From: Jan Kara <jack@xxxxxxx> 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/20200212101356.30759-1-jack@xxxxxxx Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations") Signed-off-by: Jan Kara <jack@xxxxxxx> Reported-by: Minchan Kim <minchan@xxxxxxxxxx> Reported-by: Robert Stupp <snazy@xxxxxx> Reviewed-by: Josef Bacik <josef@xxxxxxxxxxxxxx> Reviewed-by: Minchan Kim <minchan@xxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/filemap.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) --- 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--; _
