On Wed 04-06-08 19:10:42, Miklos Szeredi wrote:
> (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.
You are right, write_cache_pages() is correct - I've wrongly undrestood
what 'end' means.
> 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.
Hmm, the fact is I've seen in my tests writepage() being called on a page
which had its buffers removed. And because we attach buffers to a page in
page_mkwrite() and in write_begin() I think we should not see such page.
I've added more debug printings to the code to verify that the page has
indeed been truncated but so far I did not reproduce the problem again.
> > 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 ;).
Honza
--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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