On 07/19/2018 11:39 AM, Martin Wilck wrote:
> 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 <mwilck@xxxxxxxx>
> ---
> fs/block_dev.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
Reviewed-by: Hannes Reinecke <hare@xxxxxxxx>
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
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