Re: Block device direct read EIO handling broken?

Jens Axboe
2019-08-05T21:26:01Z

On 8/5/19 2:08 PM, Damien Le Moal wrote:
> On 2019/08/06 5:31, Jens Axboe wrote:
>> On 8/5/19 11:31 AM, Jens Axboe wrote:
>>> 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.
>>
>> How about this? The old code did:
>>
>> if (!ret)
>> ret = blk_status_to_errno(dio->bio.bi_status);
>> if (likely(!ret))
>> ret = dio->size;
>>
>> where 'ret' was just tracking the error. With 'ret' now being the
>> positive IO size, we should overwrite it if ret is >= 0, not just if
>> it's zero.
>>
>> Also kill a use-after-free.
>>
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index a6f7c892cb4a..67c8e87c9481 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -386,6 +386,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>>
>> ret = 0;
>> for (;;) {
>> + ssize_t this_size;
>> int err;
>>
>> bio_set_dev(bio, bdev);
>> @@ -433,13 +434,14 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>> polled = true;
>> }
>>
>> + this_size = bio->bi_iter.bi_size;
>> qc = submit_bio(bio);
>> if (qc == BLK_QC_T_EAGAIN) {
>> if (!ret)
>> ret = -EAGAIN;
>> goto error;
>> }
>> - ret = dio->size;
>> + ret += this_size;
>>
>> if (polled)
>> WRITE_ONCE(iocb->ki_cookie, qc);
>> @@ -460,13 +462,14 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>> atomic_inc(&dio->ref);
>> }
>>
>> + this_size = bio->bi_iter.bi_size;
>> qc = submit_bio(bio);
>> if (qc == BLK_QC_T_EAGAIN) {
>> if (!ret)
>> ret = -EAGAIN;
>> goto error;
>> }
>> - ret = dio->size;
>> + ret += this_size;
>>
>> bio = bio_alloc(gfp, nr_pages);
>> if (!bio) {
>> @@ -494,7 +497,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>> __set_current_state(TASK_RUNNING);
>>
>> out:
>> - if (!ret)
>> + if (ret >= 0)
>> ret = blk_status_to_errno(dio->bio.bi_status);
>>
>> bio_put(&dio->bio);
>>
>
> Jens,
>
> I would set "this_size" when dio->size is being incremented though, to avoid
> repeating it.
>
> if (nowait)
> bio->bi_opf |= (REQ_NOWAIT | REQ_NOWAIT_INLINE);
>
> + this_size = bio->bi_iter.bi_size;
> - dio->size += bio->bi_iter.bi_size;
> + dio->size += this_size;
> pos += bio->bi_iter.bi_size;
>
> In any case, looking again at this code, it looks like there is a
> problem with dio->size being incremented early, even for fragments
> that get BLK_QC_T_EAGAIN, because dio->size is being used in
> blkdev_bio_end_io(). So an incorrect size can be reported to user
> space in that case on completion (e.g. large asynchronous no-wait dio
> that cannot be issued in one go).
>
> So maybe something like this ? (completely untested)

I think that looks pretty good, I like not double accounting with
this_size and dio->size, and we retain the old style ordering for the
ret value.

--
Jens Axboe