Messages in this thread | | | Date | Mon, 19 Mar 2007 21:28:00 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH 1 of 2] block_page_mkwrite() Implementation V2 |
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Nick Piggin wrote: > David Chinner wrote: > >> On Mon, Mar 19, 2007 at 05:37:03PM +1100, Nick Piggin wrote: >> >>> David Chinner wrote: >>> > >>>> +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. >> >> >> >> We have to read the inode size in the normal case so that we know if >> the page is at EOF and is a partial page so we don't allocate past EOF in >> block_prepare_write(). Hence it seems like a no-brainer to me to check >> and error out on a page that we *know* is beyond EOF. >> >> I can drop the check if you see no value in it - I just don't >> like the idea of ignoring obvious boundary condition violations... > > > I would prefer it dropped, to be honest. I can see how the check does > pick up that corner case, however truncate is difficult enough (at > least, it has been an endless source of problems) that we want to keep > everyone else simple and have all the non-trivial stuff in truncate. >
Hmm, actually on second thoughts it probably is reasonable to recheck i_size under the page lock... we need to do similar in the nopage path to close the nopage vs invalidate race.
However, the already-truncated test I think can just be !page->mapping: there should be no way for the page mapping to change to something other than NULL.
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