linux-ext4 - Re: [PATCH v3] fs: Fix page

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Date:   Wed, 18 Dec 2019 11:21:44 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     Andreas Gruenbacher <agruenba@...hat.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Jeff Layton <jlayton@...nel.org>,
        Sage Weil <sage@...hat.com>, Ilya Dryomov <idryomov@...il.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jaegeuk Kim <jaegeuk@...nel.org>, Chao Yu <chao@...nel.org>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Richard Weinberger <richard@....at>,
        Artem Bityutskiy <dedekind1@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        ceph-devel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-mtd@...ts.infradead.org, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        David Sterba <dsterba@...e.com>, linux-btrfs@...r.kernel.org,
        Jan Kara <jack@...e.cz>
Subject: Re: [PATCH v3] fs: Fix page_mkwrite off-by-one errors

On Wed, Dec 18, 2019 at 10:52:16AM -0800, Darrick J. Wong wrote:
> > @@ -9016,13 +9016,11 @@ vm_fault_t btrfs_page_mkwrite(struct vm_fault *vmf)
> >  	ret = VM_FAULT_NOPAGE; /* make the VM retry the fault */
> >  again:
> >  	lock_page(page);
> > -	size = i_size_read(inode);
> >  
> > -	if ((page->mapping != inode->i_mapping) ||
> > -	    (page_start >= size)) {
> > -		/* page got truncated out from underneath us */
> > +	ret2 = page_mkwrite_check_truncate(page, inode);
> > +	if (ret2 < 0)
> >  		goto out_unlock;
> 
> ...here we try to return -EFAULT as vm_fault_t.  Notice how btrfs returns
> VM_FAULT_* values directly and never calls block_page_mkwrite_return?  I
> know dsterba acked this, but I cannot see how this is correct?

I think you misread it.  'ret2' is never returned; we'll end up returning
VM_FAULT_NOPAGE here.  Arguably it should be SIGBUS or something, but
I think retrying the fault will also end up giving a SIGBUS.