Re: [PATCH v6 19/19] mm: Use memalloc_nofs_save in readahead path

[Dave Chinner]

On Mon, Feb 17, 2020 at 10:46:13AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
> 
> Ensure that memory allocations in the readahead path do not attempt to
> reclaim file-backed pages, which could lead to a deadlock.  It is
> possible, though unlikely this is the root cause of a problem observed
> by Cong Wang.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> Reported-by: Cong Wang <xiyou.wangcong@xxxxxxxxx>
> Suggested-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/readahead.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/mm/readahead.c b/mm/readahead.c
> index 94d499cfb657..8f9c0dba24e7 100644
> --- a/mm/readahead.c
> +++ b/mm/readahead.c
> @@ -22,6 +22,7 @@
>  #include <linux/mm_inline.h>
>  #include <linux/blk-cgroup.h>
>  #include <linux/fadvise.h>
> +#include <linux/sched/mm.h>
>  
>  #include "internal.h"
>  
> @@ -174,6 +175,18 @@ void page_cache_readahead_limit(struct address_space *mapping,
>  		._nr_pages = 0,
>  	};
>  
> +	/*
> +	 * Partway through the readahead operation, we will have added
> +	 * locked pages to the page cache, but will not yet have submitted
> +	 * them for I/O.  Adding another page may need to allocate memory,
> +	 * which can trigger memory reclaim.  Telling the VM we're in
> +	 * the middle of a filesystem operation will cause it to not
> +	 * touch file-backed pages, preventing a deadlock.  Most (all?)
> +	 * filesystems already specify __GFP_NOFS in their mapping's
> +	 * gfp_mask, but let's be explicit here.
> +	 */
> +	unsigned int nofs = memalloc_nofs_save();
> +

So doesn't this largely remove the need for all the gfp flag futzing
in the readahead path? i.e. almost all readahead allocations are now
going to be GFP_NOFS | GFP_NORETRY | GFP_NOWARN ?

If so, shouldn't we just strip all the gfp flags and masking out of
the readahead path altogether?

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx