![]() | |||||||||||||
Messages in this thread |
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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
| Last update: 2007-05-16 14:03 [from the cache] ©2003-2008 | |||||||||||||