From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id E26B5C021B2 for ; Tue, 25 Feb 2025 11:05:42 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 7D0B6280002; Tue, 25 Feb 2025 06:05:42 -0500 (EST) Received: by kanga.kvack.org (Postfix, from userid 40) id 77F48280001; Tue, 25 Feb 2025 06:05:42 -0500 (EST) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 620BC280002; Tue, 25 Feb 2025 06:05:42 -0500 (EST) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 3E85F280001 for ; Tue, 25 Feb 2025 06:05:42 -0500 (EST) Received: from smtpin18.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay06.hostedemail.com (Postfix) with ESMTP id B03FAB6711 for ; Tue, 25 Feb 2025 11:05:41 +0000 (UTC) X-FDA: 83158186482.18.F137770 Received: from mail-pj1-f49.google.com (mail-pj1-f49.google.com [209.85.216.49]) by imf04.hostedemail.com (Postfix) with ESMTP id ACAD040002 for ; Tue, 25 Feb 2025 11:05:39 +0000 (UTC) Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=gmail.com header.s=20230601 header.b=BSQ20xsi; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf04.hostedemail.com: domain of 21cnbao@gmail.com designates 209.85.216.49 as permitted sender) smtp.mailfrom=21cnbao@gmail.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1740481539; a=rsa-sha256; cv=none; b=GymwQBDaUMBztwTJEdRD/GOHaaXSdxxUlFOEjMWy+6GKwFzHNzRUFFI82mh6e4Y4tirK7W 3GQGr9AGL6EI53/u4SYhm1Ff1S8YCphl7BZYV5+STLltHW3+/eVF0AdKZRNMSUFD88VcPW 5jQAayhFQIp8RcEhjQ8psT796pUptII= ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=gmail.com header.s=20230601 header.b=BSQ20xsi; dmarc=pass (policy=none) header.from=gmail.com; spf=pass (imf04.hostedemail.com: domain of 21cnbao@gmail.com designates 209.85.216.49 as permitted sender) smtp.mailfrom=21cnbao@gmail.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1740481539; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=u9fF+/KoEn4ybPiIaoVFg7y1oSf6VmVcoJLEARWll7E=; b=RCyur0RKn02zZuHOabFBARS0mLRoSet6sSrr1KBPfYI1HpE2Fx0H52h18ttnIOAL8B5NbR xyUhtKqJLodGQ0TsznOODsJ8KqrxbW5KwTCBQtmy9HKP6HQhis9NwE46PsOMqdmoz+2c6K tzknFXrLO0gRt8PiR8P6E9cIuAxmr00= Received: by mail-pj1-f49.google.com with SMTP id 98e67ed59e1d1-2fc11834404so8432754a91.0 for ; Tue, 25 Feb 2025 03:05:39 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1740481538; x=1741086338; darn=kvack.org; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=u9fF+/KoEn4ybPiIaoVFg7y1oSf6VmVcoJLEARWll7E=; b=BSQ20xsiTExFHho5kLI/gseBzYGNZDfzVXTlQ5aFshcWUZpf9dHLOb9SBdAo6y4aXT sAfUBQSd6A/TkrJ+mmHIfwTOe+OysSmDiKjvjL+HOFLzYuTRFz5wWw+jL6oxdrM4DFoc N9MaH9nI21LE6Bt6fpwI3v4V2cG34KBQQ11z4bVXsnHdQ7y7PmUYquhtIJBkyIOzERyc HUex+o67lWb69nSY3g3DCXpZRDJNzP3wY6Apl3JExIcrYR/VeNth1njiqsaZ6t122PM5 ctEr0HDDJcS+ZQuQ1Zp/h+yuaMO5VM6BX1JAuK6cWwoeW7a6+ZUHF8Ue3HvS8Oxy5DTH h7Pg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1740481538; x=1741086338; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=u9fF+/KoEn4ybPiIaoVFg7y1oSf6VmVcoJLEARWll7E=; b=WVp37XpJA+tbAjiRWTbsvyX487Y7CrrGDMxLfj5FKwrfgdTWd4YEkr821HZ6g77GtO yT1XyLy3fnJcQR2xLGS/npfB8zQQg5kGMJLCQrE03TmeXZSoH2wOyQT8ys+3neigHbyZ O+Grf4YotqXg/+JbABqq9WkTysc8NXGK/ooMKUVAYY3E/fwzxVI4NvzREhXFflR8yD59 I8Oib/qV7yirUVRyFtDaSRI9mSGjgUykvgKMYIpRfbP6eOjcX+d38KTaIEg8+8O0vUk8 Kets7tC3gZAUrttkcXkoHGv+LAalbnJfnpSkPzVQ9WNIwPypJTJhvnNz/1p+79wsNfZ9 6xTQ== X-Forwarded-Encrypted: i=1; AJvYcCV5gQCU5DqVo0D0lZQylmBKeJwCagnAauCycFN2gAXa0I5XHmvLoYx00CTMutCGo5OSNmp1/gdIFw==@kvack.org X-Gm-Message-State: AOJu0YxdDDDkMEz6WNo/DlkeZaj9B2uYoDDfmHv6qk8F6yyuLX4NcsVU zezSiPY/VNQ8kBnr4tPzwRazLeHYg5ebSbC636mbicsk6hP+iN2K6N2CBr6h X-Gm-Gg: ASbGncvrwENtJgZYcxYmMTt7nNtf0MsBBsw0EDtT7C4Y/5NQRKmwXS16HGpvMDeFyRV i4Rd0uGAMrXB1CXDKFXB18Y55H38VRgeCKQjkB9Y9fw5VSUYrk/nhke0OWFXnC78aytslHg4mzZ 4t6NPbq5XqWz96ppCwU5HlF6tnFFaxvak1ZpnAaZAlOgld8j0QLd+lJeNgqwk1/fhh94X/QbOvd XXnqLUGZBO6cAW6TGLG5p7fr9ZrmC/lz9h//RstzdIdyAxifXm4Mrp1xeOQGeZbKmxo176R6Lot icpRh8G3WMWZaxfq4oxQ4UcBer0lGagy/BUxcCqbIYk= X-Google-Smtp-Source: AGHT+IGl2ocLgwialqvNRpX1MrXvsX7kP0KR/tqxx7vHEot1rp7p2VG5BaoJ1L2wN8Kd6tw9hDmDwA== X-Received: by 2002:a17:90b:2710:b0:2fa:157e:c790 with SMTP id 98e67ed59e1d1-2fce868c836mr25184882a91.5.1740481538277; Tue, 25 Feb 2025 03:05:38 -0800 (PST) Received: from Barrys-MBP.hub ([118.92.30.135]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-2fe6a3f2121sm1257621a91.25.2025.02.25.03.05.29 (version=TLS1_3 cipher=TLS_CHACHA20_POLY1305_SHA256 bits=256/256); Tue, 25 Feb 2025 03:05:37 -0800 (PST) From: Barry Song <21cnbao@gmail.com> To: surenb@google.com Cc: 21cnbao@gmail.com, Liam.Howlett@oracle.com, aarcange@redhat.com, akpm@linux-foundation.org, axelrasmussen@google.com, bgeffon@google.com, brauner@kernel.org, david@redhat.com, hughd@google.com, jannh@google.com, kaleshsingh@google.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lokeshgidra@google.com, mhocko@suse.com, ngeoffray@google.com, peterx@redhat.com, rppt@kernel.org, ryan.roberts@arm.com, shuah@kernel.org, v-songbaohua@oppo.com, viro@zeniv.linux.org.uk, willy@infradead.org, yuzhao@google.com, zhangpeng362@huawei.com, zhengtangquan@oppo.com Subject: Re: [PATCH RFC] mm: Fix kernel BUG when userfaultfd_move encounters swapcache Date: Wed, 26 Feb 2025 00:05:25 +1300 Message-Id: <20250225110525.26732-1-21cnbao@gmail.com> X-Mailer: git-send-email 2.39.3 (Apple Git-146) In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: ACAD040002 X-Stat-Signature: te6ymskhp1cdczpaf1wzyw43yfriis7g X-Rspam-User: X-HE-Tag: 1740481539-756823 X-HE-Meta: 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 lgtuZOkF 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 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 Fri, Feb 21, 2025 at 1:36 PM Suren Baghdasaryan wrote: > > On Thu, Feb 20, 2025 at 3:52 PM Suren Baghdasaryan wrote: > > > > On Thu, Feb 20, 2025 at 3:47 PM Suren Baghdasaryan wrote: > > > > > > On Thu, Feb 20, 2025 at 2:59 PM Peter Xu wrote: > > > > > > > > On Thu, Feb 20, 2025 at 12:04:40PM +1300, Barry Song wrote: > > > > > On Thu, Feb 20, 2025 at 11:15 AM Peter Xu wrote: > > > > > > > > > > > > On Thu, Feb 20, 2025 at 09:37:50AM +1300, Barry Song wrote: > > > > > > > On Thu, Feb 20, 2025 at 7:27 AM Suren Baghdasaryan wrote: > > > > > > > > > > > > > > > > On Wed, Feb 19, 2025 at 3:25 AM 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 = 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? > > > > > > > > > > > > > > My main concern is still with large folios that require a split_folio() > > > > > > > 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_present_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 = dst_pte = NULL; > > > > > > > err = 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 = 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 swapcache. > > > > > > MOVE can happen much later after that anytime before a swapin. My > > > > > > understanding is that's also what Matthew wanted to point out. It may be > > > > > > better justified of that in a separate change with some performance > > > > > > measurements. > > > > > > > > > > The bug we’re discussing occurs precisely within the short window 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. > > > > > > > > Hmm, I see. I was expecting step 4 to be "writeback is cleared".. or at > > > > least that should be step 3.5, as IIUC "writeback" needs to be cleared > > > > before "swapcache" bit being 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. > > > > > > > > One thing I'm still not clear here is why it's a race condition, rather > > > > than more severe than that. I mean, folio->index is definitely wrong, then > > > > as long as the page still in swapcache, we should be able to move the swp > > > > entry over to dest addr of UFFDIO_MOVE, read on dest addr, then it'll see > > > > the page in swapcache with the wrong folio->index already and trigger. > > > > > > > > I wrote a quick test like that, it actually won't trigger.. > > > > > > > > I had a closer look in the code, I think it's because do_swap_page() has > > > > the logic to detect folio->index matching first, and allocate a new folio > > > > if it doesn't match in ksm_might_need_to_copy(). IIUC that was for > > > > ksm.. but it looks like it's functioning too here. > > > > > > > > ksm_might_need_to_copy: > > > > if (folio_test_ksm(folio)) { > > > > if (folio_stable_node(folio) && > > > > !(ksm_run & KSM_RUN_UNMERGE)) > > > > return folio; /* no need to copy it */ > > > > } else if (!anon_vma) { > > > > return folio; /* no need to copy it */ > > > > } else if (folio->index == linear_page_index(vma, addr) && <---------- [1] > > > > anon_vma->root == vma->anon_vma->root) { > > > > return folio; /* still no need to copy it */ > > > > } > > > > ... > > > > > > > > new_folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, addr); <---- [2] > > > > ... > > > > > > > > So I believe what I hit is at [1] it sees index doesn't match, then it > > > > decided to allocate a new folio. In this case, it won't hit your BUG > > > > because it'll be "folio != swapcache" later, so it'll setup the > > > > folio->index for the new one, rather than the sanity check. > > > > > > > > Do you know how your case got triggered, being able to bypass the above [1] > > > > which should check folio->index already? > > > > > > To understand the change I tried applying the proposed patch to both > > > mm-unstable and Linus' ToT and got conflicts for both trees. Barry, > > > which baseline are you using? > > > > Oops, never mind. My mistake. Copying from the email messed up tabs... > > It applies cleanly. > > Overall the code seems correct to me, however the new code has quite > complex logical structure IMO. Original simplified code structure is > like this: > > if (pte_present(orig_src_pte)) { > if (is_zero_pfn) { > move_zeropage_pte() > return > } > // pin and lock src_folio > spin_lock(src_ptl) > folio_get(folio) > folio_trylock(folio) > if (folio_test_large(src_folio)) > split_folio(src_folio) > anon_vma_trylock_write(src_anon_vma) > move_present_pte() > } else { > if (non_swap_entry(entry)) > if (is_migration_entry(entry)) > handle migration entry > else > move_swap_pte() > } > > The new structure looks like this: > > if (!pte_present(orig_src_pte)) { > if (is_migration_entry(entry)) { > handle migration entry > return > } > if (!non_swap_entry() || !pte_swp_exclusive()) > return > si = get_swap_device(entry); > } > if (pte_present(orig_src_pte) && is_zero_pfn(pte_pfn(orig_src_pte))) > move_zeropage_pte() > return > } > pin and lock src_folio > spin_lock(src_ptl) > if (pte_present(orig_src_pte)) > folio_get(folio) > else { > folio = filemap_get_folio(swap_entry) > if (IS_ERR(folio)) > move_swap_pte() > return > } > } > folio_trylock(folio) > if (folio_test_large(src_folio)) > split_folio(src_folio) > if (pte_present(orig_src_pte)) > anon_vma_trylock_write(src_anon_vma) > move_pte_and_folio() > > This looks more complex and harder to follow. Might be the reason > David was not in favour of treating swapcache and present pages in the > same path. And now I would agree that refactoring some common parts > and not breaking the original structure might be cleaner. Exactly, that’s the cost we’re facing in trying to share the code path for swap and present PTEs. I tried to extract some common functions for present PTE and swap entries, but I found too many detailed differences and variants. This made the common function overly complex, turning it into a real "monster." As a result, I don't think this approach would make the code any more readable or cleaner. After trying a couple of times, I feel the following is somehow more readable: (Lokesh is eager for the small folios fixes to be merged without further delay. So, I'd prefer to return -EBUSY for large folios in the hotfixes and handle the mTHP -EBUSY issue in a separate patch later.) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index 867898c4e30b..eed9286ec1f3 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -18,6 +18,7 @@ #include #include #include "internal.h" +#include "swap.h" static __always_inline bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end) @@ -1072,15 +1073,15 @@ static int move_present_pte(struct mm_struct *mm, return err; } -static int move_swap_pte(struct mm_struct *mm, +static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma, unsigned long dst_addr, unsigned long src_addr, pte_t *dst_pte, pte_t *src_pte, pte_t orig_dst_pte, pte_t orig_src_pte, pmd_t *dst_pmd, pmd_t dst_pmdval, - spinlock_t *dst_ptl, spinlock_t *src_ptl) + spinlock_t *dst_ptl, spinlock_t *src_ptl, + struct folio *src_folio) { - if (!pte_swp_exclusive(orig_src_pte)) - return -EBUSY; + int err = 0; double_pt_lock(dst_ptl, src_ptl); @@ -1090,11 +1091,22 @@ static int move_swap_pte(struct mm_struct *mm, return -EAGAIN; } + if (src_folio) { + /* Folio got pinned from under us. Put it back and fail the move. */ + if (folio_maybe_dma_pinned(src_folio)) { + err = -EBUSY; + goto out; + } + folio_move_anon_rmap(src_folio, dst_vma); + src_folio->index = linear_page_index(dst_vma, dst_addr); + } + orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte); set_pte_at(mm, dst_addr, dst_pte, orig_src_pte); - double_pt_unlock(dst_ptl, src_ptl); - return 0; +out: + double_pt_unlock(dst_ptl, src_ptl); + return err; } static int move_zeropage_pte(struct mm_struct *mm, @@ -1137,6 +1149,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, __u64 mode) { swp_entry_t entry; + struct swap_info_struct *si = NULL; pte_t orig_src_pte, orig_dst_pte; pte_t src_folio_pte; spinlock_t *src_ptl, *dst_ptl; @@ -1318,6 +1331,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval, dst_ptl, src_ptl, src_folio); } else { + struct folio *folio = NULL; + entry = pte_to_swp_entry(orig_src_pte); if (non_swap_entry(entry)) { if (is_migration_entry(entry)) { @@ -1331,9 +1346,47 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, goto out; } - err = move_swap_pte(mm, dst_addr, src_addr, dst_pte, src_pte, - orig_dst_pte, orig_src_pte, dst_pmd, - dst_pmdval, dst_ptl, src_ptl); + if (!pte_swp_exclusive(orig_src_pte)) { + err = -EBUSY; + goto out; + } + + si = get_swap_device(entry); + if (unlikely(!si)) { + err = -EAGAIN; + goto out; + } + /* + * Check if swapcache exists. If it does, the folio must be + * moved even if the PTE is a swap entry. For large folios, + * we directly return -EBUSY, as split_folio() currently + * also returns -EBUSY when attempting to split unmapped + * large folios in the swapcache. This needs to be fixed + * to allow proper handling. + */ + if (!src_folio) + folio = filemap_get_folio(swap_address_space(entry), + swap_cache_index(entry)); + if (!IS_ERR_OR_NULL(folio)) { + if (folio_test_large(folio)) { + err = -EBUSY; + folio_put(folio); + goto out; + } + src_folio = folio; + if (!folio_trylock(src_folio)) { + pte_unmap(&orig_src_pte); + pte_unmap(&orig_dst_pte); + src_pte = dst_pte = NULL; + /* now we can block and wait */ + folio_lock(src_folio); + si = NULL; + goto retry; + } + } + err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte, + orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval, + dst_ptl, src_ptl, src_folio); } out: @@ -1350,6 +1403,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, if (src_pte) pte_unmap(src_pte); mmu_notifier_invalidate_range_end(&range); + if (si) + put_swap_device(si); return err; } If there are no objections, I'll send v2 tomorrow with the above code. 12:04 AM, Time to get some sleep now! :-) > > > > > > > > > > > > > > > > > > > > My point is that if we want a proper fix for mTHP, we'd better handle writeback. > > > > > Otherwise, this isn’t much different from directly returning -EBUSY as proposed > > > > > in this RFC. > > > > > > > > > > For small folios, there’s no split_folio issue, making it relatively > > > > > simpler. Lokesh > > > > > mentioned plans to madvise NOHUGEPAGE in ART, so fixing small folios is likely > > > > > the first priority. > > > > > > > > Agreed. > > > > > > > > -- > > > > Peter Xu > > > > Thanks Barry