On Wednesday June 4, jack@xxxxxxx wrote:
> Hi,
>
> 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()?
Maybe there is an invariant that an address_space never has a dirty
page beyond the end-of-file??
Certainly 'truncate' invalidates and un-dirties such pages.
With typical writes, ->write_begin will extend EOF to include the
page, and ->write_end will mark it dirty (I think).
mmap writes are probably a bit different, but I suspect the same
principle applies.
If the page is not dirty, then
if (PageWriteback(page) ||
!clear_page_dirty_for_io(page)) {
unlock_page(page);
continue;
}
will fire, and you never get to
ret = (*writepage)(page, wbc, data);
>
> 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 ;).
My understanding is that memory mapping is always done in multiples of
the page size.
When you dirty any part of a page, you effectively dirty the whole
page, so you need to extend the file to cover the whole page.
i.e. the page_mkwrite() call must extend the file to a size of 4096.
NeilBrown
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