[v6,19/19] mm: Use memalloc_nofs_save in readahead path
diff mbox series

Message ID 20200217184613.19668-33-willy@infradead.org
State New
Headers show
Series
  • Change readahead API
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Commit Message

Matthew Wilcox Feb. 17, 2020, 6:46 p.m. UTC
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(+)

Comments

Dave Chinner Feb. 19, 2020, 3:43 a.m. UTC | #1
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.
Matthew Wilcox Feb. 19, 2020, 5:22 a.m. UTC | #2
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.

Patch
diff mbox series

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();
+
 	/*
 	 * Preallocate as many pages as we will need.
 	 */
@@ -227,6 +240,7 @@  void page_cache_readahead_limit(struct address_space *mapping,
 	if (readahead_count(&rac))
 		read_pages(&rac, &page_pool);
 	BUG_ON(!list_empty(&page_pool));
+	memalloc_nofs_restore(nofs);
 }
 EXPORT_SYMBOL_GPL(page_cache_readahead_limit);