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

Dave Chinner
2020-02-19T03:43:32Z

On Mon, Feb 17, 2020 at 10:46:13AM -0800, Matthew Wilcox wrote:
> From: "Matthew Wilcox (Oracle)" <willy@infradead.org>
>
> 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@infradead.org>
> Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
> Suggested-by: Michal Hocko <mhocko@suse.com>
> ---
> 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@fromorbit.com