linux-kernel - Re: Two questions on VFS/mm

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Date:	Wed, 04 Jun 2008 19:10:42 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jack@...e.cz
CC:	linux-kernel@...r.kernel.org, linux-ext4@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
	akpm@...ux-foundation.org
Subject: Re: Two questions on VFS/mm

(Added some CCs)

>   could some kind soul knowledgable in VFS/mm help me with the following
> two questions? I've spotted them when testing some ext4 for patches...
>   1) In write_cache_pages() we do:
> ...
> 	lock_page(page);
> 	...
> 	if (!wbc->range_cyclic && page->index > end) {
>                    done = 1;
>                    unlock_page(page);
>                    continue;
>         }
> 	...
> 	ret = (*writepage)(page, wbc, data);
> 
>   Now the problem is that if range_cyclic is set, it can happen that the
> page we give to the filesystem is beyond the current end of file (and can
> be already processed by invalidatepage()). Is the filesystem supposed to
> handle this (what would it be good for to give such a page to the fs?) or
> is it just a bug in write_cache_pages()?

There may be a bug somewhere, but write_cache_pages() looks correct.
It locks the page then checks for page->mapping to make sure the page
wasn't truncated.  And truncation (including invalidatepage()) happens
with the page locked, so that can't race with page writeback.

However the do_invalidatepage() in block_write_full_page() looks
suspicious.  It calls invalidatepage(), but doesn't perform all the
other things needed for truncation.  Maybe there's a valid reason for
that, but I really don't have any idea what.

Miklos

> 
>   2) I have the following problem with page_mkwrite() when blocksize <
> pagesize. What we want to do is to fill in a potential hole under a page
> somebody wants to write to. But consider following scenario with a
> filesystem with 1k blocksize:
>   truncate("file", 1024);
>   ptr = mmap("file");
>   *ptr = 'a'
>      -> page_mkwrite() is called.
>         but "file" is only 1k large and we cannot really allocate blocks
>         beyond end of file. So we allocate just one 1k block.
>   truncate("file", 4096);
>   *(ptr + 2048) = 'a'
>      - nothing is called and later during writepage() time we are surprised
>        we have a dirty page which is not backed by a filesystem block.
> 
>   How to solve this? One idea I have here is that when we handle truncate(),
> we mark the original last page (if it is partial) as read-only again so
> that page_mkwrite() is called on the next write to it. Is something like
> this possible? Pointers to code doing something similar are welcome, I don't
> really know these things ;).
> 
> 								Thanks
> 									Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
> --
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