Re: [PATCH 2/2] blkdev: __blkdev_direct_IO_simple: make sure to fill up the bio

On Thu, 2018-07-19 at 13:56 +0200, Jan Kara wrote:

On Thu 19-07-18 19:04:46, Ming Lei wrote:

quoted

On Thu, Jul 19, 2018 at 11:39:18AM +0200, Martin Wilck wrote:

quoted

bio_iov_iter_get_pages() returns only pages for a single non-
empty
segment of the input iov_iter's iovec. This may be much less than
the number
of pages __blkdev_direct_IO_simple() is supposed to process. Call
bio_iov_iter_get_pages() repeatedly until either the requested
number
of bytes is reached, or bio.bi_io_vec is exhausted. If this is
not done,
short writes or reads may occur for direct synchronous IOs with
multiple
iovec slots (such as generated by writev()). In that case,
__generic_file_write_iter() falls back to buffered writes, which
has been observed to cause data corruption in certain workloads.

Note: if segments aren't page-aligned in the input iovec, this
patch may
result in multiple adjacent slots of the bi_io_vec array to
reference the same
page (the byte ranges are guaranteed to be disjunct if the
preceding patch is
applied). We haven't seen problems with that in our and the
customer's
tests. It'd be possible to detect this situation and merge
bi_io_vec slots
that refer to the same page, but I prefer to keep it simple for
now.

Fixes: 72ecad22d9f1 ("block: support a full bio worth of IO for
simplified bdev direct-io")
Signed-off-by: Martin Wilck <redacted>
---
 fs/block_dev.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 0dd87aa..41643c4 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c

@@ -221,7 +221,12 @@ __blkdev_direct_IO_simple(struct kiocb

*iocb, struct iov_iter *iter,
 
 	ret = bio_iov_iter_get_pages(&bio, iter);
 	if (unlikely(ret))
-		return ret;
+		goto out;
+
+	while (ret == 0 &&
+	       bio.bi_vcnt < bio.bi_max_vecs &&
iov_iter_count(iter) > 0)
+		ret = bio_iov_iter_get_pages(&bio, iter);
+
 	ret = bio.bi_iter.bi_size;
 
 	if (iov_iter_rw(iter) == READ) {

@@ -250,6 +255,7 @@ __blkdev_direct_IO_simple(struct kiocb *iocb,

struct iov_iter *iter,
 		put_page(bvec->bv_page);
 	}
 
+out:
 	if (vecs != inline_vecs)
 		kfree(vecs);

You might put the 'vecs' leak fix into another patch, and resue the
current code block for that.

Looks all users of bio_iov_iter_get_pages() need this kind of fix,
so
what do you think about the following way?

No. AFAICT all the other users of bio_iov_iter_get_pages() are
perfectly
fine with it returning less pages and they loop appropriately.

We might consider to better fill the bios in __blkdev_direct_IO(), too,
it might be a performance gain.

Regards
Martin

-- 
Dr. Martin Wilck [off-list ref], Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)