[PATCH v1 2/2] mm/filemap: Skip non-uptodate folio if there are available folios

[Chi Zhiling <chizhiling@163.com>]

From: Chi Zhiling <chizhiling@kylinos.cn>

When reading data exceeding the maximum IO size, the operation is split
into multiple IO requests, but the data isn't immediately copied to
userspace after each IO completion.

For example, when reading 2560k data from a device with 1280k maximum IO
size, the following sequence occurs:

1. read 1280k
2. copy 41 pages and issue read ahead for next 1280k
3. copy 31 pages to user buffer
4. wait the next 1280k
5. copy 8 pages to user buffer
6. copy 20 folios(64k) to user buffer

The 8 pages in step 5 are copied after the second 1280k completes(step 4)
due to waiting for a non-uptodate folio in filemap_update_page.
We can copy the 8 pages before the second 1280k completes(step 4) to
reduce the latency of this read operation.

After applying the patch, these 8 pages will be copied before the next IO
completes:

1. read 1280k
2. copy 41 pages and issue read ahead for next 1280k
3. copy 31 pages to user buffer
4. copy 8 pages to user buffer
5. wait the next 1280k
6. copy 20 folios(64k) to user buffer

This patch drops a setting of IOCB_NOWAIT for AIO, which is fine because
filemap_read will set it again for AIO.

The final solution provided by Matthew Wilcox:
Link: https://lore.kernel.org/linux-fsdevel/aIDy076Sxt544qja@casper.infradead.org/

Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
---
 mm/filemap.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 00c30f7f7dc3..d2e07184b281 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2623,9 +2623,10 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
 			goto err;
 	}
 	if (!folio_test_uptodate(folio)) {
-		if ((iocb->ki_flags & IOCB_WAITQ) &&
-		    folio_batch_count(fbatch) > 1)
-			iocb->ki_flags |= IOCB_NOWAIT;
+		if (folio_batch_count(fbatch) > 1) {
+			err = -EAGAIN;
+			goto err;
+		}
 		err = filemap_update_page(iocb, mapping, count, folio,
 					  need_uptodate);
 		if (err)
-- 
2.43.0