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

Matthew Wilcox
2017-08-08T12:49:59Z

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:

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