linux-kernel - Re: [patch 6/10] mm: be sure to trim blocks

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Date:	Sat, 20 Jan 2007 04:50:39 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Dmitriy Monakhov <dmonakhov@...ru>
Cc:	Linux Memory Management <linux-mm@...ck.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux Filesystems <linux-fsdevel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [patch 6/10] mm: be sure to trim blocks

On Sun, Jan 14, 2007 at 05:25:44PM +0300, Dmitriy Monakhov wrote:
> Nick Piggin <npiggin@...e.de> writes:
> 
> > If prepare_write fails with AOP_TRUNCATED_PAGE, or if commit_write fails, then
> > we may have failed the write operation despite prepare_write having
> > instantiated blocks past i_size. Fix this, and consolidate the trimming into
> > one place.
> >
> > Signed-off-by: Nick Piggin <npiggin@...e.de>
> >
> > Index: linux-2.6/mm/filemap.c
> > ===================================================================
> > --- linux-2.6.orig/mm/filemap.c
> > +++ linux-2.6/mm/filemap.c
> > @@ -1911,22 +1911,9 @@ generic_file_buffered_write(struct kiocb
> >  		}
> >  
> >  		status = a_ops->prepare_write(file, page, offset, offset+bytes);
> > -		if (unlikely(status)) {
> > -			loff_t isize = i_size_read(inode);
> > +		if (unlikely(status))
> > +			goto fs_write_aop_error;
> May be it's stupid question but still..
> Why we treat non zero prepare_write() return code as error, it may be positive.
> Positive error code may be used as fine grained 'bytes' limiter in case of 
> blksize < pgsize as follows:
> 
>                 status = a_ops->prepare_write(file, page, offset, offset+bytes);
> 		if (unlikely(status)) {
>                         if (status > 0) {
>                                 bytes = min(bytes, status);
>                                 status = 0;
>                         } else {
>                 	        goto fs_write_aop_error;
>                         }
>                 }
> ---
> This is useful because fs may want to reduce 'bytes' by number of reasons,
> for example make it blksize bound. 
> Example : filesystem has 1k blksize and only two free blocks. And we try 
> write 4k bytes.
> Currently  write(fd, buff, 4096) will return -ENOSPC
> But after this fix write(fd, buff, 4096) will return as mutch as it can (2048).

It isn't a stupid question. Hmm, while it isn't documented in vfs.txt, it
seems like some filesystems actually do this. AFFS, maybe JFFS. So good
catch, thanks.
-
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