linux-kernel - Re: [PATCH][1/2]page_fault retry with NOPAGE

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Date:	Fri, 10 Apr 2009 14:48:45 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Ying Han <yinghan@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Waychison <mikew@...gle.com>,
	Rohit Seth <rohitseth@...gle.com>,
	Hugh Dickins <hugh@...itas.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"H. Peter Anvin" <hpa@...or.com>,
	Török Edwin <edwintorok@...il.com>,
	Lee Schermerhorn <lee.schermerhorn@...com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH][1/2]page_fault retry with NOPAGE_RETRY

On Fri, Apr 10, 2009 at 02:32:01PM +0800, Ying Han wrote:
> 2009/4/9 Andrew Morton <akpm@...ux-foundation.org>:
> >
> >> Subject: [PATCH][1/2]page_fault retry with NOPAGE_RETRY
> >
> > Please give each patch in the series a unique and meaningful title.
> >
> > On Wed, 8 Apr 2009 13:02:35 -0700 Ying Han <yinghan@...gle.com> wrote:
> >
> >> support for FAULT_FLAG_RETRY with no user change:
> >
> > yup, we'd prefer a complete changelog here please.
> >
> >> Signed-off-by: Ying Han <yinghan@...gle.com>
> >>              Mike Waychison <mikew@...gle.com>
> >
> > This form:
> >
> > Signed-off-by: Ying Han <yinghan@...gle.com>
> > Signed-off-by: Mike Waychison <mikew@...gle.com>
> 
> Thanks Andrew,  and i need to add Fengguang to Signed-off-by.

Thank you.

> >
> > is conventional.
> >
> >> index 4a853ef..29c2c39 100644
> >> --- a/include/linux/fs.h
> >> +++ b/include/linux/fs.h
> >> @@ -793,7 +793,7 @@ struct file_ra_state {
> >>                                          there are only # of pages ahead */
> >>
> >>       unsigned int ra_pages;          /* Maximum readahead window */
> >> -     int mmap_miss;                  /* Cache miss stat for mmap accesses */
> >> +     unsigned int mmap_miss;         /* Cache miss stat for mmap accesses */
> >
> > This change makes sense, but we're not told the reasons for making it?
> > Did it fix a bug, or is it an unrelated fixlet, or...?
> 
> Fengguang: Could you help making comments on this part? and i will
> make changes elsewhere as Andrew pointed. Thanks

Ah this may deserve a standalone patch:
---
readhead: make mmap_miss an unsigned int

This makes the performance impact of possible mmap_miss wrap around to be
temporary and tolerable: i.e. MMAP_LOTSAMISS=100 extra readarounds.

Otherwise if ever mmap_miss wraps around to negative, it takes INT_MAX
cache misses to bring it back to normal state.  During the time mmap
readaround will be _enabled_ for whatever wild random workload. That's
almost permanent performance impact.

Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
---
 include/linux/fs.h |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- mm.orig/include/linux/fs.h
+++ mm/include/linux/fs.h
@@ -824,7 +824,7 @@ struct file_ra_state {
 					   there are only # of pages ahead */
 
 	unsigned int ra_pages;		/* Maximum readahead window */
-	int mmap_miss;			/* Cache miss stat for mmap accesses */
+	unsigned int mmap_miss;		/* Cache miss stat for mmap accesses */
 	loff_t prev_pos;		/* Cache last read() position */
 };
 
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