2019-08-15 12:42:38

by Aleix Roca Nonell

Subject: [PATCH] io_uring: fix manual setup of iov_iter for fixed buffers

Commit bd11b3a391e3 ("io_uring: don't use iov_iter_advance() for fixed
buffers") introduced an optimization to avoid using the slow
iov_iter_advance by manually populating the iov_iter iterator in some
cases.

However, the computation of the iterator count field was erroneous: The
first bvec was always accounted for an extent of page size even if the
bvec length was smaller.

In consequence, some I/O operations on fixed buffers were unable to
operate on the full extent of the buffer, consistently skipping some
bytes at the end of it.

Fixes: bd11b3a391e3 ("io_uring: don't use iov_iter_advance() for fixed buffers")
Cc: [email protected]
Signed-off-by: Aleix Roca Nonell <[email protected]>
---
fs/io_uring.c | 4 +---
1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index d542f1cf4428..aa25b5bbd4ae 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1097,10 +1097,8 @@ static int io_import_fixed(struct io_ring_ctx *ctx, int rw,

iter->bvec = bvec + seg_skip;
iter->nr_segs -= seg_skip;
- iter->count -= (seg_skip << PAGE_SHIFT);
+ iter->count -= bvec->bv_len + offset;
iter->iov_offset = offset & ~PAGE_MASK;
- if (iter->iov_offset)
- iter->count -= iter->iov_offset;
}
}

--
2.12.0


http://bsc.es/disclaimer

2019-08-15 19:20:58

by Jens Axboe

Subject: Re: [PATCH] io_uring: fix manual setup of iov_iter for fixed buffers

On 8/15/19 6:03 AM, Aleix Roca Nonell wrote:
> Commit bd11b3a391e3 ("io_uring: don't use iov_iter_advance() for fixed
> buffers") introduced an optimization to avoid using the slow
> iov_iter_advance by manually populating the iov_iter iterator in some
> cases.
>
> However, the computation of the iterator count field was erroneous: The
> first bvec was always accounted for an extent of page size even if the
> bvec length was smaller.
>
> In consequence, some I/O operations on fixed buffers were unable to
> operate on the full extent of the buffer, consistently skipping some
> bytes at the end of it.

Applied, thanks.

--
Jens Axboe