[PATCH] mm/readahead: read min folio constraints under invalidate lock

[PATCH] mm/readahead: read min folio constraints under invalidate lock

[Jinchao Wang]

page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
constraints before taking the invalidate lock, allowing concurrent changes to
violate page cache invariants.

Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
allocations respect the mapping constraints.

Fixes: 47dd67532303 ("block/bdev: lift block size restrictions to 64k")
Reported-by: syzbot+4d3cc33ef7a77041efa6@syzkaller.appspotmail.com
Reported-by: syzbot+fdba5cca73fee92c69d6@syzkaller.appspotmail.com
Signed-off-by: Jinchao Wang <wangjinchao600@gmail.com>
---
 mm/readahead.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/mm/readahead.c b/mm/readahead.c
index b415c9969176..74acd6c4f87c 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -214,7 +214,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 	unsigned long index = readahead_index(ractl);
 	gfp_t gfp_mask = readahead_gfp_mask(mapping);
 	unsigned long mark = ULONG_MAX, i = 0;
-	unsigned int min_nrpages = mapping_min_folio_nrpages(mapping);
+	unsigned int min_nrpages;
 
 	/*
 	 * Partway through the readahead operation, we will have added
@@ -232,6 +232,7 @@ void page_cache_ra_unbounded(struct readahead_control *ractl,
 				      lookahead_size);
 	filemap_invalidate_lock_shared(mapping);
 	index = mapping_align_index(mapping, index);
+	min_nrpages = mapping_min_folio_nrpages(mapping);
 
 	/*
 	 * As iterator `i` is aligned to min_nrpages, round_up the
@@ -467,7 +468,7 @@ void page_cache_ra_order(struct readahead_control *ractl,
 	struct address_space *mapping = ractl->mapping;
 	pgoff_t start = readahead_index(ractl);
 	pgoff_t index = start;
-	unsigned int min_order = mapping_min_folio_order(mapping);
+	unsigned int min_order;
 	pgoff_t limit = (i_size_read(mapping->host) - 1) >> PAGE_SHIFT;
 	pgoff_t mark = index + ra->size - ra->async_size;
 	unsigned int nofs;
@@ -485,13 +486,16 @@ void page_cache_ra_order(struct readahead_control *ractl,
 
 	new_order = min(mapping_max_folio_order(mapping), new_order);
 	new_order = min_t(unsigned int, new_order, ilog2(ra->size));
-	new_order = max(new_order, min_order);
 
 	ra->order = new_order;
 
 	/* See comment in page_cache_ra_unbounded() */
 	nofs = memalloc_nofs_save();
 	filemap_invalidate_lock_shared(mapping);
+
+	min_order = mapping_min_folio_order(mapping);
+	new_order = max(new_order, min_order);
+
 	/*
 	 * If the new_order is greater than min_order and index is
 	 * already aligned to new_order, then this will be noop as index
-- 
2.43.0

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[kernel test robot]

Hi,

Thanks for your patch.

FYI: kernel test robot notices the stable kernel rule is not satisfied.

The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#option-1

Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree.
Subject: [PATCH] mm/readahead: read min folio constraints under invalidate lock
Link: https://lore.kernel.org/stable/20251215141936.1045907-1-wangjinchao600%40gmail.com

-- 
0-DAY CI Kernel Test Service
https://github.com/intel/lkp-tests/wiki

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[Matthew Wilcox]

On Mon, Dec 15, 2025 at 10:19:00PM +0800, Jinchao Wang wrote:
> page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
> constraints before taking the invalidate lock, allowing concurrent changes to
> violate page cache invariants.
> 
> Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
> allocations respect the mapping constraints.

Why are the mapping folio size constraints being changed?  They're
supposed to be set at inode instantiation and then never changed.

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[Jinchao Wang]

On Mon, Dec 15, 2025 at 02:22:23PM +0000, Matthew Wilcox wrote:
> On Mon, Dec 15, 2025 at 10:19:00PM +0800, Jinchao Wang wrote:
> > page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
> > constraints before taking the invalidate lock, allowing concurrent changes to
> > violate page cache invariants.
> > 
> > Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
> > allocations respect the mapping constraints.
> 
> Why are the mapping folio size constraints being changed?  They're
> supposed to be set at inode instantiation and then never changed.

They can change after instantiation for block devices. In the syzbot repro:
  blkdev_ioctl() -> blkdev_bszset() -> set_blocksize() ->
  mapping_set_folio_min_order()

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[Matthew Wilcox]

On Tue, Dec 16, 2025 at 09:37:51AM +0800, Jinchao Wang wrote:
> On Mon, Dec 15, 2025 at 02:22:23PM +0000, Matthew Wilcox wrote:
> > On Mon, Dec 15, 2025 at 10:19:00PM +0800, Jinchao Wang wrote:
> > > page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
> > > constraints before taking the invalidate lock, allowing concurrent changes to
> > > violate page cache invariants.
> > > 
> > > Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
> > > allocations respect the mapping constraints.
> > 
> > Why are the mapping folio size constraints being changed?  They're
> > supposed to be set at inode instantiation and then never changed.
> 
> They can change after instantiation for block devices. In the syzbot repro:
>   blkdev_ioctl() -> blkdev_bszset() -> set_blocksize() ->
>   mapping_set_folio_min_order()

Oh, this is just syzbot doing stupid things.  We should probably make
blkdev_bszset() fail if somebody else has an fd open.

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[Jinchao Wang]

On Tue, Dec 16, 2025 at 02:42:06AM +0000, Matthew Wilcox wrote:
> On Tue, Dec 16, 2025 at 09:37:51AM +0800, Jinchao Wang wrote:
> > On Mon, Dec 15, 2025 at 02:22:23PM +0000, Matthew Wilcox wrote:
> > > On Mon, Dec 15, 2025 at 10:19:00PM +0800, Jinchao Wang wrote:
> > > > page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
> > > > constraints before taking the invalidate lock, allowing concurrent changes to
> > > > violate page cache invariants.
> > > > 
> > > > Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
> > > > allocations respect the mapping constraints.
> > > 
> > > Why are the mapping folio size constraints being changed?  They're
> > > supposed to be set at inode instantiation and then never changed.
> > 
> > They can change after instantiation for block devices. In the syzbot repro:
> >   blkdev_ioctl() -> blkdev_bszset() -> set_blocksize() ->
> >   mapping_set_folio_min_order()
> 
> Oh, this is just syzbot doing stupid things.  We should probably make
> blkdev_bszset() fail if somebody else has an fd open.

Thanks, that makes sense.
Tightening blkdev_bszset() would avoid the race entirely.
This change is meant as a defensive fix to prevent BUGs.

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[Matthew Wilcox]

On Tue, Dec 16, 2025 at 11:12:21AM +0800, Jinchao Wang wrote:
> On Tue, Dec 16, 2025 at 02:42:06AM +0000, Matthew Wilcox wrote:
> > On Tue, Dec 16, 2025 at 09:37:51AM +0800, Jinchao Wang wrote:
> > > On Mon, Dec 15, 2025 at 02:22:23PM +0000, Matthew Wilcox wrote:
> > > > On Mon, Dec 15, 2025 at 10:19:00PM +0800, Jinchao Wang wrote:
> > > > > page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
> > > > > constraints before taking the invalidate lock, allowing concurrent changes to
> > > > > violate page cache invariants.
> > > > > 
> > > > > Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
> > > > > allocations respect the mapping constraints.
> > > > 
> > > > Why are the mapping folio size constraints being changed?  They're
> > > > supposed to be set at inode instantiation and then never changed.
> > > 
> > > They can change after instantiation for block devices. In the syzbot repro:
> > >   blkdev_ioctl() -> blkdev_bszset() -> set_blocksize() ->
> > >   mapping_set_folio_min_order()
> > 
> > Oh, this is just syzbot doing stupid things.  We should probably make
> > blkdev_bszset() fail if somebody else has an fd open.
> 
> Thanks, that makes sense.
> Tightening blkdev_bszset() would avoid the race entirely.
> This change is meant as a defensive fix to prevent BUGs.

Yes, but the point is that there's a lot of code which relies on
the AS_FOLIO bits not changing in the middle.  Syzbot found one of them,
but there are others.

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[Jinchao Wang]

#syz test

Re: [syzbot] [fs?] [mm?] kernel BUG in __filemap_add_folio

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
kernel BUG in __filemap_add_folio

 reset_page_owner include/linux/page_owner.h:25 [inline]
 free_pages_prepare mm/page_alloc.c:1395 [inline]
 __free_frozen_pages+0x7df/0x1170 mm/page_alloc.c:2943
 rcu_do_batch kernel/rcu/tree.c:2605 [inline]
 rcu_core+0x79c/0x15f0 kernel/rcu/tree.c:2857
 handle_softirqs+0x219/0x950 kernel/softirq.c:622
 run_ksoftirqd kernel/softirq.c:1063 [inline]
 run_ksoftirqd+0x3a/0x60 kernel/softirq.c:1055
 smpboot_thread_fn+0x3f7/0xae0 kernel/smpboot.c:160
 kthread+0x3c5/0x780 kernel/kthread.c:463
 ret_from_fork+0x983/0xb10 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:246
------------[ cut here ]------------
kernel BUG at mm/filemap.c:858!
Oops: invalid opcode: 0000 [#1] SMP KASAN NOPTI
CPU: 1 UID: 0 PID: 6821 Comm: syz.1.76 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:__filemap_add_folio+0xf29/0x11b0 mm/filemap.c:858
Code: 9b c6 ff 48 c7 c6 c0 e9 99 8b 4c 89 ef e8 0f 74 11 00 90 0f 0b e8 47 9b c6 ff 48 c7 c6 20 ea 99 8b 4c 89 ef e8 f8 73 11 00 90 <0f> 0b e8 30 9b c6 ff 90 0f 0b 90 e9 1c fc ff ff e8 22 9b c6 ff 48
RSP: 0018:ffffc900033af840 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff8880737fc980 RSI: ffffffff81f7ebf8 RDI: ffff8880737fce04
RBP: 0000000000112cc0 R08: 0000000000000001 R09: 0000000000000001
R10: ffffffff908689d7 R11: 0000000000000000 R12: 0000000000000002
R13: ffffea0001ce4980 R14: 0000000000000000 R15: 0000000000000000
FS:  000055557770b500(0000) GS:ffff888124a48000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f9aef15c000 CR3: 000000002ee4c000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 filemap_add_folio+0x19a/0x610 mm/filemap.c:966
 ra_alloc_folio mm/readahead.c:453 [inline]
 page_cache_ra_order+0x637/0xed0 mm/readahead.c:512
 do_sync_mmap_readahead mm/filemap.c:3400 [inline]
 filemap_fault+0x16ac/0x29d0 mm/filemap.c:3549
 __do_fault+0x10d/0x490 mm/memory.c:5320
 do_shared_fault mm/memory.c:5819 [inline]
 do_fault+0x302/0x1ad0 mm/memory.c:5893
 do_pte_missing mm/memory.c:4401 [inline]
 handle_pte_fault mm/memory.c:6273 [inline]
 __handle_mm_fault+0x1919/0x2bb0 mm/memory.c:6411
 handle_mm_fault+0x3fe/0xad0 mm/memory.c:6580
 do_user_addr_fault+0x60c/0x1370 arch/x86/mm/fault.c:1336
 handle_page_fault arch/x86/mm/fault.c:1476 [inline]
 exc_page_fault+0x64/0xc0 arch/x86/mm/fault.c:1532
 asm_exc_page_fault+0x26/0x30 arch/x86/include/asm/idtentry.h:618
RIP: 0033:0x7f8af1a55171
Code: 48 8b 54 24 08 48 85 d2 74 17 8b 44 24 18 0f c8 89 c0 48 89 44 24 18 48 83 fa 01 0f 85 b3 01 00 00 48 8b 44 24 10 8b 54 24 18 <89> 10 e9 15 fd ff ff 48 8b 44 24 10 8b 10 48 8b 44 24 08 48 85 c0
RSP: 002b:00007ffc7d678bf0 EFLAGS: 00010246
RAX: 0000200000000980 RBX: 0000000000000004 RCX: 0000000000000000
RDX: 0000000000004000 RSI: 0000000000000000 RDI: 000055557770b3c8
RBP: 00007ffc7d678cf8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000002 R12: 00007f8af1dd5fac
R13: 00007f8af1dd5fa0 R14: fffffffffffffffe R15: 00007ffc7d678d40
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__filemap_add_folio+0xf29/0x11b0 mm/filemap.c:858
Code: 9b c6 ff 48 c7 c6 c0 e9 99 8b 4c 89 ef e8 0f 74 11 00 90 0f 0b e8 47 9b c6 ff 48 c7 c6 20 ea 99 8b 4c 89 ef e8 f8 73 11 00 90 <0f> 0b e8 30 9b c6 ff 90 0f 0b 90 e9 1c fc ff ff e8 22 9b c6 ff 48
RSP: 0018:ffffc900033af840 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff8880737fc980 RSI: ffffffff81f7ebf8 RDI: ffff8880737fce04
RBP: 0000000000112cc0 R08: 0000000000000001 R09: 0000000000000001
R10: ffffffff908689d7 R11: 0000000000000000 R12: 0000000000000002
R13: ffffea0001ce4980 R14: 0000000000000000 R15: 0000000000000000
FS:  000055557770b500(0000) GS:ffff888124a48000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f772b5d7dac CR3: 000000002ee4c000 CR4: 00000000003526f0


Tested on:

commit:         40fbbd64 Merge tag 'pull-fixes' of git://git.kernel.or..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=10715dc2580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=495547a782e37c4f
dashboard link: https://syzkaller.appspot.com/bug?extid=4d3cc33ef7a77041efa6
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

Note: no patches were applied.

Re: [PATCH] mm/readahead: read min folio constraints under invalidate lock

[Jinchao Wang]

On Tue, Dec 16, 2025 at 03:53:17AM +0000, Matthew Wilcox wrote:
> On Tue, Dec 16, 2025 at 11:12:21AM +0800, Jinchao Wang wrote:
> > On Tue, Dec 16, 2025 at 02:42:06AM +0000, Matthew Wilcox wrote:
> > > On Tue, Dec 16, 2025 at 09:37:51AM +0800, Jinchao Wang wrote:
> > > > On Mon, Dec 15, 2025 at 02:22:23PM +0000, Matthew Wilcox wrote:
> > > > > On Mon, Dec 15, 2025 at 10:19:00PM +0800, Jinchao Wang wrote:
> > > > > > page_cache_ra_order() and page_cache_ra_unbounded() read mapping minimum folio
> > > > > > constraints before taking the invalidate lock, allowing concurrent changes to
> > > > > > violate page cache invariants.
> > > > > > 
> > > > > > Move the lookups under filemap_invalidate_lock_shared() to ensure readahead
> > > > > > allocations respect the mapping constraints.
> > > > > 
> > > > > Why are the mapping folio size constraints being changed?  They're
> > > > > supposed to be set at inode instantiation and then never changed.
> > > > 
> > > > They can change after instantiation for block devices. In the syzbot repro:
> > > >   blkdev_ioctl() -> blkdev_bszset() -> set_blocksize() ->
> > > >   mapping_set_folio_min_order()
> > > 
> > > Oh, this is just syzbot doing stupid things.  We should probably make
> > > blkdev_bszset() fail if somebody else has an fd open.
> > 
> > Thanks, that makes sense.
> > Tightening blkdev_bszset() would avoid the race entirely.
> > This change is meant as a defensive fix to prevent BUGs.
> 
> Yes, but the point is that there's a lot of code which relies on
> the AS_FOLIO bits not changing in the middle.  Syzbot found one of them,
> but there are others.

I've been thinking about this more, and I wanted to share another
perspective if that's okay.

Rather than tracking down every place that might change AS_FOLIO bits
(like blkdev_bszset() and potentially others), what if we make the
page cache layer itself robust against such changes?

The invalidate_lock was introduced for exactly this kind of protection
(commit 730633f0b7f9: "mm: Protect operations adding pages to page
cache with invalidate_lock"). This way, the page cache doesn't need
to rely on assumptions about what upper layers might do.

The readahead functions already hold filemap_invalidate_lock_shared(),
so moving the constraint reads under the lock adds no overhead. It
would protect against AS_FOLIO changes regardless of their source.

I think this separates concerns nicely: upper layers can change
constraints through the invalidate_lock protocol, and page cache
operations are automatically safe. But I'd really value your thoughts
on this approach - you have much more experience with these tradeoffs
than I do.

Thanks again for taking the time to discuss this.