mm/filemap: skip non-uptodate folio if there are available folios

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/
Link: https://lkml.kernel.org/r/20250728083952.75518-3-chizhiling@163.com
Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Suggested-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/filemap.c b/mm/filemap.c
index d6f95513241f027b68fa6f75025ac82e4465065b..6e954156bb77f4206d310c15f3a1c99f1cd4c6dc 100644 (file)
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -2622,9 +2622,10 @@ retry:
                        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)