From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <45FE2F8F.6010603@yahoo.com.au> Date: Mon, 19 Mar 2007 17:37:03 +1100 From: Nick Piggin MIME-Version: 1.0 Subject: Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2 References: <20070318233008.GA32597093@melbourne.sgi.com> In-Reply-To: <20070318233008.GA32597093@melbourne.sgi.com> Content-Type: text/plain; charset=us-ascii; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: David Chinner Cc: lkml , linux-mm , linux-fsdevel List-ID: David Chinner wrote: > Generic page_mkwrite functionality. > > Filesystems that make use of the VM ->page_mkwrite() callout will generally use > the same core code to implement it. There are several tricky truncate-related > issues that we need to deal with here as we cannot take the i_mutex as we > normally would for these paths. These issues are not documented anywhere yet > so block_page_mkwrite() seems like the best place to start. > > Version 2: > > - read inode size only once > - more comments explaining implementation restrictions > > Signed-Off-By: Dave Chinner > > --- > fs/buffer.c | 47 ++++++++++++++++++++++++++++++++++++++++++++ > include/linux/buffer_head.h | 2 + > 2 files changed, 49 insertions(+) > > Index: 2.6.x-xfs-new/fs/buffer.c > =================================================================== > --- 2.6.x-xfs-new.orig/fs/buffer.c 2007-03-17 10:55:32.291414968 +1100 > +++ 2.6.x-xfs-new/fs/buffer.c 2007-03-19 08:13:54.519909087 +1100 > @@ -2194,6 +2194,52 @@ int generic_commit_write(struct file *fi > return 0; > } > > +/* > + * block_page_mkwrite() is not allowed to change the file size as it gets > + * called from a page fault handler when a page is first dirtied. Hence we must > + * be careful to check for EOF conditions here. We set the page up correctly > + * for a written page which means we get ENOSPC checking when writing into > + * holes and correct delalloc and unwritten extent mapping on filesystems that > + * support these features. > + * > + * We are not allowed to take the i_mutex here so we have to play games to > + * protect against truncate races as the page could now be beyond EOF. Because > + * vmtruncate() writes the inode size before removing pages, once we have the > + * page lock we can determine safely if the page is beyond EOF. If it is not > + * beyond EOF, then the page is guaranteed safe against truncation until we > + * unlock the page. > + */ > +int > +block_page_mkwrite(struct vm_area_struct *vma, struct page *page, > + get_block_t get_block) > +{ > + struct inode *inode = vma->vm_file->f_path.dentry->d_inode; > + unsigned long end; > + loff_t size; > + int ret = -EINVAL; > + > + lock_page(page); > + size = i_size_read(inode); > + if ((page->mapping != inode->i_mapping) || > + ((page->index << PAGE_CACHE_SHIFT) > size)) { > + /* page got truncated out from underneath us */ > + goto out_unlock; > + } I see your explanation above, but I still don't see why this can't just follow the conventional if (!page->mapping) check for truncation. If the test happens to be performed after truncate concurrently decreases i_size, then the blocks are going to get truncated by the truncate afterwards anyway. > + > + /* page is wholly or partially inside EOF */ > + if (((page->index + 1) << PAGE_CACHE_SHIFT) > size) > + end = size & ~PAGE_CACHE_MASK; > + else > + end = PAGE_CACHE_SIZE; > + > + ret = block_prepare_write(page, 0, end, get_block); > + if (!ret) > + ret = block_commit_write(page, 0, end); > + > +out_unlock: > + unlock_page(page); > + return ret; > +} -- SUSE Labs, Novell Inc. Send instant messages to your online friends http://au.messenger.yahoo.com -- 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