Re: Block device direct read EIO handling broken?

Jens Axboe
2019-08-05T18:31:39Z

On 8/5/19 11:15 AM, Darrick J. Wong wrote:
> Hi Damien,
>
> I noticed a regression in xfs/747 (an unreleased xfstest for the
> xfs_scrub media scanning feature) on 5.3-rc3. I'll condense that down
> to a simpler reproducer:
>
> # dmsetup table
> error-test: 0 209 linear 8:48 0
> error-test: 209 1 error
> error-test: 210 6446894 linear 8:48 210
>
> Basically we have a ~3G /dev/sdd and we set up device mapper to fail IO
> for sector 209 and to pass the io to the scsi device everywhere else.
>
> On 5.3-rc3, performing a directio pread of this range with a < 1M buffer
> (in other words, a request for fewer than MAX_BIO_PAGES bytes) yields
> EIO like you'd expect:
>
> # strace -e pread64 xfs_io -d -c 'pread -b 1024k 0k 1120k' /dev/mapper/error-test
> pread64(3, 0x7f880e1c7000, 1048576, 0) = -1 EIO (Input/output error)
> pread: Input/output error
> +++ exited with 0 +++
>
> But doing it with a larger buffer succeeds(!):
>
> # strace -e pread64 xfs_io -d -c 'pread -b 2048k 0k 1120k' /dev/mapper/error-test
> pread64(3, "XFSB\0\0\20\0\0\0\0\0\0\fL\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 1146880, 0) = 1146880
> read 1146880/1146880 bytes at offset 0
> 1 MiB, 1 ops; 0.0009 sec (1.124 GiB/sec and 1052.6316 ops/sec)
> +++ exited with 0 +++
>
> (Note that the part of the buffer corresponding to the dm-error area is
> uninitialized)
>
> On 5.3-rc2, both commands would fail with EIO like you'd expect. The
> only change between rc2 and rc3 is commit 0eb6ddfb865c ("block: Fix
> __blkdev_direct_IO() for bio fragments").
>
> AFAICT we end up in __blkdev_direct_IO with a 1120K buffer, which gets
> split into two bios: one for the first BIO_MAX_PAGES worth of data (1MB)
> and a second one for the 96k after that.
>
> I think the problem is that every time we submit a bio, we increase ret
> by the size of that bio, but at the time we do that we have no idea if
> the bio is going to succeed or not. At the end of the function we do:
>
> if (!ret)
> ret = blk_status_to_errno(dio->bio.bi_status);
>
> Which means that we only pick up the IO error if we haven't already set
> ret. I suppose that was useful for being able to return a short read,
> but now that we always increment ret by the size of the bio, we act like
> the whole buffer was read. I tried a -rc2 kernel and found that 40% of
> the time I'd get an EIO and the rest of the time I got a short read.
>
> Not sure where to go from here, but something's not right...

I'll take a look.

--
Jens Axboe