On Thu, 7 Feb 2013, Andrew Morton wrote:

Date: Thu, 7 Feb 2013 15:40:42 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Lukas Czerner <redacted>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
    linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org,
    Hugh Dickins [off-list ref]
Subject: Re: [PATCH v2 10/18] mm: teach truncate_inode_pages_range() to handle
     non page aligned ranges

On Tue,  5 Feb 2013 10:12:03 +0100
Lukas Czerner [off-list ref] wrote:

quoted

This commit changes truncate_inode_pages_range() so it can handle non
page aligned regions of the truncate. Currently we can hit BUG_ON when
the end of the range is not page aligned, but we can handle unaligned
start of the range.

Being able to handle non page aligned regions of the page can help file
system punch_hole implementations and save some work, because once we're
holding the page we might as well deal with it right away.

In previous commits we've changed ->invalidatepage() prototype to accept
'length' argument to be able to specify range to invalidate. No we can
use that new ability in truncate_inode_pages_range().

...

+	/*
+	 * 'start' and 'end' always covers the range of pages to be fully
+	 * truncated. Partial pages are covered with 'partial_start' at the
+	 * start of the range and 'partial_end' at the end of the range.
+	 * Note that 'end' is exclusive while 'lend' is inclusive.
+	 */

That helped ;)  So the bytes to be truncated are

(start*PAGE_SIZE + partial_start) -> (end*PAGE_SIZE + partial_end) - 1

yes?

The start of the range is not right, because 'start' and 'end'
covers pages to be _fully_ truncated. See the while cycle and 
then 'if (partial_start)' condition where we search for the
page (start - 1) and do_invalidate within that page.

So it should be like this:


(start*PAGE_SIZE - partial_start*(PAGE_SIZE - partial_start) ->
(end*PAGE_END + partial_end) - 1


assuming that you want the range to be inclusive on both sides.

-Lukas

--
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/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>