From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <464AF224.30105@yahoo.com.au> Date: Wed, 16 May 2007 21:59:32 +1000 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2 References: <20070318233008.GA32597093@melbourne.sgi.com> <18993.1179310769@redhat.com> In-Reply-To: <18993.1179310769@redhat.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: David Howells Cc: David Chinner , lkml , linux-mm , linux-fsdevel List-ID: David Howells wrote: > David Chinner 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org