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Message-ID: <CAOQ4uxhBM-rKQsV-S78G0b_aTTYbeXdt-mCiy7GcC4WSdn-NnQ@mail.gmail.com>
Date: Sun, 4 Jan 2026 20:44:13 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Gao Xiang <hsiangkao@...ux.alibaba.com>
Cc: linux-erofs@...ts.ozlabs.org, LKML <linux-kernel@...r.kernel.org>,
Alexander Larsson <alexl@...hat.com>, Christian Brauner <brauner@...nel.org>,
Miklos Szeredi <mszeredi@...hat.com>, linux-fsdevel <linux-fsdevel@...r.kernel.org>,
overlayfs <linux-unionfs@...r.kernel.org>
Subject: Re: [PATCH] erofs: don't bother with s_stack_depth increasing for now
On Sun, Jan 4, 2026 at 11:42 AM Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:
>
>
>
> On 2026/1/4 18:01, Amir Goldstein wrote:
> > [+fsdevel][+overlayfs]
> >
> > On Sun, Jan 4, 2026 at 4:56 AM Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:
> >>
> >> Hi Amir,
> >>
> >> On 2026/1/1 23:52, Amir Goldstein wrote:
> >>> On Wed, Dec 31, 2025 at 9:42 PM Gao Xiang <hsiangkao@...ux.alibaba.com> wrote:
> >>>>
> >>>> Previously, commit d53cd891f0e4 ("erofs: limit the level of fs stacking
> >>>> for file-backed mounts") bumped `s_stack_depth` by one to avoid kernel
> >>>> stack overflow, but it breaks composefs mounts, which need erofs+ovl^2
> >>>> sometimes (and such setups are already used in production for quite long
> >>>> time) since `s_stack_depth` can be 3 (i.e., FILESYSTEM_MAX_STACK_DEPTH
> >>>> needs to change from 2 to 3).
> >>>>
> >>>> After a long discussion on GitHub issues [1] about possible solutions,
> >>>> it seems there is no need to support nesting file-backed mounts as one
> >>>> conclusion (especially when increasing FILESYSTEM_MAX_STACK_DEPTH to 3).
> >>>> So let's disallow this right now, since there is always a way to use
> >>>> loopback devices as a fallback.
> >>>>
> >>>> Then, I started to wonder about an alternative EROFS quick fix to
> >>>> address the composefs mounts directly for this cycle: since EROFS is the
> >>>> only fs to support file-backed mounts and other stacked fses will just
> >>>> bump up `FILESYSTEM_MAX_STACK_DEPTH`, just check that `s_stack_depth`
> >>>> != 0 and the backing inode is not from EROFS instead.
> >>>>
> >>>> At least it works for all known file-backed mount use cases (composefs,
> >>>> containerd, and Android APEX for some Android vendors), and the fix is
> >>>> self-contained.
> >>>>
> >>>> Let's defer increasing FILESYSTEM_MAX_STACK_DEPTH for now.
> >>>>
> >>>> Fixes: d53cd891f0e4 ("erofs: limit the level of fs stacking for file-backed mounts")
> >>>> Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1]
> >>>> Closes: https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0QXwCpec9sXtg@mail.gmail.com
> >>>> Cc: Amir Goldstein <amir73il@...il.com>
> >>>> Cc: Alexander Larsson <alexl@...hat.com>
> >>>> Cc: Christian Brauner <brauner@...nel.org>
> >>>> Cc: Miklos Szeredi <mszeredi@...hat.com>
> >>>> Signed-off-by: Gao Xiang <hsiangkao@...ux.alibaba.com>
> >>>> ---
> >>>
> >>> Acked-by: Amir Goldstein <amir73il@...il.com>
> >>>
> >>> But you forgot to include details of the stack usage analysis you ran
> >>> with erofs+ovl^2 setup.
> >>>
> >>> I am guessing people will want to see this information before relaxing
> >>> s_stack_depth in this case.
> >>
> >> Sorry I didn't check emails these days, I'm not sure if posting
> >> detailed stack traces are useful, how about adding the following
> >> words:
> >
> > Didn't mean detailed stack traces, but you did some tests with the
> > new possible setup and you reached stack usage < 8K so I think this is
>
> The issue is that my limited stress test setup cannot cover
> every cases:
>
> - I cannot find a way to make direct reclaim reliably in the
> deep memory allocation, is there some suggestion on this?
>
> - I'm not sure what's the perfered way to evaluate the worst
> stack usage below the block layer, but we should care more
> about increasing delta just out of one more overlayfs I
> guess?
>
> I can only say what I've seen is the peak stack usage of my
> fsstress for an erofs+ovl^2 setup on x86_64 is < 8K (7184 bytes,
> but I don't think the peak value absolutely useful), which
> evaluates RW workloads in the upperdir, and for such workloads,
> the stack depth won't be impacted by FILESYSTEM_MAX_STACK_DEPTH,
> I don't see such workload is harmful.
>
> And then I manually copyup some files (because I didn't find any
> available tool to stress overlayfs copyups) and I could see the
> delta is (I think "ovl_copy_up_" is the only one path to do
> copyups):
>
> 0) 6688 48 mempool_alloc_slab+0x9/0x20
> 1) 6640 56 mempool_alloc_noprof+0x65/0xd0
> 2) 6584 72 __sg_alloc_table+0x128/0x190
> 3) 6512 40 sg_alloc_table_chained+0x46/0xa0
> 4) 6472 64 scsi_alloc_sgtables+0x91/0x2c0
> 5) 6408 72 sd_init_command+0x263/0x930
> 6) 6336 88 scsi_queue_rq+0x54a/0xb70
> 7) 6248 144 blk_mq_dispatch_rq_list+0x265/0x6c0
> 8) 6104 144 __blk_mq_sched_dispatch_requests+0x399/0x5c0
> 9) 5960 16 blk_mq_sched_dispatch_requests+0x2d/0x70
> 10) 5944 56 blk_mq_run_hw_queue+0x208/0x290
> 11) 5888 96 blk_mq_dispatch_list+0x13f/0x460
> 12) 5792 48 blk_mq_flush_plug_list+0x4b/0x180
> 13) 5744 32 blk_add_rq_to_plug+0x3d/0x160
> 14) 5712 136 blk_mq_submit_bio+0x4f4/0x760
> 15) 5576 120 __submit_bio+0x9b/0x240
> 16) 5456 88 submit_bio_noacct_nocheck+0x271/0x330
> 17) 5368 72 iomap_bio_read_folio_range+0xde/0x1d0
> 18) 5296 112 iomap_read_folio_iter+0x1ee/0x2d0
> 19) 5184 264 iomap_readahead+0xb9/0x290
> 20) 4920 48 xfs_vm_readahead+0x4a/0x70
> 21) 4872 112 read_pages+0x6c/0x1b0
> 22) 4760 104 page_cache_ra_unbounded+0x12c/0x210
> 23) 4656 80 filemap_readahead.isra.0+0x78/0xb0
> 24) 4576 192 filemap_get_pages+0x3a6/0x820
> 25) 4384 376 filemap_read+0xde/0x380
> 26) 4008 32 xfs_file_buffered_read+0xa6/0xd0
> 27) 3976 16 xfs_file_read_iter+0x6a/0xd0
> 28) 3960 48 vfs_iocb_iter_read+0xdb/0x140
> 29) 3912 88 erofs_fileio_rq_submit+0x136/0x190
> 30) 3824 368 z_erofs_runqueue+0x1ce/0x9f0
> 31) 3456 232 z_erofs_readahead+0x16c/0x220
> 32) 3224 112 read_pages+0x6c/0x1b0
> 33) 3112 104 page_cache_ra_unbounded+0x12c/0x210
> 34) 3008 80 filemap_readahead.isra.0+0x78/0xb0
> 35) 2928 192 filemap_get_pages+0x3a6/0x820
> 36) 2736 400 filemap_splice_read+0x12c/0x2f0
> 37) 2336 48 backing_file_splice_read+0x3f/0x90
> 38) 2288 128 ovl_splice_read+0xef/0x170
> 39) 2160 104 splice_direct_to_actor+0xb9/0x260
> 40) 2056 88 do_splice_direct+0x76/0xc0
> 41) 1968 120 ovl_copy_up_file+0x1a8/0x2b0
> 42) 1848 840 ovl_copy_up_one+0x14b0/0x1610
> 43) 1008 72 ovl_copy_up_flags+0xd7/0x110
> 44) 936 56 ovl_open+0x72/0x110
> 45) 880 56 do_dentry_open+0x16c/0x480
> 46) 824 40 vfs_open+0x2e/0xf0
> 47) 784 152 path_openat+0x80a/0x12e0
> 48) 632 296 do_filp_open+0xb8/0x160
> 49) 336 80 do_sys_openat2+0x72/0xf0
> 50) 256 40 __x64_sys_openat+0x57/0xa0
> 51) 216 40 do_syscall_64+0xa4/0x310
> 52) 176 176 entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> And it's still far from the stack overflow of 16k stacks,
> because the difference seems only how many (
> ovl_splice_read + backing_file_splice_read), and there only takes
> hundreds of bytes for each layer.
>
> Finally I used my own rostress to stress RO workloads, and the
> deepest stack so far is as below (5456 bytes):
>
> 0) 5456 48 arch_scale_cpu_capacity+0x9/0x30
> 1) 5408 16 cpu_util.constprop.0+0x7e/0xe0
> 2) 5392 392 sched_balance_find_src_group+0x29f/0xd30
> 3) 5000 280 sched_balance_rq+0x1b2/0xf10
> 4) 4720 120 pick_next_task_fair+0x23b/0x7b0
> 5) 4600 104 __schedule+0x2bc/0xda0
> 6) 4496 16 schedule+0x27/0xd0
> 7) 4480 24 io_schedule+0x46/0x70
> 8) 4456 120 blk_mq_get_tag+0x11b/0x280
> 9) 4336 96 __blk_mq_alloc_requests+0x2a1/0x410
> 10) 4240 136 blk_mq_submit_bio+0x59c/0x760
> 11) 4104 120 __submit_bio+0x9b/0x240
> 12) 3984 88 submit_bio_noacct_nocheck+0x271/0x330
> 13) 3896 72 iomap_bio_read_folio_range+0xde/0x1d0
> 14) 3824 112 iomap_read_folio_iter+0x1ee/0x2d0
> 15) 3712 264 iomap_readahead+0xb9/0x290
> 16) 3448 48 xfs_vm_readahead+0x4a/0x70
> 17) 3400 112 read_pages+0x6c/0x1b0
> 18) 3288 104 page_cache_ra_unbounded+0x12c/0x210
> 19) 3184 80 filemap_readahead.isra.0+0x78/0xb0
> 20) 3104 192 filemap_get_pages+0x3a6/0x820
> 21) 2912 376 filemap_read+0xde/0x380
> 22) 2536 32 xfs_file_buffered_read+0xa6/0xd0
> 23) 2504 16 xfs_file_read_iter+0x6a/0xd0
> 24) 2488 48 vfs_iocb_iter_read+0xdb/0x140
> 25) 2440 88 erofs_fileio_rq_submit+0x136/0x190
> 26) 2352 368 z_erofs_runqueue+0x1ce/0x9f0
> 27) 1984 232 z_erofs_readahead+0x16c/0x220
> 28) 1752 112 read_pages+0x6c/0x1b0
> 29) 1640 104 page_cache_ra_unbounded+0x12c/0x210
> 30) 1536 40 force_page_cache_ra+0x96/0xc0
> 31) 1496 192 filemap_get_pages+0x123/0x820
> 32) 1304 376 filemap_read+0xde/0x380
> 33) 928 72 do_iter_readv_writev+0x1b9/0x220
> 34) 856 56 vfs_iter_read+0xde/0x140
> 35) 800 64 backing_file_read_iter+0x193/0x1e0
> 36) 736 56 ovl_read_iter+0x98/0xa0
> 37) 680 72 do_iter_readv_writev+0x1b9/0x220
> 38) 608 56 vfs_iter_read+0xde/0x140
> 39) 552 64 backing_file_read_iter+0x193/0x1e0
> 40) 488 56 ovl_read_iter+0x98/0xa0
> 41) 432 152 vfs_read+0x21a/0x350
> 42) 280 64 __x64_sys_pread64+0x92/0xc0
> 43) 216 40 do_syscall_64+0xa4/0x310
> 44) 176 176 entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> > something worth mentioning.
> >
> >>
> >> Note: There are some observations while evaluating the erofs + ovl^2
> >> setup with an XFS backing fs:
> >>
> >> - Regular RW workloads traverse only one overlayfs layer regardless of
> >> the value of FILESYSTEM_MAX_STACK_DEPTH, because `upperdir=` cannot
> >> point to another overlayfs. Therefore, for pure RW workloads, the
> >> typical stack is always just:
> >> overlayfs + upper fs + underlay storage
> >>
> >> - For read-only workloads and the copy-up read part (ovl_splice_read),
> >> the difference can lie in how many overlays are nested.
> >> The stack just looks like either:
> >> ovl + ovl [+ erofs] + backing fs + underlay storage
> >> or
> >> ovl [+ erofs] + ext4/xfs + underlay storage
> >>
> >> - The fs reclaim path should be entered only once, so the writeback
> >> path will not re-enter.
> >>
> >> Sorry about my English, and I'm not sure if it's enough (e.g. FUSE
> >> passthrough part). I will look for your further inputs (and other
> >> acks) before sending this patch upstream.
> >>
> >
> > I think that most people will have problems understanding this
> > rationale not because of the English, but because of the tech ;)
> > this is a bit too hand wavy IMO.
>
> Honestly, I don't have better way to describe it, I think we'd
> better just to focus more on the increment of one more overlayfs:
>
ok. but are we talking about one more overlayfs?
This patch is adding just one erofs, so what am I missing?
> FILESYSTEM_MAX_STACK_DEPTH 2 already works for 8k kstacks on
> 32-bit arches, so I don't think FILESYSTEM_MAX_STACK_DEPTH from
> 2 to 3, which causes hundreds-more-byte additional stack usage
> out of mediate overlayfs on 16k kstacks on 64-bit arches is
> harmful (and only RO workloads and copyups are impacted).
>
> And if hundreds-more-byte additional stack usage can overflow
> the 16k kstack, I do think then the kernel stack can be
> overflowed randomly everywhere in the storage stack, not just
> because this FILESYSTEM_MAX_STACK_DEPTH modification.
>
Fine by me, but does that mean that you only want to allow
erofs backing files with >8K stack size?
Otherwise, I do not follow your argument.
Thanks,
Amir.
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