Re: [RFC][PATCH] mm: balance_dirty_pages() vs throttle_vm_writeout() deadlock

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 20 Feb 2007 16:49:24 +0100
Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:

> 
> If we have a lot of dirty memory and hit the throttle in balance_dirty_pages()
> we (potentially) generate a lot of writeback and unstable pages, if however
> during this writeback we need to reclaim a bit, we might hit
> throttle_vm_writeout(), which might delay us until the combined total of
> NR_UNSTABLE_NFS + NR_WRITEBACK falls below the dirty limit.
> 
> However unstable pages don't go away automagickally, they need a push. While
> balance_dirty_pages() does this push, throttle_vm_writeout() doesn't. So we can
> sit here ad infintum.
> 
> Hence I propose to remove the NR_UNSTABLE_NFS count from throttle_vm_writeout().
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  mm/page-writeback.c |    3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> Index: linux-2.6-git/mm/page-writeback.c
> ===================================================================
> --- linux-2.6-git.orig/mm/page-writeback.c	2007-02-20 15:07:43.000000000 +0100
> +++ linux-2.6-git/mm/page-writeback.c	2007-02-20 16:42:45.000000000 +0100
> @@ -310,8 +310,7 @@ void throttle_vm_writeout(void)
>                   */
>                  dirty_thresh += dirty_thresh / 10;      /* wheeee... */
>  
> -                if (global_page_state(NR_UNSTABLE_NFS) +
> -			global_page_state(NR_WRITEBACK) <= dirty_thresh)
> +                if (global_page_state(NR_WRITEBACK) <= dirty_thresh)
>                          	break;
>                  congestion_wait(WRITE, HZ/10);
>          }

I think we need the below.  It is to address a deadlock which usb-storage
triggered doing a GFP_NOIO allocation, but I suspect it'll fix NFS too?



From: Andrew Morton <akpm@linux-foundation.org>

throttle_vm_writeout() is designed to wait for the dirty levels to subside. 
But if the caller holds IO or FS locks, we might be holding up that writeout.

So change it to take a single nap to give other devices a chance to clean some
memory, then return.

Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: Pete Zaitcev <zaitcev@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 include/linux/writeback.h |    2 +-
 mm/page-writeback.c       |   13 +++++++++++--
 mm/vmscan.c               |    2 +-
 3 files changed, 13 insertions(+), 4 deletions(-)

diff -puN mm/vmscan.c~throttle_vm_writeout-dont-loop-on-gfp_nofs-and-gfp_noio-allocations mm/vmscan.c
--- a/mm/vmscan.c~throttle_vm_writeout-dont-loop-on-gfp_nofs-and-gfp_noio-allocations
+++ a/mm/vmscan.c
@@ -952,7 +952,7 @@ static unsigned long shrink_zone(int pri
 		}
 	}
 
-	throttle_vm_writeout();
+	throttle_vm_writeout(sc->gfp_mask);
 
 	atomic_dec(&zone->reclaim_in_progress);
 	return nr_reclaimed;
diff -puN mm/page-writeback.c~throttle_vm_writeout-dont-loop-on-gfp_nofs-and-gfp_noio-allocations mm/page-writeback.c
--- a/mm/page-writeback.c~throttle_vm_writeout-dont-loop-on-gfp_nofs-and-gfp_noio-allocations
+++ a/mm/page-writeback.c
@@ -296,11 +296,21 @@ void balance_dirty_pages_ratelimited_nr(
 }
 EXPORT_SYMBOL(balance_dirty_pages_ratelimited_nr);
 
-void throttle_vm_writeout(void)
+void throttle_vm_writeout(gfp_t gfp_mask)
 {
 	long background_thresh;
 	long dirty_thresh;
 
+	if ((gfp_mask & (__GFP_FS|__GFP_IO)) != ) {
+		/*
+		 * The caller might hold locks which can prevent IO completion
+		 * or progress in the filesystem.  So we cannot just sit here
+		 * waiting for IO to complete.
+		 */
+		congestion_wait(WRITE, HZ/10);
+		return;
+	}
+
         for ( ; ; ) {
 		get_dirty_limits(&background_thresh, &dirty_thresh, NULL);
 
@@ -317,7 +327,6 @@ void throttle_vm_writeout(void)
         }
 }
 
-
 /*
  * writeback at least _min_pages, and keep writing until the amount of dirty
  * memory is less than the background threshold, or until we're all clean.
diff -puN include/linux/writeback.h~throttle_vm_writeout-dont-loop-on-gfp_nofs-and-gfp_noio-allocations include/linux/writeback.h
--- a/include/linux/writeback.h~throttle_vm_writeout-dont-loop-on-gfp_nofs-and-gfp_noio-allocations
+++ a/include/linux/writeback.h
@@ -84,7 +84,7 @@ static inline void wait_on_inode(struct 
 int wakeup_pdflush(long nr_pages);
 void laptop_io_completion(void);
 void laptop_sync_completion(void);
-void throttle_vm_writeout(void);
+void throttle_vm_writeout(gfp_t gfp_mask);
 
 /* These are exported to sysctl. */
 extern int dirty_background_ratio;
_

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