Re: [PATCH v1 2/6] fs: use on-stack-bio if backing device has BDI_CAP_SYNC capability

[Matthew Wilcox]

On Tue, Aug 08, 2017 at 03:50:20PM +0900, Minchan Kim wrote:
> There is no need to use dynamic bio allocation for BDI_CAP_SYNC
> devices. They can with on-stack-bio without concern about waiting
> bio allocation from mempool under heavy memory pressure.

This seems ... more complex than necessary?  Why not simply do this:

diff --git a/fs/mpage.c b/fs/mpage.c
index baff8f820c29..6db6bf5131ed 100644
--- a/fs/mpage.c
+++ b/fs/mpage.c
@@ -157,6 +157,8 @@ do_mpage_readpage(struct bio *bio, struct page *page, unsigned nr_pages,
 	unsigned page_block;
 	unsigned first_hole = blocks_per_page;
 	struct block_device *bdev = NULL;
+	struct bio sbio;
+	struct bio_vec sbvec;
 	int length;
 	int fully_mapped = 1;
 	unsigned nblocks;
@@ -281,10 +283,17 @@ 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;
+			bio_init(bio, &sbvec, nr_pages);
+			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;
@@ -301,6 +310,8 @@ 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: