mm: allow read-ahead with IOCB_NOWAIT set

Readahead support for IOCB_NOWAIT was introduced in commit 2e85abf
("mm: allow read-ahead with IOCB_NOWAIT set").  However, this
implementation broke the semantics of IOCB_NOWAIT by potentially causing
it to wait on I/O during memory reclamation.  This behavior was later
modified in commit efa8480 ("fs: RWF_NOWAIT should imply IOCB_NOIO").

To resolve the blocking issue during memory reclamation, we can use
memalloc_noio_{save,restore} to ensure non-blocking behavior.  This change
restores the original functionality, allowing preadv2(IOCB_NOWAIT) to
trigger readahead if the file content is not present in the page cache.

While this process may trigger direct memory reclamation, the
__GFP_NORETRY flag is set in the readahead GFP flags, ensuring it won't
block.

A use case for this change is when we want to trigger readahead in the
preadv2(2) syscall if the file cache is absent, but without waiting for
certain filesystem locks, like xfs_ilock.  A simple example is as follows:

retry:
    if (preadv2(fd, iovec, cnt, offset, RWF_NOWAIT) < 0) {
        do_other_work();
        goto retry;
    }

Link: https://lore.gnuweeb.org/io-uring/20200624164127.GP21350@casper.infradead.org/
Link: https://lkml.kernel.org/r/20240820022639.89562-1-laoar.shao@gmail.com
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>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/fs.h

@@ -3461,7 +3461,6 @@ 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)

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"
@@ -2519,6 +2520,7 @@ 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 */
@@ -2531,8 +2533,12 @@ 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_noio_save();
 		page_cache_sync_readahead(mapping, ra, filp, index,
 				last_index - index);
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			memalloc_noio_restore(flags);
 		filemap_get_read_batch(mapping, index, last_index - 1, fbatch);
 	}
 	if (!folio_batch_count(fbatch)) {