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From: Eric Biggers <ebiggers@kernel.org>
To: Satya Tangirala <satyat@google.com>
Cc: "Theodore Y . Ts'o" <tytso@mit.edu>,
	Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <chao@kernel.org>,
	Jens Axboe <axboe@kernel.dk>,
	"Darrick J . Wong" <darrick.wong@oracle.com>,
	linux-kernel@vger.kernel.org, linux-fscrypt@vger.kernel.org,
	linux-f2fs-devel@lists.sourceforge.net,
	linux-xfs@vger.kernel.org, linux-block@vger.kernel.org,
	linux-ext4@vger.kernel.org
Subject: Re: [PATCH v9 5/9] block: Make bio_iov_iter_get_pages() respect bio_required_sector_alignment()
Date: Fri, 23 Jul 2021 14:33:02 -0700	[thread overview]
Message-ID: <YPs1jlAsvXLomSJJ@gmail.com> (raw)
In-Reply-To: <20210604210908.2105870-6-satyat@google.com>

On Fri, Jun 04, 2021 at 09:09:04PM +0000, Satya Tangirala wrote:
> Previously, bio_iov_iter_get_pages() wasn't used with bios that could have
> an encryption context. However, direct I/O support using blk-crypto
> introduces this possibility, so this function must now respect
> bio_required_sector_alignment() (otherwise, xfstests like generic/465 with
> ext4 will fail).

Can you be more clear that the fscrypt direct I/O support only requires this in
order to support I/O segments that aren't fs-block aligned?

I do still wonder if we should just not support that...  Dave is the only person
who has asked for it, and it's a lot of trouble to support.

I also noticed that f2fs has always only supported direct I/O that is *fully*
fs-block aligned (including the I/O segments) anyway.  So presumably that
limitation is not really that important after all...

Does anyone else have thoughts on this?

One more comment on this patch below:

> 
> Signed-off-by: Satya Tangirala <satyat@google.com>
> ---
>  block/bio.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/block/bio.c b/block/bio.c
> index 32f75f31bb5c..99c510f706e2 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -1099,7 +1099,8 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
>   * The function tries, but does not guarantee, to pin as many pages as
>   * fit into the bio, or are requested in @iter, whatever is smaller. If
>   * MM encounters an error pinning the requested pages, it stops. Error
> - * is returned only if 0 pages could be pinned.
> + * is returned only if 0 pages could be pinned. It also ensures that the number
> + * of sectors added to the bio is aligned to bio_required_sector_alignment().
>   *
>   * It's intended for direct IO, so doesn't do PSI tracking, the caller is
>   * responsible for setting BIO_WORKINGSET if necessary.
> @@ -1107,6 +1108,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
>  int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  {
>  	int ret = 0;
> +	unsigned int aligned_sectors;
>  
>  	if (iov_iter_is_bvec(iter)) {
>  		if (bio_op(bio) == REQ_OP_ZONE_APPEND)
> @@ -1121,6 +1123,15 @@ int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
>  			ret = __bio_iov_iter_get_pages(bio, iter);
>  	} while (!ret && iov_iter_count(iter) && !bio_full(bio, 0));
>  
> +	/*
> +	 * Ensure that number of sectors in bio is aligned to
> +	 * bio_required_sector_align()
> +	 */
> +	aligned_sectors = round_down(bio_sectors(bio),
> +				     bio_required_sector_alignment(bio));
> +	iov_iter_revert(iter, (bio_sectors(bio) - aligned_sectors) << SECTOR_SHIFT);
> +	bio_truncate(bio, aligned_sectors << SECTOR_SHIFT);
> +
>  	/* don't account direct I/O as memory stall */
>  	bio_clear_flag(bio, BIO_WORKINGSET);
>  	return bio->bi_vcnt ? 0 : ret;

Doesn't this need to return an error if the bio's size gets rounded down to 0?
For example if logical_block_size=512 and data_unit_size=4096, and the iov_iter
points to 4096 bytes in 8 512-byte segments but the last one isn't mapped, then
7 pages would be pinned and the last one would fail.  This would then truncate
the bio's size to 0, but bio->bi_vcnt would be 7, so this would still return 0.
It would also be necessary to release the pages before returning an error.

- Eric

  parent reply	other threads:[~2021-07-23 21:33 UTC|newest]

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-04 21:08 [PATCH v9 0/9] add support for direct I/O with fscrypt using blk-crypto Satya Tangirala
2021-06-04 21:09 ` [PATCH v9 1/9] block: blk-crypto-fallback: handle data unit split across multiple bvecs Satya Tangirala
2021-06-04 21:09 ` [PATCH v9 2/9] block: blk-crypto: relax alignment requirements for bvecs in bios Satya Tangirala
2021-06-04 21:09 ` [PATCH v9 3/9] fscrypt: add functions for direct I/O support Satya Tangirala
2021-07-23 20:42   ` Eric Biggers
2021-06-04 21:09 ` [PATCH v9 4/9] direct-io: add support for fscrypt using blk-crypto Satya Tangirala
2021-06-04 21:09 ` [PATCH v9 5/9] block: Make bio_iov_iter_get_pages() respect bio_required_sector_alignment() Satya Tangirala
2021-06-04 22:51   ` kernel test robot
2021-06-05  2:55   ` kernel test robot
2021-07-23 21:33   ` Eric Biggers [this message]
2021-07-24  7:19     ` Christoph Hellwig
2021-06-04 21:09 ` [PATCH v9 6/9] iomap: support direct I/O with fscrypt using blk-crypto Satya Tangirala
2021-07-22 19:04   ` Darrick J. Wong
2021-06-04 21:09 ` [PATCH v9 7/9] ext4: " Satya Tangirala
2021-06-04 21:09 ` [PATCH v9 8/9] f2fs: " Satya Tangirala
2021-06-04 21:09 ` [PATCH v9 9/9] fscrypt: update documentation for direct I/O support Satya Tangirala
     [not found] ` <CAF2Aj3h-Gt3bOxH4wXB7aeQ3jVzR3TEqd3uLsh4T9Q=e6W6iqQ@mail.gmail.com>
2021-07-22 14:48   ` [PATCH v9 0/9] add support for direct I/O with fscrypt using blk-crypto Eric Biggers
2021-08-04  8:26     ` Lee Jones

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