[PATCH 1/1] block: Check the queue limit before bio submitting

[Ming Lei]

On Mon, Nov 06, 2023 at 12:53:31PM +0800, Ming Lei wrote:
> On Mon, Nov 06, 2023 at 01:40:12AM +0000, Ed Tsai (蔡宗軒) wrote:
> > On Mon, 2023-11-06 at 09:33 +0800, Ed Tsai wrote:
> > > On Sat, 2023-11-04 at 11:43 +0800, Ming Lei wrote:
> 
> ...
> 
> > Sorry for missing out on my dd command. Here it is:
> > dd if=/data/test_file of=/dev/null bs=64m count=1 iflag=direct
> 
> OK, thanks for the sharing.
> 
> I understand the issue now, but not sure if it is one good idea to check
> queue limit in __bio_iov_iter_get_pages():
> 
> 1) bio->bi_bdev may not be set
> 
> 2) what matters is actually bio's alignment, and bio size still can
> be big enough
> 
> So I cooked one patch, and it should address your issue:

The following one fixes several bugs, and is verified to be capable of
making big & aligned bios, feel free to run your test against this one:

 block/bio.c | 28 +++++++++++++++++++++++++++-
 1 file changed, 27 insertions(+), 1 deletion(-)

diff --git a/block/bio.c b/block/bio.c
index 816d412c06e9..80b36ce57510 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1211,6 +1211,7 @@ static int bio_iov_add_zone_append_page(struct bio *bio, struct page *page,
 }
 
 #define PAGE_PTRS_PER_BVEC     (sizeof(struct bio_vec) / sizeof(struct page *))
+#define BIO_CHUNK_SIZE	(256U << 10)
 
 /**
  * __bio_iov_iter_get_pages - pin user or kernel pages and add them to a bio
@@ -1266,6 +1267,31 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 		size -= trim;
 	}
 
+	/*
+	 * Try to make bio aligned with 128KB if it isn't the last one, so
+	 * we can avoid small bio in case of big chunk sequential IO because
+	 * of bio split and multipage bvec.
+	 *
+	 * If nothing is added to this bio, simply allow unaligned since we
+	 * have chance to add more bytes
+	 */
+	if (iov_iter_count(iter) && bio->bi_iter.bi_size) {
+		unsigned int aligned_size = (bio->bi_iter.bi_size + size) &
+			~(BIO_CHUNK_SIZE - 1);
+
+		if (aligned_size <= bio->bi_iter.bi_size) {
+			/* stop to add page if this bio can't keep aligned */
+			if (!(bio->bi_iter.bi_size & (BIO_CHUNK_SIZE - 1))) {
+				ret = left = size;
+				goto revert;
+			}
+		} else {
+			aligned_size -= bio->bi_iter.bi_size;
+			iov_iter_revert(iter, size - aligned_size);
+			size = aligned_size;
+		}
+	}
+
 	if (unlikely(!size)) {
 		ret = -EFAULT;
 		goto out;
@@ -1285,7 +1311,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 
 		offset = 0;
 	}
-
+revert:
 	iov_iter_revert(iter, left);
 out:
 	while (i < nr_pages)
-- 
2.41.0



Thanks, 
Ming