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Re: [PATCH 3/3] writeback: limit write_cache_pages integrity scanning to

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: [PATCH 3/3] writeback: limit write_cache_pages integrity scanning to current EOF
From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Date: Wed, 9 Jun 2010 14:12:08 -0700
Cc: torvalds@xxxxxxxxxxxxxxxxxxxx, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx, stable@xxxxxxxxxx
In-reply-to: <1276043840-1946-4-git-send-email-david@xxxxxxxxxxxxx>
References: <1276043840-1946-1-git-send-email-david@xxxxxxxxxxxxx> <1276043840-1946-4-git-send-email-david@xxxxxxxxxxxxx>
On Wed,  9 Jun 2010 10:37:20 +1000
Dave Chinner <david@xxxxxxxxxxxxx> 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. That requires more complex intervention while this patch only
> addresses the common append-case of this sync holdoff.
> 

I don't know if this regression sucks enough to warrant backporting,
but the fix is simple and is independent of [1/3] and [2/3].

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