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([2605:e000:100e:83a1:3cf5:36ed:899e:8d54]) by smtp.gmail.com with ESMTPSA id u128sm97527386pfu.48.2019.08.05.13.31.46 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 05 Aug 2019 13:31:47 -0700 (PDT) Subject: Re: Block device direct read EIO handling broken? From: Jens Axboe To: "Darrick J. Wong" , Damien Le Moal Cc: "linux-fsdevel@vger.kernel.org" , linux-block@vger.kernel.org, xfs References: <20190805181524.GE7129@magnolia> <66bd785d-7598-5cc2-5e98-447fd128c153@kernel.dk> Message-ID: <36973a52-e876-fc09-7a63-2fc16b855f8d@kernel.dk> Date: Mon, 5 Aug 2019 13:31:45 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <66bd785d-7598-5cc2-5e98-447fd128c153@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org 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 Axboe