[029/155] mm/filemap.c: don't bother dropping mmap_sem for zero size readahead

Commit Message

Andrew Morton April 2, 2020, 4:04 a.m. UTC

From: Jan Kara <[e-mail]>
Subject: mm/filemap.c: don't bother dropping mmap_sem for zero size readahead

When handling a page fault, we drop mmap_sem to start async readahead so
that we don't block on IO submission with mmap_sem held.  However there's
no point to drop mmap_sem in case readahead is disabled.  Handle that case
to avoid pointless dropping of mmap_sem and retrying the fault.  This was
actually reported to block mlockall(MCL_CURRENT) indefinitely.

Link: http://lkml.kernel.org/r/[e-mail]
Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations")
Signed-off-by: Jan Kara <[e-mail]>
Reported-by: Minchan Kim <[e-mail]>
Reported-by: Robert Stupp <[e-mail]>
Reviewed-by: Josef Bacik <[e-mail]>
Reviewed-by: Minchan Kim <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
---

 mm/filemap.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Patch

--- a/mm/filemap.c~mm-dont-bother-dropping-mmap_sem-for-zero-size-readahead
+++ a/mm/filemap.c
@@ -2416,7 +2416,7 @@  static struct file *do_async_mmap_readah
 	pgoff_t offset = vmf->pgoff;
 
 	/* If we don't want any read-ahead, don't bother */
-	if (vmf->vma->vm_flags & VM_RAND_READ)
+	if (vmf->vma->vm_flags & VM_RAND_READ || !ra->ra_pages)
 		return fpin;
 	if (ra->mmap_miss > 0)
 		ra->mmap_miss--;