[PATCH v2 0/3] introduce uncharged file mapped folios

[PATCH v2 0/3] introduce uncharged file mapped folios

[Boris Burkov]

I would like to revisit Qu's proposal to not charge btrfs extent_buffer
allocations to the user's cgroup.

https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/

I believe it is detrimental to randomly account these global pages to
the cgroup using them, basically at random. A bit more justification
and explanation in the patches themselves.

---
Changelog:
v2:
- switch from filemap_add_folio_nocharge() to AS_UNCHARGED on the
  address_space.
- fix an interrupt safety bug in the vmstat patch.
- fix some foolish build errors for CONFIG_MEMCG=n

Boris Burkov (3):
  mm/filemap: add AS_UNCHARGED
  mm: add vmstat for cgroup uncharged pages
  btrfs: set AS_UNCHARGED on the btree_inode

 fs/btrfs/disk-io.c      |  1 +
 include/linux/mmzone.h  |  3 +++
 include/linux/pagemap.h |  1 +
 mm/filemap.c            | 29 +++++++++++++++++++++++++----
 mm/vmstat.c             |  3 +++
 5 files changed, 33 insertions(+), 4 deletions(-)

-- 
2.50.1

[PATCH v2 1/3] mm/filemap: add AS_UNCHARGED

[Boris Burkov]

Btrfs currently tracks its metadata pages in the page cache, using a
fake inode (fs_info->btree_inode) with offsets corresponding to where
the metadata is stored in the filesystem's full logical address space.

A consequence of this is that when btrfs uses filemap_add_folio(), this
usage is charged to the cgroup of whichever task happens to be running
at the time. These folios don't belong to any particular user cgroup, so
I don't think it makes much sense for them to be charged in that way.
Some negative consequences as a result:
- A task can be holding some important btrfs locks, then need to lookup
  some metadata and go into reclaim, extending the duration it holds
  that lock for, and unfairly pushing its own reclaim pain onto other
  cgroups.
- If that cgroup goes into reclaim, it might reclaim these folios a
  different non-reclaiming cgroup might need soon. This is naturally
  offset by LRU reclaim, but still.

A very similar proposal to use the root cgroup was previously made by
Qu, where he eventually proposed the idea of setting it per
address_space. This makes good sense for the btrfs use case, as the
uncharged behavior should apply to all use of the address_space, not
select allocations. I.e., if someone adds another filemap_add_folio()
call using btrfs's btree_inode, we would almost certainly want the
uncharged behavior.

Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/
Suggested-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Boris Burkov <boris@bur.io>
---
 include/linux/pagemap.h |  1 +
 mm/filemap.c            | 12 ++++++++----
 2 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
index 12a12dae727d..1ca63761a13a 100644
--- a/include/linux/pagemap.h
+++ b/include/linux/pagemap.h
@@ -211,6 +211,7 @@ enum mapping_flags {
 				   folio contents */
 	AS_INACCESSIBLE = 8,	/* Do not attempt direct R/W access to the mapping */
 	AS_WRITEBACK_MAY_DEADLOCK_ON_RECLAIM = 9,
+	AS_UNCHARGED = 10,	/* Do not charge usage to a cgroup */
 	/* Bits 16-25 are used for FOLIO_ORDER */
 	AS_FOLIO_ORDER_BITS = 5,
 	AS_FOLIO_ORDER_MIN = 16,
diff --git a/mm/filemap.c b/mm/filemap.c
index 751838ef05e5..6046e7f27709 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -960,15 +960,19 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
 {
 	void *shadow = NULL;
 	int ret;
+	bool charge_mem_cgroup = !test_bit(AS_UNCHARGED, &mapping->flags);
 
-	ret = mem_cgroup_charge(folio, NULL, gfp);
-	if (ret)
-		return ret;
+	if (charge_mem_cgroup) {
+		ret = mem_cgroup_charge(folio, NULL, gfp);
+		if (ret)
+			return ret;
+	}
 
 	__folio_set_locked(folio);
 	ret = __filemap_add_folio(mapping, folio, index, gfp, &shadow);
 	if (unlikely(ret)) {
-		mem_cgroup_uncharge(folio);
+		if (charge_mem_cgroup)
+			mem_cgroup_uncharge(folio);
 		__folio_clear_locked(folio);
 	} else {
 		/*
-- 
2.50.1

Re: [PATCH v2 1/3] mm/filemap: add AS_UNCHARGED

[Shakeel Butt]

On Fri, Aug 15, 2025 at 04:40:31PM -0700, Boris Burkov wrote:
> Btrfs currently tracks its metadata pages in the page cache, using a
> fake inode (fs_info->btree_inode) with offsets corresponding to where
> the metadata is stored in the filesystem's full logical address space.
> 
> A consequence of this is that when btrfs uses filemap_add_folio(), this
> usage is charged to the cgroup of whichever task happens to be running
> at the time. These folios don't belong to any particular user cgroup, so
> I don't think it makes much sense for them to be charged in that way.
> Some negative consequences as a result:
> - A task can be holding some important btrfs locks, then need to lookup
>   some metadata and go into reclaim, extending the duration it holds
>   that lock for, and unfairly pushing its own reclaim pain onto other
>   cgroups.
> - If that cgroup goes into reclaim, it might reclaim these folios a
>   different non-reclaiming cgroup might need soon. This is naturally
>   offset by LRU reclaim, but still.
> 
> A very similar proposal to use the root cgroup was previously made by
> Qu, where he eventually proposed the idea of setting it per
> address_space. This makes good sense for the btrfs use case, as the
> uncharged behavior should apply to all use of the address_space, not
> select allocations. I.e., if someone adds another filemap_add_folio()
> call using btrfs's btree_inode, we would almost certainly want the
> uncharged behavior.
> 
> Link: https://lore.kernel.org/linux-mm/b5fef5372ae454a7b6da4f2f75c427aeab6a07d6.1727498749.git.wqu@suse.com/
> Suggested-by: Qu Wenruo <wqu@suse.com>
> Signed-off-by: Boris Burkov <boris@bur.io>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

[PATCH v2 2/3] mm: add vmstat for cgroup uncharged pages

[Boris Burkov]

Uncharged pages are tricky to track by their essential "uncharged"
nature. To maintain good accounting, introduce a vmstat counter tracking
all uncharged pages. Since this is only meaningful when cgroups are
configured, only expose the counter when CONFIG_MEMCG is set.

Confirmed that these work as expected at a high level by mounting a
btrfs using AS_UNCHARGED for metadata pages, and seeing the counter rise
with fs usage then go back to a minimal level after drop_caches and
finally down to 0 after unmounting the fs.

Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
Signed-off-by: Boris Burkov <boris@bur.io>
---
 include/linux/mmzone.h |  3 +++
 mm/filemap.c           | 17 +++++++++++++++++
 mm/vmstat.c            |  3 +++
 3 files changed, 23 insertions(+)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 0c5da9141983..f6d885c97e99 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -245,6 +245,9 @@ enum node_stat_item {
 	NR_HUGETLB,
 #endif
 	NR_BALLOON_PAGES,
+#ifdef CONFIG_MEMCG
+	NR_UNCHARGED_FILE_PAGES,
+#endif
 	NR_VM_NODE_STAT_ITEMS
 };
 
diff --git a/mm/filemap.c b/mm/filemap.c
index 6046e7f27709..cd5af44a838c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -146,6 +146,19 @@ static void page_cache_delete(struct address_space *mapping,
 	mapping->nrpages -= nr;
 }
 
+#ifdef CONFIG_MEMCG
+static void filemap_mod_uncharged_vmstat(struct folio *folio, int sign)
+{
+	long nr = folio_nr_pages(folio) * sign;
+
+	lruvec_stat_mod_folio(folio, NR_UNCHARGED_FILE_PAGES, nr);
+}
+#else
+static void filemap_mod_uncharged_vmstat(struct folio *folio, int sign)
+{
+}
+#endif
+
 static void filemap_unaccount_folio(struct address_space *mapping,
 		struct folio *folio)
 {
@@ -190,6 +203,8 @@ static void filemap_unaccount_folio(struct address_space *mapping,
 		__lruvec_stat_mod_folio(folio, NR_FILE_THPS, -nr);
 		filemap_nr_thps_dec(mapping);
 	}
+	if (test_bit(AS_UNCHARGED, &folio->mapping->flags))
+		filemap_mod_uncharged_vmstat(folio, -1);
 
 	/*
 	 * At this point folio must be either written or cleaned by
@@ -987,6 +1002,8 @@ int filemap_add_folio(struct address_space *mapping, struct folio *folio,
 		if (!(gfp & __GFP_WRITE) && shadow)
 			workingset_refault(folio, shadow);
 		folio_add_lru(folio);
+		if (!charge_mem_cgroup)
+			filemap_mod_uncharged_vmstat(folio, 1);
 	}
 	return ret;
 }
diff --git a/mm/vmstat.c b/mm/vmstat.c
index 71cd1ceba191..c8c8b5831f2d 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1289,6 +1289,9 @@ const char * const vmstat_text[] = {
 	[I(NR_HUGETLB)]				= "nr_hugetlb",
 #endif
 	[I(NR_BALLOON_PAGES)]			= "nr_balloon_pages",
+#ifdef CONFIG_MEMCG
+	[I(NR_UNCHARGED_FILE_PAGES)]		= "nr_uncharged_file_pages",
+#endif
 #undef I
 
 	/* system-wide enum vm_stat_item counters */
-- 
2.50.1

Re: [PATCH v2 2/3] mm: add vmstat for cgroup uncharged pages

[Shakeel Butt]

On Fri, Aug 15, 2025 at 04:40:32PM -0700, Boris Burkov wrote:
> Uncharged pages are tricky to track by their essential "uncharged"
> nature. To maintain good accounting, introduce a vmstat counter tracking
> all uncharged pages. Since this is only meaningful when cgroups are
> configured, only expose the counter when CONFIG_MEMCG is set.
> 
> Confirmed that these work as expected at a high level by mounting a
> btrfs using AS_UNCHARGED for metadata pages, and seeing the counter rise
> with fs usage then go back to a minimal level after drop_caches and
> finally down to 0 after unmounting the fs.
> 
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Signed-off-by: Boris Burkov <boris@bur.io>
> ---
>  include/linux/mmzone.h |  3 +++
>  mm/filemap.c           | 17 +++++++++++++++++
>  mm/vmstat.c            |  3 +++
>  3 files changed, 23 insertions(+)
> 
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index 0c5da9141983..f6d885c97e99 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -245,6 +245,9 @@ enum node_stat_item {
>  	NR_HUGETLB,
>  #endif
>  	NR_BALLOON_PAGES,
> +#ifdef CONFIG_MEMCG
> +	NR_UNCHARGED_FILE_PAGES,
> +#endif
>  	NR_VM_NODE_STAT_ITEMS
>  };
>  
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 6046e7f27709..cd5af44a838c 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -146,6 +146,19 @@ static void page_cache_delete(struct address_space *mapping,
>  	mapping->nrpages -= nr;
>  }
>  
> +#ifdef CONFIG_MEMCG
> +static void filemap_mod_uncharged_vmstat(struct folio *folio, int sign)
> +{
> +	long nr = folio_nr_pages(folio) * sign;
> +
> +	lruvec_stat_mod_folio(folio, NR_UNCHARGED_FILE_PAGES, nr);

Since we expect to add this metric to memory.stat, I think we should use
mod_node_page_state() instead here.

With that you can add:

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

Re: [PATCH v2 2/3] mm: add vmstat for cgroup uncharged pages

[Shakeel Butt]

On Fri, Aug 15, 2025 at 05:48:33PM -0700, Shakeel Butt wrote:
> On Fri, Aug 15, 2025 at 04:40:32PM -0700, Boris Burkov wrote:
> > Uncharged pages are tricky to track by their essential "uncharged"
> > nature. To maintain good accounting, introduce a vmstat counter tracking
> > all uncharged pages. Since this is only meaningful when cgroups are
> > configured, only expose the counter when CONFIG_MEMCG is set.
> > 
> > Confirmed that these work as expected at a high level by mounting a
> > btrfs using AS_UNCHARGED for metadata pages, and seeing the counter rise
> > with fs usage then go back to a minimal level after drop_caches and
> > finally down to 0 after unmounting the fs.
> > 
> > Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> > Signed-off-by: Boris Burkov <boris@bur.io>
> > ---
> >  include/linux/mmzone.h |  3 +++
> >  mm/filemap.c           | 17 +++++++++++++++++
> >  mm/vmstat.c            |  3 +++
> >  3 files changed, 23 insertions(+)
> > 
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 0c5da9141983..f6d885c97e99 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -245,6 +245,9 @@ enum node_stat_item {
> >  	NR_HUGETLB,
> >  #endif
> >  	NR_BALLOON_PAGES,
> > +#ifdef CONFIG_MEMCG
> > +	NR_UNCHARGED_FILE_PAGES,
> > +#endif
> >  	NR_VM_NODE_STAT_ITEMS
> >  };
> >  
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 6046e7f27709..cd5af44a838c 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -146,6 +146,19 @@ static void page_cache_delete(struct address_space *mapping,
> >  	mapping->nrpages -= nr;
> >  }
> >  
> > +#ifdef CONFIG_MEMCG
> > +static void filemap_mod_uncharged_vmstat(struct folio *folio, int sign)
> > +{
> > +	long nr = folio_nr_pages(folio) * sign;
> > +
> > +	lruvec_stat_mod_folio(folio, NR_UNCHARGED_FILE_PAGES, nr);
> 
> Since we 

*never*

> expect to add this metric to memory.stat, I think we should use
> mod_node_page_state() instead here.
> 
> With that you can add:
> 
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> 

[PATCH v2 3/3] btrfs: set AS_UNCHARGED on the btree_inode

[Boris Burkov]

extent_buffers are global and shared so their pages should not belong to
any particular cgroup (currently whichever cgroups happens to allocate
the extent_buffer).

Btrfs tree operations should not arbitrarily block on cgroup reclaim or
have the shared extent_buffer pages on a cgroup's reclaim lists.

Signed-off-by: Boris Burkov <boris@bur.io>
---
 fs/btrfs/disk-io.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 123c397ca8f8..6a6ed8c1389c 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -1930,6 +1930,7 @@ static int btrfs_init_btree_inode(struct super_block *sb)
 	BTRFS_I(inode)->root = btrfs_grab_root(fs_info->tree_root);
 	set_bit(BTRFS_INODE_DUMMY, &BTRFS_I(inode)->runtime_flags);
 	__insert_inode_hash(inode, hash);
+	set_bit(AS_UNCHARGED, &inode->i_mapping->flags);
 	fs_info->btree_inode = inode;
 
 	return 0;
-- 
2.50.1

Re: [PATCH v2 3/3] btrfs: set AS_UNCHARGED on the btree_inode

[Shakeel Butt]

On Fri, Aug 15, 2025 at 04:40:33PM -0700, Boris Burkov wrote:
> extent_buffers are global and shared so their pages should not belong to
> any particular cgroup (currently whichever cgroups happens to allocate
> the extent_buffer).
> 
> Btrfs tree operations should not arbitrarily block on cgroup reclaim or
> have the shared extent_buffer pages on a cgroup's reclaim lists.
> 
> Signed-off-by: Boris Burkov <boris@bur.io>

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>

I think mm-tree is better for this series.

Re: [PATCH v2 3/3] btrfs: set AS_UNCHARGED on the btree_inode

[David Sterba]

On Fri, Aug 15, 2025 at 05:50:28PM -0700, Shakeel Butt wrote:
> On Fri, Aug 15, 2025 at 04:40:33PM -0700, Boris Burkov wrote:
> > extent_buffers are global and shared so their pages should not belong to
> > any particular cgroup (currently whichever cgroups happens to allocate
> > the extent_buffer).
> > 
> > Btrfs tree operations should not arbitrarily block on cgroup reclaim or
> > have the shared extent_buffer pages on a cgroup's reclaim lists.
> > 
> > Signed-off-by: Boris Burkov <boris@bur.io>
> 
> Acked-by: Shakeel Butt <shakeel.butt@linux.dev>
> 
> I think mm-tree is better for this series.

Agreed, majority of the changes are in MM code and the btrfs side is not
something that would affect other testing.

Acked-by: David Sterba <dsterba@suse.com>

[syzbot ci] Re: introduce uncharged file mapped folios

[syzbot ci]

syzbot ci has tested the following series

[v2] introduce uncharged file mapped folios
https://lore.kernel.org/all/cover.1755300815.git.boris@bur.io
* [PATCH v2 1/3] mm/filemap: add AS_UNCHARGED
* [PATCH v2 2/3] mm: add vmstat for cgroup uncharged pages
* [PATCH v2 3/3] btrfs: set AS_UNCHARGED on the btree_inode

and found the following issue:
WARNING in folio_lruvec_lock_irqsave

Full report is available here:
https://ci.syzbot.org/series/15fd2538-1138-43c0-b4d6-6d7f53b0be69

***

WARNING in folio_lruvec_lock_irqsave

tree:      torvalds
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
base:      dfc0f6373094dd88e1eaf76c44f2ff01b65db851
arch:      amd64
compiler:  Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
config:    https://ci.syzbot.org/builds/5b3f7bc8-d0c2-4985-8ae6-d51ea4e2baed/config
C repro:   https://ci.syzbot.org/findings/f02552bd-e7e6-4c3f-8cc3-04a5a946771d/c_repro
syz repro: https://ci.syzbot.org/findings/f02552bd-e7e6-4c3f-8cc3-04a5a946771d/syz_repro

 do_new_mount+0x2a2/0x9e0 fs/namespace.c:3805
 do_mount fs/namespace.c:4133 [inline]
 __do_sys_mount fs/namespace.c:4344 [inline]
 __se_sys_mount+0x317/0x410 fs/namespace.c:4321
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
------------[ cut here ]------------
WARNING: CPU: 0 PID: 5951 at ./include/linux/memcontrol.h:734 folio_lruvec include/linux/memcontrol.h:734 [inline]
WARNING: CPU: 0 PID: 5951 at ./include/linux/memcontrol.h:734 folio_lruvec_lock_irqsave+0x184/0x1d0 mm/memcontrol.c:1252
Modules linked in:
CPU: 0 UID: 0 PID: 5951 Comm: syz-executor Not tainted 6.17.0-rc1-syzkaller-00036-gdfc0f6373094-dirty #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
RIP: 0010:folio_lruvec include/linux/memcontrol.h:734 [inline]
RIP: 0010:folio_lruvec_lock_irqsave+0x184/0x1d0 mm/memcontrol.c:1252
Code: 74 0c 4c 89 f7 e8 cc 53 f8 ff 48 8b 04 24 49 89 06 eb a0 48 89 df 48 c7 c6 60 4e 98 8b e8 a4 36 fd fe c6 05 52 a1 62 0d 01 90 <0f> 0b 90 e9 a5 fe ff ff 44 89 e1 80 e1 07 80 c1 03 38 c1 0f 8c 22
RSP: 0018:ffffc9000359f540 EFLAGS: 00010246
RAX: f0f9782cf8520600 RBX: ffffea0000807140 RCX: f0f9782cf8520600
RDX: 0000000000000002 RSI: ffffffff8dba6067 RDI: ffff888020a73980
RBP: ffffc9000359f5e0 R08: ffff88804b024253 R09: 1ffff1100960484a
R10: dffffc0000000000 R11: ffffed100960484b R12: ffff88804b032fe8
R13: ffff88801ba80918 R14: ffff888106950000 R15: 0000000000000000
FS:  0000555587fa3500(0000) GS:ffff8880b861c000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000c00003e720 CR3: 0000000029a9c000 CR4: 00000000000006f0
Call Trace:
 <TASK>
 folio_lruvec_relock_irqsave include/linux/memcontrol.h:1544 [inline]
 folio_batch_move_lru+0x20a/0x3a0 mm/swap.c:167
 lru_add_drain_cpu+0x119/0x880 mm/swap.c:647
 lru_add_drain+0x122/0x3e0 mm/swap.c:735
 __folio_batch_release+0x48/0x90 mm/swap.c:1054
 folio_batch_release include/linux/pagevec.h:101 [inline]
 invalidate_inode_pages2_range+0x889/0xa80 mm/truncate.c:707
 close_ctree+0x6ff/0x1380 fs/btrfs/disk-io.c:4408
 generic_shutdown_super+0x135/0x2c0 fs/super.c:643
 kill_anon_super+0x3b/0x70 fs/super.c:1282
 btrfs_kill_super+0x41/0x50 fs/btrfs/super.c:2114
 deactivate_locked_super+0xbc/0x130 fs/super.c:474
 cleanup_mnt+0x425/0x4c0 fs/namespace.c:1378
 task_work_run+0x1d4/0x260 kernel/task_work.c:227
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 exit_to_user_mode_loop+0xec/0x110 kernel/entry/common.c:43
 exit_to_user_mode_prepare include/linux/irq-entry-common.h:225 [inline]
 syscall_exit_to_user_mode_work include/linux/entry-common.h:175 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:210 [inline]
 do_syscall_64+0x2bd/0x3b0 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f4a78b8ff17
Code: a8 ff ff ff f7 d8 64 89 01 48 83 c8 ff c3 0f 1f 44 00 00 31 f6 e9 09 00 00 00 66 0f 1f 84 00 00 00 00 00 b8 a6 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 01 c3 48 c7 c2 a8 ff ff ff f7 d8 64 89 02 b8
RSP: 002b:00007ffe33fe80a8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
RAX: 0000000000000000 RBX: 00007f4a78c11c05 RCX: 00007f4a78b8ff17
RDX: 0000000000000000 RSI: 0000000000000009 RDI: 00007ffe33fe8160
RBP: 00007ffe33fe8160 R08: 0000000000000000 R09: 0000000000000000
R10: 00000000ffffffff R11: 0000000000000246 R12: 00007ffe33fe91f0
R13: 00007f4a78c11c05 R14: 000000000000f471 R15: 00007ffe33fe9230
 </TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
Tested-by: syzbot@syzkaller.appspotmail.com

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.

Re: [syzbot ci] Re: introduce uncharged file mapped folios

[Shakeel Butt]

On Sun, Aug 17, 2025 at 01:34:44AM -0700, syzbot ci wrote:
> syzbot ci has tested the following series
> 
> [v2] introduce uncharged file mapped folios
> https://lore.kernel.org/all/cover.1755300815.git.boris@bur.io
> * [PATCH v2 1/3] mm/filemap: add AS_UNCHARGED
> * [PATCH v2 2/3] mm: add vmstat for cgroup uncharged pages
> * [PATCH v2 3/3] btrfs: set AS_UNCHARGED on the btree_inode
> 
> and found the following issue:
> WARNING in folio_lruvec_lock_irqsave

Ok that is expected and we need to fix the warning instead. We can
either remove this warning or make is only applicable to non-file
folios.