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@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 ? 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.
