Re: [PATCH v2] mm/rmap: fix incorrect pte restoration for lazyfree folios

["David Hildenbrand (Arm)" <david@kernel.org>]

On 2/28/26 19:34, Barry Song wrote:
> On Sun, Mar 1, 2026 at 3:06 AM Dev Jain <dev.jain@arm.com> 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";
>>     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 <stable@kernel.org>
>> Signed-off-by: Dev Jain <dev.jain@arm.com>
>> ---
>> v1->v2:
>>  - Just respect the writable bit instead of hacking in a pte_wrprotect() in
>>    failure path
>>  - Also handle soft-dirty bit
>>
>> Based on mm-unstable (df9c51269a5e).
>>
>>  mm/rmap.c | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>>
>> diff --git a/mm/rmap.c b/mm/rmap.c
>> index bff8f222004e4..fb64829913052 100644
>> --- a/mm/rmap.c
>> +++ b/mm/rmap.c
>> @@ -1955,7 +1955,17 @@ 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 (!folio_test_anon(folio))
>> +               return folio_pte_batch(folio, pvmw->pte, pte, max_nr);
>> +
>> +       /*
>> +        * For anon folios, 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);
> 
> Do we really need to differentiate between file and anon?
> I’d rather just return unconditionally by removing the
> if (!folio_test_anon(folio)) check above.
> 
> If we do want to keep two branches, why not use a flag variant instead?

I suspect Dev's code might generate better code, as the compiler might
not want to provide two variants of folio_pte_batch_flags() where it
propagates all constants; and even if it does, we'd end up with two
essentially identical functions in the kernel binary.

So if we want to special-case anon folios, I think we should use Dev's
variant.

But I also wonder whether we just want to keep it simple for now and
just unconditionally check FPB_RESPECT_WRITE | FPB_RESPECT_SOFT_DIRTY.

I'd vote for simplicity at this point.

-- 
Cheers,

David