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SubjectRe: [PATCH 2/2] [PATCH] bcache: __write_super to handle page sizes other than 4k
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On 2019/12/6 4:55 下午, Liang Chen wrote:
> __write_super assumes super block data starts at offset 0 of the page
> read in with __bread from read_super, which is not true when page size
> is not 4k. We encountered the issue on system with 64K page size - commonly
> seen on aarch64 architecture.
>
> Instead of making any assumption on the offset of the data within the page,
> this patch calls __bread again to locate the data. That should not introduce
> an extra io since the page has been held when it's read in from read_super,
> and __write_super is not on performance critical code path.
>
> Signed-off-by: Liang Chen <liangchen.linux@gmail.com>

In general the patch is good for me. Just two minor requests I add them
in line the email.

Thanks.

> ---
> drivers/md/bcache/super.c | 32 +++++++++++++++++++++++++++-----
> 1 file changed, 27 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index a573ce1d85aa..a39450c9bc34 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -207,15 +207,27 @@ static void write_bdev_super_endio(struct bio *bio)
> closure_put(&dc->sb_write);
> }
>
> -static void __write_super(struct cache_sb *sb, struct bio *bio)
> +static int __write_super(struct cache_sb *sb, struct bio *bio,
> + struct block_device *bdev)
> {
> - struct cache_sb *out = page_address(bio_first_page_all(bio));
> + struct cache_sb *out;
> unsigned int i;
> + struct buffer_head *bh;
> +
> + /*
> + * The page is held since read_super, this __bread * should not
> + * cause an extra io read.
> + */
> + bh = __bread(bdev, 1, SB_SIZE);
> + if (!bh)
> + goto out_bh;
> +
> + out = (struct cache_sb *) bh->b_data;

This is quite tricky here. Could you please to move this code piece into
an inline function and add code comments to explain why a read is
necessary for a write.


>
> bio->bi_iter.bi_sector = SB_SECTOR;
> bio->bi_iter.bi_size = SB_SIZE;
> bio_set_op_attrs(bio, REQ_OP_WRITE, REQ_SYNC|REQ_META);
> - bch_bio_map(bio, NULL);
> + bch_bio_map(bio, bh->b_data);
>
> out->offset = cpu_to_le64(sb->offset);
> out->version = cpu_to_le64(sb->version);
> @@ -239,7 +251,14 @@ static void __write_super(struct cache_sb *sb, struct bio *bio)
> pr_debug("ver %llu, flags %llu, seq %llu",
> sb->version, sb->flags, sb->seq);
>
> + /* The page will still be held without this bh.*/
> + put_bh(bh);
> submit_bio(bio);
> + return 0;
> +
> +out_bh:
> + pr_err("Couldn't read super block, __write_super failed");
> + return -1;
> }
>
> static void bch_write_bdev_super_unlock(struct closure *cl)
> @@ -264,7 +283,8 @@ void bch_write_bdev_super(struct cached_dev *dc, struct closure *parent)
>
> closure_get(cl);
> /* I/O request sent to backing device */
> - __write_super(&dc->sb, bio);
> + if(__write_super(&dc->sb, bio, dc->bdev))
> + closure_put(cl);
>
> closure_return_with_destructor(cl, bch_write_bdev_super_unlock);
> }
> @@ -312,7 +332,9 @@ void bcache_write_super(struct cache_set *c)
> bio->bi_private = ca;
>
> closure_get(cl);
> - __write_super(&ca->sb, bio);
> + if(__write_super(&ca->sb, bio, ca->bdev))

And here, please add code comments for why closure_put() is necessary here.

> + closure_put(cl);
> +
> }
>
> closure_return_with_destructor(cl, bcache_write_super_unlock);
>


--

Coly Li

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