linux-ext4 - Re: Problem with direct IO

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Date:   Mon, 25 Oct 2021 17:57:13 +0200
From:   Jan Kara <jack@...e.cz>
To:     Zhengyuan Liu <liuzhengyuang521@...il.com>
Cc:     Jan Kara <jack@...e.cz>, viro@...iv.linux.org.uk,
        Andrew Morton <akpm@...ux-foundation.org>, tytso@....edu,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-ext4@...r.kernel.org,
        刘云 <liuyun01@...inos.cn>,
        Zhengyuan Liu <liuzhengyuan@...inos.cn>
Subject: Re: Problem with direct IO

On Sat 23-10-21 10:06:24, Zhengyuan Liu wrote:
> On Fri, Oct 22, 2021 at 5:31 PM Jan Kara <jack@...e.cz> wrote:
> > > > > > Can you post output of "dumpe2fs -h <device>" for the filesystem where the
> > > > > > problem happens? Thanks!
> > > > >
> > > > > Sure, the output is:
> > > > >
> > > > > # dumpe2fs -h /dev/sda3
> > > > > dumpe2fs 1.45.3 (14-Jul-2019)
> > > > > Filesystem volume name:   <none>
> > > > > Last mounted on:          /data
> > > > > Filesystem UUID:          09a51146-b325-48bb-be63-c9df539a90a1
> > > > > Filesystem magic number:  0xEF53
> > > > > Filesystem revision #:    1 (dynamic)
> > > > > Filesystem features:      has_journal ext_attr resize_inode dir_index
> > > > > filetype needs_recovery sparse_super large_file
> > > >
> > > > Thanks for the data. OK, a filesystem without extents. Does your test by
> > > > any chance try to do direct IO to a hole in a file? Because that is not
> > > > (and never was) supported without extents. Also the fact that you don't see
> > > > the problem with ext4 (which means extents support) would be pointing in
> > > > that direction.
> > >
> > > I am not sure if it trys to do direct IO to a hole or not, is there any
> > > way to check?  If you have a simple test to reproduce please let me know,
> > > we are glad to try.
> >
> > Can you enable following tracing?
> 
> Sure, but let's confirm before doing that, it seems Ext4 doesn't
> support iomap in
> V4.19 which could also reproduce the problem, so if it is necessary to
> do the following
> tracing? or should we modify the tracing if under V4.19?

Well, iomap is just a different generic framework for doing direct IO. The
fact that you can observe the problem both with iomap and the old direct IO
framework is one of the reasons why I think the problem is actually that
the file has holes (unallocated space in the middle).
 
> > echo 1 >/sys/kernel/debug/tracing/events/ext4/ext4_ind_map_blocks_exit/enable
> > echo iomap_dio_rw >/sys/kernel/debug/tracing/set_ftrace_filter
> > echo "function_graph" >/sys/kernel/debug/tracing/current_tracer

If you want to trace a kernel were ext4 direct IO path is not yet
converted to iomap framework you need to replace tracing of iomap_dio_rw
with:

echo __blockdev_direct_IO >/sys/kernel/debug/tracing/set_ftrace_filter

> > And then gather output from /sys/kernel/debug/tracing/trace_pipe. Once the
> > problem reproduces, you can gather the problematic file name from dmesg, find
> > inode number from "stat <filename>" and provide that all to me? Thanks!

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR