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

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Date:	Mon, 01 Jun 2009 16:46:28 +0200
From:	Goswin von Brederlow <goswin-v-b@....de>
To:	Jan Kara <jack@...e.cz>
Cc:	Goswin von Brederlow <goswin-v-b@....de>,
	Pavel Machek <pavel@....cz>,
	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

Jan Kara <jack@...e.cz> writes:

> On Mon 01-06-09 13:33:08, Goswin von Brederlow wrote:
>> Jan Kara <jack@...e.cz> writes:
>> 
>> > On Sat 30-05-09 13:23:24, Pavel Machek wrote:
>> >> 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? ...?
>> >   Depends on the filesystem. For example on ext4, you'll see a WARN_ON and the data
>> > won't be written. Some filesystems may just try to map blocks and possibly
>> > hit deadlock or something like that. Filesystems like ext2 / ext3 /
>> > reiserfs generally don't care because so far they allocate blocks on writepage
>> > time (which has the problem that you can write data via mmap and kernel
>> > will later discard them because it hits ENOSPC or quota limit). That's
>> > actually what I was trying to fix originally.
>> >
>> > 										Honza
>> 
>> man mmap:
>>        A file is mapped in multiples of the page size.  For a file that is not
>>        a  multiple  of  the  page  size,  the  remaining memory is zeroed when
>>        mapped, and writes to that region are not written out to the file.  The
>>        effect  of changing the size of the underlying file of a mapping on the
>>        pages that correspond to added  or  removed  regions  of  the  file  is
>>        unspecified.
>> 
>> Whatever happens happens. The above code is just wrong, as in
>> unspecified behaviour.
>> What happens if you ftruncate() before mmap()?
>   OK, I admit I didn't realize mmap() has so weak requirements. Doing mmap
> after ftruncate() should work fine because before you write via that new
> mmap page_mkwrite() will be called anyway.

But the ftruncate would only allocate a block at position 10000. The
file still has a big hole from 1024-4095.

>   So what we could alternatively do is that we just discard dirty bits from
> buffers that don't have underlying blocks allocated. That would satisfy the
> specification as well. But I have to say I'm a bit afraid of discarding
> dirty bits like that. Also we'd have to handle the case where someone does
> mremap() after ftruncate().
>   What other memory management people think?

As said above the file still has a big hole after ftruncate. So not
having underlying blocks allocated can't be the deciding factor.

If possible I would make ftruncate() after mmap() work just like
ftruncate() before mmap(). That is write any dirty page completly up
to the current filesize. Allocate disk blocks for the file as needed
(you need to do that anyway). Wouldn't it be more work to remember the
filesize at the time of the mmap() to limit updates to that than using
the current file size?

MfG
        Goswin
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