Messages in this thread | | | Date | Wed, 16 May 2007 21:59:32 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2 |
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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).
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