Re: [PATCH] mm: Don't bother dropping mmap_sem for zero size readahead

[SeongJae Park <sjpark@amazon.com>]

Hello,


The commit fixed by this patch[1] was merged in v5.1 and this patch was merged
in the mainline in v5.7 (5c72feee3e45b40a3c96c7145ec422899d0e8964).  Thus, the
issue affects [v5.1, v5.6].  I was also able to reproduce the issue and confirm
the fix works on v5.4 based kernels.

However, I couldn't find this fix in neither latest stable/linux-5.4.y, nor
stable-queue/master.  Could you please put this patch in the queue?

[1] https://lore.kernel.org/linux-mm/20200212101356.30759-1-jack@suse.cz/


Thanks,
SeongJae Park

On Wed, 12 Feb 2020 11:13:56 +0100 Jan Kara <jack@suse.cz> wrote:

> 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.
> 
> Fixes: 6b4c9f446981 ("filemap: drop the mmap_sem for all blocking operations")
> Reported-by: Minchan Kim <minchan@kernel.org>
> Reported-by: Robert Stupp <snazy@gmx.de>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>  mm/filemap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> Andrew, could you please pick up this patch? Minchan also tripped over this
> bug...
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 1146fcfa3215..3d39c437b07e 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2458,7 +2458,7 @@ static struct file *do_async_mmap_readahead(struct vm_fault *vmf,
>  	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--;
> -- 
> 2.16.4