From: Jan Kara Subject: Re: [PATCH 1/2] ext4: Close race between direct IO and ext4_break_layouts() Date: Wed, 8 Aug 2018 10:49:13 +0200 Message-ID: <20180808084913.GB15413@quack2.suse.cz> References: <153367989755.37314.6889218648604435494.stgit@djiang5-desk3.ch.intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: lczerner-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, jack-AlSwsSmVLrQ@public.gmane.org, linux-nvdimm-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org, darrick.wong-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org, david-FqsqvQoI3Ljby3iVrkZq2A@public.gmane.org, linux-xfs-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, zwisler-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org, linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, tytso-3s7WtUTddSA@public.gmane.org, linux-ext4-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, hch-jcswGhMUV9g@public.gmane.org To: Dave Jiang Return-path: Content-Disposition: inline In-Reply-To: <153367989755.37314.6889218648604435494.stgit-Cxk7aZI4ujnJARH06PadV2t3HXsI98Cx0E9HWUfgJXw@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-ext4.vger.kernel.org On Tue 07-08-18 15:11:37, Dave Jiang wrote: > From: Ross Zwisler > > If the refcount of a page is lowered between the time that it is returned > by dax_busy_page() and when the refcount is again checked in > ext4_break_layouts() => ___wait_var_event(), the waiting function > ext4_wait_dax_page() will never be called. This means that > ext4_break_layouts() will still have 'retry' set to false, so we'll stop > looping and never check the refcount of other pages in this inode. > > Instead, always continue looping as long as dax_layout_busy_page() gives us > a page which it found with an elevated refcount. > > Note that this works around the race exposed by my unit test, but I think > that there is another race that needs to be addressed, probably with > additional synchronization added between direct I/O and > {ext4,xfs}_break_layouts(). I'd just note that the race Ross suspected should be properly handled by dax_layout_busy_page() so I think this last paragraph from the changelog can go. Also Ted, this fixes a problem in the DAX truncate patches you currently carry in your tree so you can consider just pushing it with them during the merge window. It's not necessary though - the patches already make the problematic behavior much less likely, this patch just hopefully completely closes the race window. > Signed-off-by: Ross Zwisler > Reviewed-by: Jan Kara Honza > --- > fs/ext4/inode.c | 9 +++------ > 1 file changed, 3 insertions(+), 6 deletions(-) > > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c > index 8f6ad7667974..d2663a1e3ec2 100644 > --- a/fs/ext4/inode.c > +++ b/fs/ext4/inode.c > @@ -4191,9 +4191,8 @@ int ext4_update_disksize_before_punch(struct inode *inode, loff_t offset, > return 0; > } > > -static void ext4_wait_dax_page(struct ext4_inode_info *ei, bool *did_unlock) > +static void ext4_wait_dax_page(struct ext4_inode_info *ei) > { > - *did_unlock = true; > up_write(&ei->i_mmap_sem); > schedule(); > down_write(&ei->i_mmap_sem); > @@ -4203,14 +4202,12 @@ int ext4_break_layouts(struct inode *inode) > { > struct ext4_inode_info *ei = EXT4_I(inode); > struct page *page; > - bool retry; > int error; > > if (WARN_ON_ONCE(!rwsem_is_locked(&ei->i_mmap_sem))) > return -EINVAL; > > do { > - retry = false; > page = dax_layout_busy_page(inode->i_mapping); > if (!page) > return 0; > @@ -4218,8 +4215,8 @@ int ext4_break_layouts(struct inode *inode) > error = ___wait_var_event(&page->_refcount, > atomic_read(&page->_refcount) == 1, > TASK_INTERRUPTIBLE, 0, 0, > - ext4_wait_dax_page(ei, &retry)); > - } while (error == 0 && retry); > + ext4_wait_dax_page(ei)); > + } while (error == 0); > > return error; > } > -- Jan Kara SUSE Labs, CR