(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@xxxxxxx>
> SUSE Labs, CR
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