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[PATCH] fs/ceph/addr: always call ceph_shift_unused_folios_left()

Max Kellermann posted 1 patch 1 month ago 
fs/ceph/addr.c | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)
[PATCH] fs/ceph/addr: always call ceph_shift_unused_folios_left()
Posted by Max Kellermann 1 month ago 
The function ceph_process_folio_batch() sets folio_batch entries to
NULL, which is an illegal state.  Before folio_batch_release() crashes
due to this API violation, the function
ceph_shift_unused_folios_left() is supposed to remove those NULLs from
the array.

However, since commit ce80b76dd327 ("ceph: introduce
ceph_process_folio_batch() method"), this shifting doesn't happen
anymore because the "for" loop got moved to
ceph_process_folio_batch(), and now the `i` variable that remains in
ceph_writepages_start() doesn't get incremented anymore, making the
shifting effectively unreachable much of the time.

Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write()
method") added more preconditions for doing the shift, replacing the
`i` check (with something that is still just as broken):

- if ceph_process_folio_batch() fails, shifting never happens

- if ceph_move_dirty_page_in_page_array() was never called (because
  ceph_process_folio_batch() has returned early for some of various
  reasons), shifting never happens

- if `processed_in_fbatch` is zero (because ceph_process_folio_batch()
  has returned early for some of the reasons mentioned above or
  because ceph_move_dirty_page_in_page_array() has failed), shifting
  never happens

Since those two commits, any problem in ceph_process_folio_batch()
could crash the kernel, e.g. this way:

 BUG: kernel NULL pointer dereference, address: 0000000000000034
 #PF: supervisor write access in kernel mode
 #PF: error_code(0x0002) - not-present page
 PGD 0 P4D 0
 Oops: Oops: 0002 [#1] SMP NOPTI
 CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE
 Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023
 Workqueue: writeback wb_workfn (flush-ceph-1)
 RIP: 0010:folios_put_refs+0x85/0x140
 Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 >
 RSP: 0018:ffffb880af8db778 EFLAGS: 00010207
 RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003
 RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0
 RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f
 R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0
 R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000
 FS:  0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0
 PKRU: 55555554
 Call Trace:
  <TASK>
  ceph_writepages_start+0xeb9/0x1410

The crash can be reproduced easily by changing the
ceph_check_page_before_write() return value to `-E2BIG`.

(Interestingly, the crash happens only if `huge_zero_folio` has
already been allocated; without `huge_zero_folio`,
is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL
entries instead of dereferencing them.  That makes reproducing the bug
somewhat unreliable.  See
https://lore.kernel.org/20250826231626.218675-1-max.kellermann@ionos.com
for a discussion of this detail.)

My suggestion is to move the ceph_shift_unused_folios_left() to right
after ceph_process_folio_batch() to ensure it always gets called to
fix up the illegal folio_batch state.

Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method")
Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/
Cc: stable@vger.kernel.org
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
---
 fs/ceph/addr.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
index 8b202d789e93..8bc66b45dade 100644
--- a/fs/ceph/addr.c
+++ b/fs/ceph/addr.c
@@ -1687,6 +1687,7 @@ static int ceph_writepages_start(struct address_space *mapping,
 
 process_folio_batch:
 		rc = ceph_process_folio_batch(mapping, wbc, &ceph_wbc);
+		ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
 		if (rc)
 			goto release_folios;
 
@@ -1695,8 +1696,6 @@ static int ceph_writepages_start(struct address_space *mapping,
 			goto release_folios;
 
 		if (ceph_wbc.processed_in_fbatch) {
-			ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
-
 			if (folio_batch_count(&ceph_wbc.fbatch) == 0 &&
 			    ceph_wbc.locked_pages < ceph_wbc.max_pages) {
 				doutc(cl, "reached end fbatch, trying for more\n");
-- 
2.47.2
Re: [PATCH] fs/ceph/addr: always call ceph_shift_unused_folios_left()
Posted by Ilya Dryomov 3 weeks, 4 days ago 
On Wed, Aug 27, 2025 at 8:17 PM Max Kellermann <max.kellermann@ionos.com> wrote:
>
> The function ceph_process_folio_batch() sets folio_batch entries to
> NULL, which is an illegal state.  Before folio_batch_release() crashes
> due to this API violation, the function
> ceph_shift_unused_folios_left() is supposed to remove those NULLs from
> the array.
>
> However, since commit ce80b76dd327 ("ceph: introduce
> ceph_process_folio_batch() method"), this shifting doesn't happen
> anymore because the "for" loop got moved to
> ceph_process_folio_batch(), and now the `i` variable that remains in
> ceph_writepages_start() doesn't get incremented anymore, making the
> shifting effectively unreachable much of the time.
>
> Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write()
> method") added more preconditions for doing the shift, replacing the
> `i` check (with something that is still just as broken):
>
> - if ceph_process_folio_batch() fails, shifting never happens
>
> - if ceph_move_dirty_page_in_page_array() was never called (because
>   ceph_process_folio_batch() has returned early for some of various
>   reasons), shifting never happens
>
> - if `processed_in_fbatch` is zero (because ceph_process_folio_batch()
>   has returned early for some of the reasons mentioned above or
>   because ceph_move_dirty_page_in_page_array() has failed), shifting
>   never happens
>
> Since those two commits, any problem in ceph_process_folio_batch()
> could crash the kernel, e.g. this way:
>
>  BUG: kernel NULL pointer dereference, address: 0000000000000034
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0002) - not-present page
>  PGD 0 P4D 0
>  Oops: Oops: 0002 [#1] SMP NOPTI
>  CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE
>  Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023
>  Workqueue: writeback wb_workfn (flush-ceph-1)
>  RIP: 0010:folios_put_refs+0x85/0x140
>  Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 >
>  RSP: 0018:ffffb880af8db778 EFLAGS: 00010207
>  RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003
>  RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0
>  RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f
>  R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0
>  R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000
>  FS:  0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0
>  PKRU: 55555554
>  Call Trace:
>   <TASK>
>   ceph_writepages_start+0xeb9/0x1410
>
> The crash can be reproduced easily by changing the
> ceph_check_page_before_write() return value to `-E2BIG`.
>
> (Interestingly, the crash happens only if `huge_zero_folio` has
> already been allocated; without `huge_zero_folio`,
> is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL
> entries instead of dereferencing them.  That makes reproducing the bug
> somewhat unreliable.  See
> https://lore.kernel.org/20250826231626.218675-1-max.kellermann@ionos.com
> for a discussion of this detail.)
>
> My suggestion is to move the ceph_shift_unused_folios_left() to right
> after ceph_process_folio_batch() to ensure it always gets called to
> fix up the illegal folio_batch state.
>
> Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method")
> Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> ---
>  fs/ceph/addr.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 8b202d789e93..8bc66b45dade 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1687,6 +1687,7 @@ static int ceph_writepages_start(struct address_space *mapping,
>
>  process_folio_batch:
>                 rc = ceph_process_folio_batch(mapping, wbc, &ceph_wbc);
> +               ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
>                 if (rc)
>                         goto release_folios;
>
> @@ -1695,8 +1696,6 @@ static int ceph_writepages_start(struct address_space *mapping,
>                         goto release_folios;
>
>                 if (ceph_wbc.processed_in_fbatch) {
> -                       ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
> -
>                         if (folio_batch_count(&ceph_wbc.fbatch) == 0 &&
>                             ceph_wbc.locked_pages < ceph_wbc.max_pages) {
>                                 doutc(cl, "reached end fbatch, trying for more\n");
> --
> 2.47.2
>

Queued up for 6.17-rc6.

Thanks,

                Ilya
Re: [PATCH] fs/ceph/addr: always call ceph_shift_unused_folios_left()
Posted by Viacheslav Dubeyko 1 month ago 
On Wed, 2025-08-27 at 20:17 +0200, Max Kellermann wrote:
> The function ceph_process_folio_batch() sets folio_batch entries to
> NULL, which is an illegal state.  Before folio_batch_release() crashes
> due to this API violation, the function
> ceph_shift_unused_folios_left() is supposed to remove those NULLs from
> the array.
> 
> However, since commit ce80b76dd327 ("ceph: introduce
> ceph_process_folio_batch() method"), this shifting doesn't happen
> anymore because the "for" loop got moved to
> ceph_process_folio_batch(), and now the `i` variable that remains in
> ceph_writepages_start() doesn't get incremented anymore, making the
> shifting effectively unreachable much of the time.
> 
> Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write()
> method") added more preconditions for doing the shift, replacing the
> `i` check (with something that is still just as broken):
> 
> - if ceph_process_folio_batch() fails, shifting never happens
> 
> - if ceph_move_dirty_page_in_page_array() was never called (because
>   ceph_process_folio_batch() has returned early for some of various
>   reasons), shifting never happens
> 
> - if `processed_in_fbatch` is zero (because ceph_process_folio_batch()
>   has returned early for some of the reasons mentioned above or
>   because ceph_move_dirty_page_in_page_array() has failed), shifting
>   never happens
> 
> Since those two commits, any problem in ceph_process_folio_batch()
> could crash the kernel, e.g. this way:
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000034
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0002) - not-present page
>  PGD 0 P4D 0
>  Oops: Oops: 0002 [#1] SMP NOPTI
>  CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE
>  Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023
>  Workqueue: writeback wb_workfn (flush-ceph-1)
>  RIP: 0010:folios_put_refs+0x85/0x140
>  Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 >
>  RSP: 0018:ffffb880af8db778 EFLAGS: 00010207
>  RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003
>  RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0
>  RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f
>  R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0
>  R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000
>  FS:  0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0
>  PKRU: 55555554
>  Call Trace:
>   <TASK>
>   ceph_writepages_start+0xeb9/0x1410
> 
> The crash can be reproduced easily by changing the
> ceph_check_page_before_write() return value to `-E2BIG`.
> 

I cannot reproduce the crash/issue. If ceph_check_page_before_write() returns
`-E2BIG`, then nothing happens. There is no crush and no write operations could
be processed by file system driver anymore. So, it doesn't look like recipe to
reproduce the issue. I cannot confirm that the patch fixes the issue without
clear way to reproduce the issue.

Could you please provide more clear explanation of the issue reproduction path?

Thanks,
Slava.


> (Interestingly, the crash happens only if `huge_zero_folio` has
> already been allocated; without `huge_zero_folio`,
> is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL
> entries instead of dereferencing them.  That makes reproducing the bug
> somewhat unreliable.  See
> https://lore.kernel.org/20250826231626.218675-1-max.kellermann@ionos.com  
> for a discussion of this detail.)
> 
> My suggestion is to move the ceph_shift_unused_folios_left() to right
> after ceph_process_folio_batch() to ensure it always gets called to
> fix up the illegal folio_batch state.
> 
> Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method")
> Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/  
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> ---
>  fs/ceph/addr.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 8b202d789e93..8bc66b45dade 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1687,6 +1687,7 @@ static int ceph_writepages_start(struct address_space *mapping,
>  
>  process_folio_batch:
>  		rc = ceph_process_folio_batch(mapping, wbc, &ceph_wbc);
> +		ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
>  		if (rc)
>  			goto release_folios;
>  
> @@ -1695,8 +1696,6 @@ static int ceph_writepages_start(struct address_space *mapping,
>  			goto release_folios;
>  
>  		if (ceph_wbc.processed_in_fbatch) {
> -			ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
> -
>  			if (folio_batch_count(&ceph_wbc.fbatch) == 0 &&
>  			    ceph_wbc.locked_pages < ceph_wbc.max_pages) {
>  				doutc(cl, "reached end fbatch, trying for more\n");
Re: [PATCH] fs/ceph/addr: always call ceph_shift_unused_folios_left()
Posted by Ilya Dryomov 1 month ago 
On Thu, Aug 28, 2025 at 8:55 PM Viacheslav Dubeyko
<Slava.Dubeyko@ibm.com> wrote:
>
> On Wed, 2025-08-27 at 20:17 +0200, Max Kellermann wrote:
> > The function ceph_process_folio_batch() sets folio_batch entries to
> > NULL, which is an illegal state.  Before folio_batch_release() crashes
> > due to this API violation, the function
> > ceph_shift_unused_folios_left() is supposed to remove those NULLs from
> > the array.
> >
> > However, since commit ce80b76dd327 ("ceph: introduce
> > ceph_process_folio_batch() method"), this shifting doesn't happen
> > anymore because the "for" loop got moved to
> > ceph_process_folio_batch(), and now the `i` variable that remains in
> > ceph_writepages_start() doesn't get incremented anymore, making the
> > shifting effectively unreachable much of the time.
> >
> > Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write()
> > method") added more preconditions for doing the shift, replacing the
> > `i` check (with something that is still just as broken):
> >
> > - if ceph_process_folio_batch() fails, shifting never happens
> >
> > - if ceph_move_dirty_page_in_page_array() was never called (because
> >   ceph_process_folio_batch() has returned early for some of various
> >   reasons), shifting never happens
> >
> > - if `processed_in_fbatch` is zero (because ceph_process_folio_batch()
> >   has returned early for some of the reasons mentioned above or
> >   because ceph_move_dirty_page_in_page_array() has failed), shifting
> >   never happens
> >
> > Since those two commits, any problem in ceph_process_folio_batch()
> > could crash the kernel, e.g. this way:
> >
> >  BUG: kernel NULL pointer dereference, address: 0000000000000034
> >  #PF: supervisor write access in kernel mode
> >  #PF: error_code(0x0002) - not-present page
> >  PGD 0 P4D 0
> >  Oops: Oops: 0002 [#1] SMP NOPTI
> >  CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE
> >  Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023
> >  Workqueue: writeback wb_workfn (flush-ceph-1)
> >  RIP: 0010:folios_put_refs+0x85/0x140
> >  Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 >
> >  RSP: 0018:ffffb880af8db778 EFLAGS: 00010207
> >  RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003
> >  RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0
> >  RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f
> >  R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0
> >  R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000
> >  FS:  0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000
> >  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> >  CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0
> >  PKRU: 55555554
> >  Call Trace:
> >   <TASK>
> >   ceph_writepages_start+0xeb9/0x1410
> >
> > The crash can be reproduced easily by changing the
> > ceph_check_page_before_write() return value to `-E2BIG`.
> >
>
> I cannot reproduce the crash/issue. If ceph_check_page_before_write() returns
> `-E2BIG`, then nothing happens. There is no crush and no write operations could
> be processed by file system driver anymore. So, it doesn't look like recipe to
> reproduce the issue. I cannot confirm that the patch fixes the issue without
> clear way to reproduce the issue.
>
> Could you please provide more clear explanation of the issue reproduction path?

Hi Slava,

Was this bit taken into account?

  (Interestingly, the crash happens only if `huge_zero_folio` has
  already been allocated; without `huge_zero_folio`,
  is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL
  entries instead of dereferencing them.  That makes reproducing the bug
  somewhat unreliable.  See
  https://lore.kernel.org/20250826231626.218675-1-max.kellermann@ionos.com
  for a discussion of this detail.)

Thanks,

                Ilya
Re: [PATCH] fs/ceph/addr: always call ceph_shift_unused_folios_left()
Posted by Viacheslav Dubeyko 1 month ago 
On Wed, 2025-08-27 at 20:17 +0200, Max Kellermann wrote:
> The function ceph_process_folio_batch() sets folio_batch entries to
> NULL, which is an illegal state.  Before folio_batch_release() crashes
> due to this API violation, the function
> ceph_shift_unused_folios_left() is supposed to remove those NULLs from
> the array.
> 
> However, since commit ce80b76dd327 ("ceph: introduce
> ceph_process_folio_batch() method"), this shifting doesn't happen
> anymore because the "for" loop got moved to
> ceph_process_folio_batch(), and now the `i` variable that remains in
> ceph_writepages_start() doesn't get incremented anymore, making the
> shifting effectively unreachable much of the time.
> 
> Later, commit 1551ec61dc55 ("ceph: introduce ceph_submit_write()
> method") added more preconditions for doing the shift, replacing the
> `i` check (with something that is still just as broken):
> 
> - if ceph_process_folio_batch() fails, shifting never happens
> 
> - if ceph_move_dirty_page_in_page_array() was never called (because
>   ceph_process_folio_batch() has returned early for some of various
>   reasons), shifting never happens
> 
> - if `processed_in_fbatch` is zero (because ceph_process_folio_batch()
>   has returned early for some of the reasons mentioned above or
>   because ceph_move_dirty_page_in_page_array() has failed), shifting
>   never happens
> 
> Since those two commits, any problem in ceph_process_folio_batch()
> could crash the kernel, e.g. this way:
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000034
>  #PF: supervisor write access in kernel mode
>  #PF: error_code(0x0002) - not-present page
>  PGD 0 P4D 0
>  Oops: Oops: 0002 [#1] SMP NOPTI
>  CPU: 172 UID: 0 PID: 2342707 Comm: kworker/u778:8 Not tainted 6.15.10-cm4all1-es #714 NONE
>  Hardware name: Dell Inc. PowerEdge R7615/0G9DHV, BIOS 1.6.10 12/08/2023
>  Workqueue: writeback wb_workfn (flush-ceph-1)
>  RIP: 0010:folios_put_refs+0x85/0x140
>  Code: 83 c5 01 39 e8 7e 76 48 63 c5 49 8b 5c c4 08 b8 01 00 00 00 4d 85 ed 74 05 41 8b 44 ad 00 48 8b 15 b0 >
>  RSP: 0018:ffffb880af8db778 EFLAGS: 00010207
>  RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000003
>  RDX: ffffe377cc3b0000 RSI: 0000000000000000 RDI: ffffb880af8db8c0
>  RBP: 0000000000000000 R08: 000000000000007d R09: 000000000102b86f
>  R10: 0000000000000001 R11: 00000000000000ac R12: ffffb880af8db8c0
>  R13: 0000000000000000 R14: 0000000000000000 R15: ffff9bd262c97000
>  FS:  0000000000000000(0000) GS:ffff9c8efc303000(0000) knlGS:0000000000000000
>  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>  CR2: 0000000000000034 CR3: 0000000160958004 CR4: 0000000000770ef0
>  PKRU: 55555554
>  Call Trace:
>   <TASK>
>   ceph_writepages_start+0xeb9/0x1410
> 
> The crash can be reproduced easily by changing the
> ceph_check_page_before_write() return value to `-E2BIG`.
> 
> (Interestingly, the crash happens only if `huge_zero_folio` has
> already been allocated; without `huge_zero_folio`,
> is_huge_zero_folio(NULL) returns true and folios_put_refs() skips NULL
> entries instead of dereferencing them.  That makes reproducing the bug
> somewhat unreliable.  See
> https://lore.kernel.org/20250826231626.218675-1-max.kellermann@ionos.com  
> for a discussion of this detail.)
> 
> My suggestion is to move the ceph_shift_unused_folios_left() to right
> after ceph_process_folio_batch() to ensure it always gets called to
> fix up the illegal folio_batch state.
> 
> Fixes: ce80b76dd327 ("ceph: introduce ceph_process_folio_batch() method")
> Link: https://lore.kernel.org/ceph-devel/aK4v548CId5GIKG1@swift.blarg.de/  
> Cc: stable@vger.kernel.org
> Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
> ---
>  fs/ceph/addr.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/fs/ceph/addr.c b/fs/ceph/addr.c
> index 8b202d789e93..8bc66b45dade 100644
> --- a/fs/ceph/addr.c
> +++ b/fs/ceph/addr.c
> @@ -1687,6 +1687,7 @@ static int ceph_writepages_start(struct address_space *mapping,
>  
>  process_folio_batch:
>  		rc = ceph_process_folio_batch(mapping, wbc, &ceph_wbc);
> +		ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
>  		if (rc)
>  			goto release_folios;
>  
> @@ -1695,8 +1696,6 @@ static int ceph_writepages_start(struct address_space *mapping,
>  			goto release_folios;
>  
>  		if (ceph_wbc.processed_in_fbatch) {
> -			ceph_shift_unused_folios_left(&ceph_wbc.fbatch);
> -
>  			if (folio_batch_count(&ceph_wbc.fbatch) == 0 &&
>  			    ceph_wbc.locked_pages < ceph_wbc.max_pages) {
>  				doutc(cl, "reached end fbatch, trying for more\n");

Let us try to reproduce the issue and to test the patch.

Thanks,
Slava.

Patchew 2.1-devel-b67e4c2

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