000004000 000000060= 0000000 ffff00000405f000 > > > > > > [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff8000= 8507b538 ffff000006260361 > > > > > > [ 13.339204] head: 0000000ffffaf150 0000000000004000 00000006= 00000000 ffff00000405f000 > > > > > > [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff= 00000000 0000000000000001 > > > > > > [ 13.339546] head: 0000000000000004 0000000000000000 00000000= ffffffff 0000000000000000 > > > > > > [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(f= olio, page) !=3D 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=3D--) > > > > > > [ 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 (d4210= 000) > > > > > > [ 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 handl= ing > > > > > > 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 =3D linear_page_index like in move_present_pte() or > > > > > something more? > > > > > > > > My main concern is still with large folios that require a split_fol= io() > > > > during move_pages(), as the entire folio shares the same index and > > > > anon_vma. However, userfaultfd_move() moves pages individually, > > > > making a split necessary. > > > > > > > > However, in split_huge_page_to_list_to_order(), there is a: > > > > > > > > if (folio_test_writeback(folio)) > > > > return -EBUSY; > > > > > > > > This is likely true for swapcache, right? However, even for move_pr= esent_pte(), > > > > it simply returns -EBUSY: > > > > > > > > move_pages_pte() > > > > { > > > > /* at this point we have src_folio locked */ > > > > if (folio_test_large(src_folio)) { > > > > /* split_folio() can block */ > > > > pte_unmap(&orig_src_pte); > > > > pte_unmap(&orig_dst_pte); > > > > src_pte =3D dst_pte =3D NULL; > > > > err =3D split_folio(src_folio); > > > > if (err) > > > > goto out; > > > > > > > > /* have to reacquire the folio after it got= split */ > > > > folio_unlock(src_folio); > > > > folio_put(src_folio); > > > > src_folio =3D NULL; > > > > goto retry; > > > > } > > > > } > > > > > > > > Do we need a folio_wait_writeback() before calling split_folio()? > > > > > > Maybe no need in the first version to fix the immediate bug? > > > > > > It's also not always the case to hit writeback here. IIUC, writeback = only > > > happens for a short window when the folio was just added into swapcac= he. > > > MOVE can happen much later after that anytime before a swapin. My > > > understanding is that's also what Matthew wanted to point out. It ma= y be > > > better justified of that in a separate change with some performance > > > measurements. > > > > The bug we=E2=80=99re discussing occurs precisely within the short wind= ow you > > mentioned. > > > > 1. add_to_swap: The folio is added to swapcache. > > 2. try_to_unmap: PTEs are converted to swap entries. > > 3. pageout > > 4. Swapcache is cleared. > > > > The issue happens between steps 2 and 4, where the PTE is not present, = but > > the folio is still in swapcache - the current code does move_swap_pte()= but does > > not fixup folio->index within swapcache. > > > > My point is that if we want a proper fix for mTHP, we'd better handle w= riteback. > > Otherwise, this isn=E2=80=99t much different from directly returning -E= BUSY as proposed > > in this RFC. > > > > For small folios, there=E2=80=99s no split_folio issue, making it relat= ively > > simpler. Lokesh > > mentioned plans to madvise NOHUGEPAGE in ART, so fixing small folios is= likely > > the first priority. > > Fixing for the non-mTHP case first sounds good to me. For a large > folio in swap cache maybe you can return EBUSY for now? > > But for that, I believe the cleanest and simplest would be to > restructure move_pages_pte() such that folio is retrieved from > vm_normal_foliio() if src_pte is present, otherwise, from > filemap_get_folio() and then incorporate the check you have in this > FRC in there. This way the entire locking dance logic in there can be > reused for swap-cache case as well. Yep, let me give it a try in v2. > > > > > > > > Thanks, > > > > > > -- > > > Peter Xu > > > > Thanks > > Barry