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

Matthew Wilcox
2020-02-19T05:22:32Z

On Wed, Feb 19, 2020 at 02:43:24PM +1100, Dave Chinner wrote:
> 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 ?

I don't think it ensures the GFP_NORETRY | GFP_NOWARN, just the GFP_NOFS
part. IOW, we'll still need a readahead_gfp() macro at some point ... I
don't want to add that to this already large series though.

Michal also wants to kill mapping->gfp_mask, btw.