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Re: bio_add_page rw mode check by merge_bvec_fn

To: "linux-fsdevel@xxxxxxxxxxxxxxx" <linux-fsdevel@xxxxxxxxxxxxxxx>, xfs@xxxxxxxxxxx, linux-ext4@xxxxxxxxxxxxxxx
Subject: Re: bio_add_page rw mode check by merge_bvec_fn
From: Bernd Schubert <bernd.schubert@xxxxxxxxxxxxxxxxxx>
Date: Thu, 21 Nov 2013 19:08:25 +0100
Cc: Kent Overstreet <kmo@xxxxxxxxxxxxx>, NeilBrown <neilb@xxxxxxx>, Jens Axboe <axboe@xxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <528E4A50.7040609@xxxxxxxxxxxxxxxxxx>
References: <528E4A50.7040609@xxxxxxxxxxxxxxxxxx>
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.0
[CCing linux-fsdevel instead of accidental fhgfs cc]

On 11/21/2013 07:00 PM, Bernd Schubert wrote:
Hi all,

raid5_mergeable_bvec() (q->merge_bvec_fn) checks for the rw-type and
makes decisions based on that if the bio is mergable or not. But so far
this value is only initialized on calling submit_bio(),
at least not by ext4 and xfs. I have not checked other file system so far.

Question 1: merge_bvec_fn is supposed to be removed, is is
still worth to fix the usage of merge_bvec_fn?

Question 2: If it is still supposed to be fixed, how do want to have
it? So far I have two patches for xfs and ext4 to set the rw type
directly there, but maybe bio_add_page should get an int rw_type
parameter and set bio->bi_rw itself?

diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
index d488f80..4cddc15 100644
--- a/fs/ext4/page-io.c
+++ b/fs/ext4/page-io.c
@@ -370,6 +370,7 @@ static int io_submit_init_bio(struct ext4_io_submit *io,
         bio->bi_bdev = bh->b_bdev;
         bio->bi_end_io = ext4_end_bio;
         bio->bi_private = ext4_get_io_end(io->io_end);
+       bio->bi_rw |= io->io_op;
         io->io_bio = bio;
         io->io_next_block = bh->b_blocknr;
         return 0;

diff --git a/fs/xfs/xfs_aops.c b/fs/xfs/xfs_aops.c
index 71c8c9d..b48048f 100644
--- a/fs/xfs/xfs_aops.c
+++ b/fs/xfs/xfs_aops.c
@@ -401,7 +401,8 @@ xfs_submit_ioend_bio(

  STATIC struct bio *
-       struct buffer_head      *bh)
+       struct buffer_head      *bh,
+       struct writeback_control *wbc)
         int                     nvecs = bio_get_nr_vecs(bh->b_bdev);
         struct bio              *bio = bio_alloc(GFP_NOIO, nvecs);
@@ -409,6 +410,7 @@ xfs_alloc_ioend_bio(
         ASSERT(bio->bi_private == NULL);
         bio->bi_sector = bh->b_blocknr * (bh->b_size >> 9);
         bio->bi_bdev = bh->b_bdev;
+       bio->bi_rw |= (wbc->sync_mode == WB_SYNC_ALL) ? WRITE_SYNC : WRITE;
         return bio;

@@ -511,7 +513,8 @@ xfs_submit_ioend(

                         if (!bio) {
-                               bio = xfs_alloc_ioend_bio(bh);
+                               bio = xfs_alloc_ioend_bio(bh, wbc);
                         } else if (bh->b_blocknr != lastblock + 1) {
                                 xfs_submit_ioend_bio(wbc, ioend, bio);
                                 goto retry;


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