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

Brian Foster
2025-10-27T12:12:15Z

On Fri, Oct 24, 2025 at 02:55:20PM -0700, Joanne Koong wrote:
> On Fri, Oct 24, 2025 at 12:48 PM Joanne Koong <joannelkoong@gmail.com> wrote:
> >
> > On Fri, Oct 24, 2025 at 10:10 AM Brian Foster <bfoster@redhat.com> wrote:
> > >
> > > On Fri, Oct 24, 2025 at 09:25:13AM -0700, Joanne Koong wrote:
> > > > On Thu, Oct 23, 2025 at 5:01 PM Joanne Koong <joannelkoong@gmail.com> wrote:
> > > > >
> > > > > 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.
> > > >
> > > > This is the fix:
> > > >
> > > > diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
> > > > index 4e6258fdb915..aa46fec8362d 100644
> > > > --- a/fs/iomap/buffered-io.c
> > > > +++ b/fs/iomap/buffered-io.c
> > > > @@ -445,6 +445,9 @@ static void iomap_read_end(struct folio *folio,
> > > > size_t bytes_pending)
> > > > bool end_read, uptodate;
> > > > size_t bytes_accounted = folio_size(folio) - bytes_pending;
> > > >
> > > > + if (!bytes_accounted)
> > > > + return;
> > > > +
> > > > spin_lock_irq(&ifs->state_lock);
> > > >
> > > >
> > > > What I missed was that if all the bytes in the folio are non-uptodate
> > > > and need to read in by the filesystem, then there's a bug where the
> > > > read will be ended on the folio twice (in iomap_read_end() and when
> > > > the filesystem calls iomap_finish_folio_write(), when only the
> > > > filesystem should end the read), which does 2 folio unlocks which ends
> > > > up locking the folio. Looking at the writeback patch that does a
> > > > similar optimization [1], I miss the same thing there.
> > > >
> > >
> > > Makes sense.. though a short comment wouldn't hurt in there. ;) I found
> > > myself a little confused by the accounted vs. pending naming when
> > > reading through that code. If I follow correctly, the intent is to refer
> > > to the additional bytes accounted to read_bytes_pending via the init
> > > (where it just accounts the whole folio up front) and pending refers to
> > > submitted I/O.
> > >
> > > Presumably that extra accounting doubly serves as the typical "don't
> > > complete the op before the submitter is done processing" extra
> > > reference, except in this full submit case of course. If so, that's
> > > subtle enough in my mind that a sentence or two on it wouldn't hurt..
> >
> > I will add some a comment about this :) That's a good point about the
> > naming, maybe "bytes_submitted" and "bytes_unsubmitted" is a lot less
> > confusing than "bytes_pending" and "bytes_accounted".
>
> Thinking about this some more, bytes_unsubmitted sounds even more
> confusing, so maybe bytes_nonsubmitted or bytes_not_submitted. I'll
> think about this some more but kept it as pending/accounted for now.
>

bytes_submitted sounds better than pending to me, not sure about
unsubmitted or whatever. As long as there's a sentence or two that
explains what accounted means in the end helper, though, that seems
reasonable enough to me.

Brian

> The fix for this bug is here [1].
>
> Thanks,
> Joanne
>
> [1] https://lore.kernel.org/linux-fsdevel/20251024215008.3844068-1-joannelkoong@gmail.com/
>
> >
> > Thanks,
> > Joanne
> >
> > >
> > > > I'll fix up both. Thanks for catching this and bisecting it down to
> > > > this patch. Sorry for the trouble.
> > > >
> > >
> > > No prob. Thanks for the fix!
> > >
> > > Brian
> > >
> > > > Thanks,
> > > > Joanne
> > > >
> > > > [1] https://lore.kernel.org/linux-fsdevel/20251009225611.3744728-4-joannelkoong@gmail.com/
> > > > >
> > > > > Thanks,
> > > > > Joanne
> > > > > >
> > > >
> > >
>