Re: [PATCH v5 07/14] iomap: track pending read bytes more optimally

Joanne Koong
2025-10-24T00:01:28Z

On Thu, Oct 23, 2025 at 12:30 PM Brian Foster <bfoster@redhat.com> wrote:
>
> On Thu, Sep 25, 2025 at 05:26:02PM -0700, Joanne Koong wrote:
> > Instead of incrementing read_bytes_pending for every folio range read in
> > (which requires acquiring the spinlock to do so), set read_bytes_pending
> > to the folio size when the first range is asynchronously read in, keep
> > track of how many bytes total are asynchronously read in, and adjust
> > read_bytes_pending accordingly after issuing requests to read in all the
> > necessary ranges.
> >
> > iomap_read_folio_ctx->cur_folio_in_bio can be removed since a non-zero
> > value for pending bytes necessarily indicates the folio is in the bio.
> >
> > Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
> > Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
> > ---
>
> Hi Joanne,
>
> I was throwing some extra testing at the vfs-6.19.iomap branch since the
> little merge conflict thing with iomap_iter_advance(). I end up hitting
> what appears to be a lockup on XFS with 1k FSB (-bsize=1k) running
> generic/051. It reproduces fairly reliably within a few iterations or so
> and seems to always stall during a read for a dedupe operation:
>
> task:fsstress state:D stack:0 pid:12094 tgid:12094 ppid:12091 task_flags:0x400140 flags:0x00080003
> Call Trace:
> <TASK>
> __schedule+0x2fc/0x7a0
> schedule+0x27/0x80
> io_schedule+0x46/0x70
> folio_wait_bit_common+0x12b/0x310
> ? __pfx_wake_page_function+0x10/0x10
> ? __pfx_xfs_vm_read_folio+0x10/0x10 [xfs]
> filemap_read_folio+0x85/0xd0
> ? __pfx_xfs_vm_read_folio+0x10/0x10 [xfs]
> do_read_cache_folio+0x7c/0x1b0
> vfs_dedupe_file_range_compare.constprop.0+0xaf/0x2d0
> __generic_remap_file_range_prep+0x276/0x2a0
> generic_remap_file_range_prep+0x10/0x20
> xfs_reflink_remap_prep+0x22c/0x300 [xfs]
> xfs_file_remap_range+0x84/0x360 [xfs]
> vfs_dedupe_file_range_one+0x1b2/0x1d0
> ? remap_verify_area+0x46/0x140
> vfs_dedupe_file_range+0x162/0x220
> do_vfs_ioctl+0x4d1/0x940
> __x64_sys_ioctl+0x75/0xe0
> do_syscall_64+0x84/0x800
> ? do_syscall_64+0xbb/0x800
> ? avc_has_perm_noaudit+0x6b/0xf0
> ? _copy_to_user+0x31/0x40
> ? cp_new_stat+0x130/0x170
> ? __do_sys_newfstat+0x44/0x70
> ? do_syscall_64+0xbb/0x800
> ? do_syscall_64+0xbb/0x800
> ? clear_bhb_loop+0x30/0x80
> ? clear_bhb_loop+0x30/0x80
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> RIP: 0033:0x7fe6bbd9a14d
> RSP: 002b:00007ffde72cd4e0 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
> RAX: ffffffffffffffda RBX: 0000000000000068 RCX: 00007fe6bbd9a14d
> RDX: 000000000a1394b0 RSI: 00000000c0189436 RDI: 0000000000000004
> RBP: 00007ffde72cd530 R08: 0000000000001000 R09: 000000000a11a3fc
> R10: 000000000001d6c0 R11: 0000000000000246 R12: 000000000a12cfb0
> R13: 000000000a12ba10 R14: 000000000a14e610 R15: 0000000000019000
> </TASK>
>
> It wasn't immediately clear to me what the issue was so I bisected and
> it landed on this patch. It kind of looks like we're failing to unlock a
> folio at some point and then tripping over it later..? I can kill the
> fsstress process but then the umount ultimately gets stuck tossing
> pagecache [1], so the mount still ends up stuck indefinitely. Anyways,
> I'll poke at it some more but I figure you might be able to make sense
> of this faster than I can.
>
> Brian

Hi Brian,

Thanks for your report and the repro instructions. I will look into
this and report back what I find.

Thanks,
Joanne
>
> [1] umount stack trace:
>
> task:umount state:D stack:0 pid:12216 tgid:12216 ppid:2514 task_flags:0x400100 flags:0x00080001
> Call Trace:
> <TASK>
> __schedule+0x2fc/0x7a0
> schedule+0x27/0x80
> io_schedule+0x46/0x70
> folio_wait_bit_common+0x12b/0x310
> ? __pfx_wake_page_function+0x10/0x10
> truncate_inode_pages_range+0x42a/0x4d0
> xfs_fs_evict_inode+0x1f/0x30 [xfs]
> evict+0x112/0x290
> evict_inodes+0x209/0x230
> generic_shutdown_super+0x42/0x100
> kill_block_super+0x1a/0x40
> xfs_kill_sb+0x12/0x20 [xfs]
> deactivate_locked_super+0x33/0xb0
> cleanup_mnt+0xba/0x150
> task_work_run+0x5c/0x90
> exit_to_user_mode_loop+0x12f/0x170
> do_syscall_64+0x1af/0x800
> ? vfs_statx+0x80/0x160
> ? do_statx+0x62/0xa0
> ? __x64_sys_statx+0xaf/0x100
> ? do_syscall_64+0xbb/0x800
> ? __x64_sys_statx+0xaf/0x100
> ? do_syscall_64+0xbb/0x800
> ? count_memcg_events+0xdd/0x1b0
> ? handle_mm_fault+0x220/0x340
> ? do_user_addr_fault+0x2c3/0x7f0
> ? clear_bhb_loop+0x30/0x80
> ? clear_bhb_loop+0x30/0x80
> entry_SYSCALL_64_after_hwframe+0x76/0x7e
> RIP: 0033:0x7fdd641ed5ab
> RSP: 002b:00007ffd671182e8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6
> RAX: 0000000000000000 RBX: 0000559b3e2056b0 RCX: 00007fdd641ed5ab
> RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000559b3e205ac0
> RBP: 00007ffd671183c0 R08: 0000000000000000 R09: 0000000000000000
> R10: 0000000000000103 R11: 0000000000000246 R12: 0000559b3e2057b8
> R13: 0000000000000000 R14: 0000559b3e205ac0 R15: 0000000000000000
> </TASK>
>