Re: [PATCH v3] mm/rmap: fix incorrect pte restoration for lazyfree folios

[Lance Yang <lance.yang@linux.dev>]

On Tue,  3 Mar 2026 11:45:28 +0530, 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:
>
>[   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 <stdio.h>
>#include <unistd.h>
>#include <stdlib.h>
>#include <sys/mman.h>
>#include <string.h>
>#include <sys/wait.h>
>#include <sched.h>
>#include <fcntl.h>
>
>void write_to_reclaim() {
>    const char *path = "/sys/devices/system/node/node0/reclaim";

I wasn't able to get this reproducer working with node reclaim, but using
memcg v1 memory.force_empty worked fine for me.

[...]

>
>Fixes: 354dffd29575 ("mm: support batched unmap for lazyfree large folios during reclamation")
>Cc: stable <stable@kernel.org>
>Signed-off-by: Dev Jain <dev.jain@arm.com>
>---

Thanks!

Tested-by: Lance Yang <lance.yang@linux.dev>