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Re: [PATCH 6/6] writeback: limit write_cache_pages integrity scanning to

To: Nick Piggin <npiggin@xxxxxxx>
Subject: Re: [PATCH 6/6] writeback: limit write_cache_pages integrity scanning to current EOF
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Tue, 8 Jun 2010 16:59:22 +1000
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, akpm@xxxxxxxxxxxxxxxxxxxx
In-reply-to: <20100608053831.GR26335@laptop>
References: <1275957487-23633-1-git-send-email-david@xxxxxxxxxxxxx> <1275957487-23633-7-git-send-email-david@xxxxxxxxxxxxx> <20100608053831.GR26335@laptop>
User-agent: Mutt/1.5.20 (2009-06-14)
On Tue, Jun 08, 2010 at 03:38:31PM +1000, Nick Piggin wrote:
> On Tue, Jun 08, 2010 at 10:38:07AM +1000, Dave Chinner wrote:
> > From: Dave Chinner <dchinner@xxxxxxxxxx>
> > 
> > sync can currently take a really long time if a concurrent writer is
> > extending a file. The problem is that the dirty pages on the address
> > space grow in the same direction as write_cache_pages scans, so if
> > the writer keeps ahead of writeback, the writeback will not
> > terminate until the writer stops adding dirty pages.
> > 
> > For a data integrity sync, we only need to write the pages dirty at
> > the time we start the writeback, so we can stop scanning once we get
> > to the page that was at the end of the file at the time the scan
> > started.
> > 
> > This will prevent operations like copying a large file preventing
> > sync from completing as it will not write back pages that were
> > dirtied after the sync was started. This does not impact the
> > existing integrity guarantees, as any dirty page (old or new)
> > within the EOF range at the start of the scan will still be
> > captured.
> > 
> > This patch will not prevent sync from blocking on large writes into
> > holes.
> The writes don't have to be into holes to cause this starvation
> problem, do they?

No, they don't.

> > That requires more complex intervention while this patch only
> > addresses the common append-case of this sync holdoff.
> Jan's tagging patch looks pretty good to me and isn't so complex.
> I think we should just take that.

I don't care which one we take as long as it is actually tested by
more than the submitter and we get everything in for 2.6.35...


Dave Chinner

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