From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail137.messagelabs.com (mail137.messagelabs.com [216.82.249.19]) by kanga.kvack.org (Postfix) with SMTP id B00C76B002D for ; Mon, 21 Nov 2011 18:01:18 -0500 (EST) Date: Mon, 21 Nov 2011 15:01:16 -0800 From: Andrew Morton Subject: Re: [PATCH 3/8] readahead: replace ra->mmap_miss with ra->ra_flags Message-Id: <20111121150116.094cf194.akpm@linux-foundation.org> In-Reply-To: <20111121093846.378529145@intel.com> References: <20111121091819.394895091@intel.com> <20111121093846.378529145@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Wu Fengguang Cc: Linux Memory Management List , linux-fsdevel@vger.kernel.org, Andi Kleen , Steven Whitehouse , Rik van Riel , LKML On Mon, 21 Nov 2011 17:18:22 +0800 Wu Fengguang wrote: > Introduce a readahead flags field and embed the existing mmap_miss in it > (mainly to save space). What an ugly patch. > It will be possible to lose the flags in race conditions, however the > impact should be limited. For the race to happen, there must be two > threads sharing the same file descriptor to be in page fault or > readahead at the same time. > > Note that it has always been racy for "page faults" at the same time. > > And if ever the race happen, we'll lose one mmap_miss++ or mmap_miss--. > Which may change some concrete readahead behavior, but won't really > impact overall I/O performance. > > CC: Andi Kleen > CC: Steven Whitehouse > Acked-by: Rik van Riel > Signed-off-by: Wu Fengguang > --- > include/linux/fs.h | 31 ++++++++++++++++++++++++++++++- > mm/filemap.c | 9 ++------- > 2 files changed, 32 insertions(+), 8 deletions(-) > > --- linux-next.orig/include/linux/fs.h 2011-11-20 11:30:55.000000000 +0800 > +++ linux-next/include/linux/fs.h 2011-11-20 11:48:53.000000000 +0800 > @@ -945,10 +945,39 @@ struct file_ra_state { > there are only # of pages ahead */ > > unsigned int ra_pages; /* Maximum readahead window */ > - unsigned int mmap_miss; /* Cache miss stat for mmap accesses */ > + unsigned int ra_flags; And it doesn't actually save any space, unless ra_flags gets used for something else in a subsequent patch. And if it does, perhaps ra_flags should be ulong, which is compatible with the bitops.h code. Or perhaps we should use a bitfield and let the compiler do the work. > loff_t prev_pos; /* Cache last read() position */ > }; > > +/* ra_flags bits */ > +#define READAHEAD_MMAP_MISS 0x000003ff /* cache misses for mmap access */ > + > +/* > + * Don't do ra_flags++ directly to avoid possible overflow: > + * the ra fields can be accessed concurrently in a racy way. > + */ > +static inline unsigned int ra_mmap_miss_inc(struct file_ra_state *ra) > +{ > + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS; > + > + /* the upper bound avoids banging the cache line unnecessarily */ > + if (miss < READAHEAD_MMAP_MISS) { > + miss++; > + ra->ra_flags = miss | (ra->ra_flags & ~READAHEAD_MMAP_MISS); > + } > + return miss; > +} > + > +static inline void ra_mmap_miss_dec(struct file_ra_state *ra) > +{ > + unsigned int miss = ra->ra_flags & READAHEAD_MMAP_MISS; > + > + if (miss) { > + miss--; > + ra->ra_flags = miss | (ra->ra_flags & ~READAHEAD_MMAP_MISS); > + } > +} It's strange that ra_mmap_miss_inc() returns the new value whereas ra_mmap_miss_dec() returns void. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org