Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2

Nick Piggin
2007-05-16T12:00:24Z

David Howells wrote:
> David Chinner <dgc@sgi.com> wrote:
>
>
>>+ ret = block_prepare_write(page, 0, end, get_block);
>
>
> As I understand the way prepare_write() works, this is incorrect.

I think it is actually OK.

> The start and end points passed to block_prepare_write() delimit the region of
> the page that is going to be modified. This means that prepare_write()
> doesn't need to fill it in if the page is not up to date. It does, however,
> need to fill in the region before (if present) and the region after (if
> present). Look at it like this:
>
> +---------------+
> | |
> | | <-- Filled in by prepare_write()
> | |
> to-> |:::::::::::::::|
> | |
> | | <-- Filled in by caller
> | |
> offset->|:::::::::::::::|
> | |
> | | <-- Filled in by prepare_write()
> | |
> page-> +---------------+
>
> However, page_mkwrite() isn't told which bit of the page is going to be
> written to. This means it has to ask prepare_write() to make sure the whole
> page is filled in. In other words, offset and to must be equal (in AFS I set
> them both to 0).

Dave is using prepare_write here to ensure blocks are allocated in the
given range. The filesystem's ->nopage function must ensure it is uptodate
before allowing it to be mapped.

> With what you've got, if, say, 'offset' is 0 and 'to' is calculated at
> PAGE_SIZE, then if the page is not up to date for any reason, then none of the
> page will be updated before the page is written on by the faulting code.

Consider that the code currently works OK today _without_ page_mkwrite.
page_mkwrite is being added to do block allocation / reservation.

> You probably get away with this in a blockdev-based filesystem because it's
> unlikely that the page will cease to be up to date.
>
> However, if someone adds a syscall to punch holes in files, this may change...

We have one. Strangely enough, it is done with madvise(MADV_REMOVE).

--
SUSE Labs, Novell Inc.