linux-ext4 - Re: [PATCH 03/11] vfs: Add better VFS support for page

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Message-ID: <20090530112324.GD1395@ucw.cz>
Date:	Sat, 30 May 2009 13:23:24 +0200
From:	Pavel Machek <pavel@....cz>
To:	Jan Kara <jack@...e.cz>
Cc:	LKML <linux-kernel@...r.kernel.org>, npiggin@...e.de,
	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 03/11] vfs: Add better VFS support for page_mkwrite
	when blocksize < pagesize

Hi!

> On filesystems where blocksize < pagesize the situation is more complicated.
> Think for example that blocksize = 1024, pagesize = 4096 and a process does:
>   ftruncate(fd, 0);
>   pwrite(fd, buf, 1024, 0);
>   map = mmap(NULL, 4096, PROT_WRITE, MAP_SHARED, fd, 0);
>   map[0] = 'a';  ----> page_mkwrite() for index 0 is called
>   ftruncate(fd, 10000); /* or even pwrite(fd, buf, 1, 10000) */
>   fsync(fd); ----> writepage() for index 0 is called
> 
> At the moment page_mkwrite() is called, filesystem can allocate only one block
> for the page because i_size == 1024. Otherwise it would create blocks beyond
> i_size which is generally undesirable. But later at writepage() time, we would
> like to have blocks allocated for the whole page (and in principle we have to
> allocate them because user could have filled the page with data after the
> second ftruncate()). This patch introduces a framework which allows filesystems
> to handle this with a reasonable effort.

What happens when you do above sequence on today's kernels? Oops? 3000
bytes of random junk in file? ...?


-- 
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