[PATCH 2/2] mm: allow read-ahead with IOCB_NOWAIT set

[Yafang Shao <laoar.shao@gmail.com>]

Readahead support for IOCB_NOWAIT was introduced in commit 2e85abf053b9
("mm: allow read-ahead with IOCB_NOWAIT set"). However, this behavior was
altered in commit efa8480a8316 ("fs: RWF_NOWAIT should imply IOCB_NOIO") to
prevent potential blocking during memory reclamation.

To address the blocking issue during memory reclamation, we can use
memalloc_nowait_{save,restore} to ensure non-blocking behavior. With this
change, we can restore the original functionality, allowing
preadv2(IOCB_NOWAIT) to read data directly from the disk if it's not
already in the page cache.

Link: https://lore.gnuweeb.org/io-uring/20200624164127.GP21350@casper.infradead.org/
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Christian Brauner <brauner@kernel.org>
---
 include/linux/fs.h | 1 -
 mm/filemap.c       | 6 ++++++
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index fd34b5755c0b..ced74b1b350d 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -3455,7 +3455,6 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags,
 	if (flags & RWF_NOWAIT) {
 		if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
 			return -EOPNOTSUPP;
-		kiocb_flags |= IOCB_NOIO;
 	}
 	if (flags & RWF_ATOMIC) {
 		if (rw_type != WRITE)
diff --git a/mm/filemap.c b/mm/filemap.c
index 657bcd887fdb..93c690e1d1fd 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -46,6 +46,7 @@
 #include <linux/pipe_fs_i.h>
 #include <linux/splice.h>
 #include <linux/rcupdate_wait.h>
+#include <linux/sched/mm.h>
 #include <asm/pgalloc.h>
 #include <asm/tlbflush.h>
 #include "internal.h"
@@ -2514,6 +2515,7 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
 	pgoff_t index = iocb->ki_pos >> PAGE_SHIFT;
 	pgoff_t last_index;
 	struct folio *folio;
+	unsigned int flags;
 	int err = 0;
 
 	/* "last_index" is the index of the page beyond the end of the read */
@@ -2526,8 +2528,12 @@ static int filemap_get_pages(struct kiocb *iocb, size_t count,
 	if (!folio_batch_count(fbatch)) {
 		if (iocb->ki_flags & IOCB_NOIO)
 			return -EAGAIN;
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			flags = memalloc_nowait_save();
 		page_cache_sync_readahead(mapping, ra, filp, index,
 				last_index - index);
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			memalloc_nowait_restore(flags);
 		filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
 	}
 	if (!folio_batch_count(fbatch)) {
-- 
2.43.5