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imf19.hostedemail.com; dkim=none; spf=pass (imf19.hostedemail.com: domain of dev.jain@arm.com designates 217.140.110.172 as permitted sender) smtp.mailfrom=dev.jain@arm.com; dmarc=pass (policy=none) header.from=arm.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1772533937; a=rsa-sha256; cv=none; b=yyoHKPkV2Fplz5dIfmIQhdfrPQB5VLKN5ejFoGgszlDrT1LlD8UDXYd3IQPo356q6DqGU6 27TmuG5Oyz+wR0NFawHtV8pO868PNUjACXhBO2xn8EXZ2jpJ4xi8eZ1B3xU1f7x48m0jPm 7Y7LosooHCSrVhsW1gbtSwCD4pRbluY= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A34D7497; Tue, 3 Mar 2026 02:32:09 -0800 (PST) Received: from [10.164.19.52] (unknown [10.164.19.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id BB6553F7BD; Tue, 3 Mar 2026 02:32:11 -0800 (PST) Message-ID: <56d607c0-dc72-431a-bc86-6dae100f1708@arm.com> Date: Tue, 3 Mar 2026 16:02:08 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v3] mm/rmap: fix incorrect pte restoration for lazyfree folios To: akpm@linux-foundation.org, david@kernel.org, lorenzo.stoakes@oracle.com Cc: riel@surriel.com, Liam.Howlett@oracle.com, vbabka@kernel.org, harry.yoo@oracle.com, jannh@google.com, baohua@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, ryan.roberts@arm.com, anshuman.khandual@arm.com, stable References: <20260303061528.2429162-1-dev.jain@arm.com> Content-Language: en-US From: Dev Jain In-Reply-To: <20260303061528.2429162-1-dev.jain@arm.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: D9C101A000B X-Stat-Signature: p7zuc5d6buxaayhcx6xmdw657yrhkoy9 X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1772533936-258760 X-HE-Meta: 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 ELbNyuJ6 4KldjTiF3AJ+Vjy9c7gpwzIDEyhrEl49d6FfqYkjkpAsuGs0P9FMSRIf1ktFWIoWZJndga+r40uq6kacbHoPSxMJm4RoBhs3Xd0ZqIUGmWWOt5fzZBN3nhNWCj1YpGrbVLucUKQGzJpei4DRpD8VkmiYa2zSOWbXADyGYobeHQdewq1TDhYLsHKlSq62kvemehJ97hAOqRH3R3FN6x8Hp0/f4r36CRGgsdI7A Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 03/03/26 11:45 am, Dev Jain wrote: > We batch unmap anonymous lazyfree folios by folio_unmap_pte_batch. > If the batch has a mix of writable and non-writable bits, we may end up > setting the entire batch writable. Fix this by respecting writable bit > during batching. > Although on a successful unmap of a lazyfree folio, the soft-dirty bit is > lost, preserve it on pte restoration by respecting the bit during batching, > to make the fix consistent w.r.t both writable bit and soft-dirty bit. > > I was able to write the below reproducer and crash the kernel. > Explanation of reproducer (set 64K mTHP to always): > > Fault in a 64K large folio. Split the VMA at mid-point with MADV_DONTFORK. > fork() - parent points to the folio with 8 writable ptes and 8 non-writable > ptes. Merge the VMAs with MADV_DOFORK so that folio_unmap_pte_batch() can > determine all the 16 ptes as a batch. Do MADV_FREE on the range to mark > the folio as lazyfree. Write to the memory to dirty the pte, eventually > rmap will dirty the folio. Then trigger reclaim, we will hit the pte > restoration path, and the kernel will crash with the following trace: Andrew, could you convert the last line to the following: ..restoration path, and the kernel will crash with the trace given below. The BUG happens at: BUG_ON(atomic_inc_return(&ptc->anon_map_count) > 1 && rw); The code path is asking for anonymous page to be mapped writable into the pagetable. The BUG_ON() firing implies that such a writable page has been mapped into the pagetables of more than one process, which breaks anonymous memory/CoW semantics. > > [ 21.134473] kernel BUG at mm/page_table_check.c:118! > [ 21.134497] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP > [ 21.135917] Modules linked in: > [ 21.136085] CPU: 1 UID: 0 PID: 1735 Comm: dup-lazyfree Not tainted 7.0.0-rc1-00116-g018018a17770 #1028 PREEMPT > [ 21.136858] Hardware name: linux,dummy-virt (DT) > [ 21.137019] pstate: 21400005 (nzCv daif +PAN -UAO -TCO +DIT -SSBS BTYPE=--) > [ 21.137308] pc : page_table_check_set+0x28c/0x2a8 > [ 21.137607] lr : page_table_check_set+0x134/0x2a8 > [ 21.137885] sp : ffff80008a3b3340 > [ 21.138124] x29: ffff80008a3b3340 x28: fffffdffc3d14400 x27: ffffd1a55e03d000 > [ 21.138623] x26: 0040000000000040 x25: ffffd1a55f7dd000 x24: 0000000000000001 > [ 21.139045] x23: 0000000000000001 x22: 0000000000000001 x21: ffffd1a55f217f30 > [ 21.139629] x20: 0000000000134521 x19: 0000000000134519 x18: 005c43e000040000 > [ 21.140027] x17: 0001400000000000 x16: 0001700000000000 x15: 000000000000ffff > [ 21.140578] x14: 000000000000000c x13: 005c006000000000 x12: 0000000000000020 > [ 21.140828] x11: 0000000000000000 x10: 005c000000000000 x9 : ffffd1a55c079ee0 > [ 21.141077] x8 : 0000000000000001 x7 : 005c03e000040000 x6 : 000000004000ffff > [ 21.141490] x5 : ffff00017fffce00 x4 : 0000000000000001 x3 : 0000000000000002 > [ 21.141741] x2 : 0000000000134510 x1 : 0000000000000000 x0 : ffff0000c08228c0 > [ 21.141991] Call trace: > [ 21.142093] page_table_check_set+0x28c/0x2a8 (P) > [ 21.142265] __page_table_check_ptes_set+0x144/0x1e8 > [ 21.142441] __set_ptes_anysz.constprop.0+0x160/0x1a8 > [ 21.142766] contpte_set_ptes+0xe8/0x140 > [ 21.142907] try_to_unmap_one+0x10c4/0x10d0 > [ 21.143177] rmap_walk_anon+0x100/0x250 > [ 21.143315] try_to_unmap+0xa0/0xc8 > [ 21.143441] shrink_folio_list+0x59c/0x18a8 > [ 21.143759] shrink_lruvec+0x664/0xbf0 > [ 21.144043] shrink_node+0x218/0x878 > [ 21.144285] __node_reclaim.constprop.0+0x98/0x338 > [ 21.144763] user_proactive_reclaim+0x2a4/0x340 > [ 21.145056] reclaim_store+0x3c/0x60 > [ 21.145216] dev_attr_store+0x20/0x40 > [ 21.145585] sysfs_kf_write+0x84/0xa8 > [ 21.145835] kernfs_fop_write_iter+0x130/0x1c8 > [ 21.145994] vfs_write+0x2b8/0x368 > [ 21.146119] ksys_write+0x70/0x110 > [ 21.146240] __arm64_sys_write+0x24/0x38 > [ 21.146380] invoke_syscall+0x50/0x120 > [ 21.146513] el0_svc_common.constprop.0+0x48/0xf8 > [ 21.146679] do_el0_svc+0x28/0x40 > [ 21.146798] el0_svc+0x34/0x110 > [ 21.146926] el0t_64_sync_handler+0xa0/0xe8 > [ 21.147074] el0t_64_sync+0x198/0x1a0 > [ 21.147225] Code: f9400441 b4fff241 17ffff94 d4210000 (d4210000) > [ 21.147440] ---[ end trace 0000000000000000 ]--- > > > #define _GNU_SOURCE > #include > #include > #include > #include > #include > #include > #include > #include > > void write_to_reclaim() { > const char *path = "/sys/devices/system/node/node0/reclaim"; > const char *value = "409600000000"; > int fd = open(path, O_WRONLY); > if (fd == -1) { > perror("open"); > exit(EXIT_FAILURE); > } > > if (write(fd, value, sizeof("409600000000") - 1) == -1) { > perror("write"); > close(fd); > exit(EXIT_FAILURE); > } > > printf("Successfully wrote %s to %s\n", value, path); > close(fd); > } > > int main() > { > char *ptr = mmap((void *)(1UL << 30), 1UL << 16, PROT_READ | PROT_WRITE, > MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > if ((unsigned long)ptr != (1UL << 30)) { > perror("mmap"); > return 1; > } > > /* a 64K folio gets faulted in */ > memset(ptr, 0, 1UL << 16); > > /* 32K half will not be shared into child */ > if (madvise(ptr, 1UL << 15, MADV_DONTFORK)) { > perror("madvise madv dontfork"); > return 1; > } > > pid_t pid = fork(); > > if (pid < 0) { > perror("fork"); > return 1; > } else if (pid == 0) { > sleep(15); > } else { > /* merge VMAs. now first half of the 16 ptes are writable, the other half not. */ > if (madvise(ptr, 1UL << 15, MADV_DOFORK)) { > perror("madvise madv fork"); > return 1; > } > if (madvise(ptr, (1UL << 16), MADV_FREE)) { > perror("madvise madv free"); > return 1; > } > > /* dirty the large folio */ > (*ptr) += 10; > > write_to_reclaim(); > // sleep(10); > waitpid(pid, NULL, 0); > > } > } > > Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation") > Cc: stable > Signed-off-by: Dev Jain > --- > Patch applies on mm-unstable (9af4957ef127). > > v2->v3: > - Don't special case for anon folios > > v1->v2: > - Just respect the writable bit instead of hacking in a pte_wrprotect() in > failure path > - Also handle soft-dirty bit > > mm/rmap.c | 9 ++++++++- > 1 file changed, 8 insertions(+), 1 deletion(-) > > diff --git a/mm/rmap.c b/mm/rmap.c > index bff8f222004e4..5a3e408e3f179 100644 > --- a/mm/rmap.c > +++ b/mm/rmap.c > @@ -1955,7 +1955,14 @@ static inline unsigned int folio_unmap_pte_batch(struct folio *folio, > if (userfaultfd_wp(vma)) > return 1; > > - return folio_pte_batch(folio, pvmw->pte, pte, max_nr); > + /* > + * If unmap fails, we need to restore the ptes. To avoid accidentally > + * upgrading write permissions for ptes that were not originally > + * writable, and to avoid losing the soft-dirty bit, use the > + * appropriate FPB flags. > + */ > + return folio_pte_batch_flags(folio, vma, pvmw->pte, &pte, max_nr, > + FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY); > } > > /*