There is no need to use dynamic bio allocation for BDI_CAP_SYNCHRONOUS
devices. They can with on-stack bio without concern about waiting
bio allocation from mempool under heavy memory pressure.

Cc: Matthew Wilcox <wi...@infradead.org>
Signed-off-by: Minchan Kim <minc...@kernel.org>
---

Hi Mattew,

I didn't use sbvec[nr_pages] as you suggested[1] because I don't think
it's pointless in do_mpage_readpage which works per-page base as
I replied to you.
If I misunderstood something, please correct me.

[1] http://lkml.kernel.org/r/<20170808132904.gc31...@bombadil.infradead.org>

 fs/mpage.c | 47 ++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 40 insertions(+), 7 deletions(-)

diff --git a/fs/mpage.c b/fs/mpage.c
index 2e4c41ccb5c9..d3b777fdfd5a 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -162,6 +162,9 @@ do_mpage_readpage(struct bio *bio, struct page *page, 
unsigned nr_pages,
        int fully_mapped = 1;
        unsigned nblocks;
        unsigned relative_block;
+       /* on-stack bio for synchronous devices */
+       struct bio sbio;
+       struct bio_vec sbvec;
 
        if (page_has_buffers(page))
                goto confused;
@@ -282,10 +285,22 @@ do_mpage_readpage(struct bio *bio, struct page *page, 
unsigned nr_pages,
                                                                page))
                                goto out;
                }
-               bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
+
+               if (bdi_cap_synchronous_io(inode_to_bdi(inode))) {
+                       bio = &sbio;
+                       /* mpage_end_io calls bio_put unconditionally */
+                       bio_get(&sbio);
+
+                       bio_init(&sbio, &sbvec, 1);
+                       sbio.bi_bdev = bdev;
+                       sbio.bi_iter.bi_sector = blocks[0] << (blkbits - 9);
+               } else {
+
+                       bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
                                min_t(int, nr_pages, BIO_MAX_PAGES), gfp);
-               if (bio == NULL)
-                       goto confused;
+                       if (bio == NULL)
+                               goto confused;
+               }
        }
 
        length = first_hole << blkbits;
@@ -302,6 +317,8 @@ do_mpage_readpage(struct bio *bio, struct page *page, 
unsigned nr_pages,
        else
                *last_block_in_bio = blocks[blocks_per_page - 1];
 out:
+       if (bio == &sbio)
+               bio = mpage_bio_submit(REQ_OP_READ, 0, bio);
        return bio;
 
 confused:
@@ -492,6 +509,9 @@ static int __mpage_writepage(struct page *page, struct 
writeback_control *wbc,
        loff_t i_size = i_size_read(inode);
        int ret = 0;
        int op_flags = wbc_to_write_flags(wbc);
+       /* on-stack-bio */
+       struct bio sbio;
+       struct bio_vec sbvec;
 
        if (page_has_buffers(page)) {
                struct buffer_head *head = page_buffers(page);
@@ -610,10 +630,21 @@ static int __mpage_writepage(struct page *page, struct 
writeback_control *wbc,
                                goto out;
                        }
                }
-               bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
-                               BIO_MAX_PAGES, GFP_NOFS|__GFP_HIGH);
-               if (bio == NULL)
-                       goto confused;
+
+               if (bdi_cap_synchronous_io(inode_to_bdi(inode))) {
+                       bio = &sbio;
+                       /* mpage_end_io calls bio_put unconditionally */
+                       bio_get(&sbio);
+
+                       bio_init(&sbio, &sbvec, 1);
+                       sbio.bi_bdev = bdev;
+                       sbio.bi_iter.bi_sector = blocks[0] << (blkbits - 9);
+               } else {
+                       bio = mpage_alloc(bdev, blocks[0] << (blkbits - 9),
+                                       BIO_MAX_PAGES, GFP_NOFS|__GFP_HIGH);
+                       if (bio == NULL)
+                               goto confused;
+               }
 
                wbc_init_bio(wbc, bio);
                bio->bi_write_hint = inode->i_write_hint;
@@ -662,6 +693,8 @@ static int __mpage_writepage(struct page *page, struct 
writeback_control *wbc,
         */
        mapping_set_error(mapping, ret);
 out:
+       if (bio == &sbio)
+               bio = mpage_bio_submit(REQ_OP_WRITE, op_flags, bio);
        mpd->bio = bio;
        return ret;
 }
-- 
2.7.4

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