From mboxrd@z Thu Jan 1 00:00:00 1970 From: Boaz Harrosh Subject: Re: [PATCH, RFC 2/2] dax: use range_lock instead of i_mmap_lock Date: Wed, 12 Aug 2015 11:51:05 +0300 Message-ID: <55CB08F9.6030901@plexistor.com> References: <1439219664-88088-1-git-send-email-kirill.shutemov@linux.intel.com> <1439219664-88088-3-git-send-email-kirill.shutemov@linux.intel.com> <20150811081909.GD2650@quack.suse.cz> <20150811093708.GB906@dastard> <20150811135004.GC2659@quack.suse.cz> <55CA0728.7060001@plexistor.com> <100D68C7BA14664A8938383216E40DE040914C3E@FMSMSX114.amr.corp.intel.com> <20150811214822.GA20596@dastard> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Cc: Boaz Harrosh , Jan Kara , "Kirill A. Shutemov" , Andrew Morton , "linux-mm@kvack.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , Davidlohr Bueso To: Dave Chinner , "Wilcox, Matthew R" Return-path: In-Reply-To: <20150811214822.GA20596@dastard> Sender: owner-linux-mm@kvack.org List-Id: linux-fsdevel.vger.kernel.org On 08/12/2015 12:48 AM, Dave Chinner wrote: > On Tue, Aug 11, 2015 at 04:51:22PM +0000, Wilcox, Matthew R wrote: >> The race that you're not seeing is page fault vs page fault. Two >> threads each attempt to store a byte to different locations on the >> same page. With a read-mutex to exclude truncates, each thread >> calls ->get_block. One of the threads gets back a buffer marked >> as BH_New and calls memset() to clear the page. The other thread >> gets back a buffer which isn't marked as BH_New and simply inserts >> the mapping, returning to userspace, which stores the byte ... >> just in time for the other thread's memset() to write a zero over >> the top of it. > > So, this is not a truncate race that the XFS MMAPLOCK solves. > > However, that doesn't mean that the DAX code needs to add locking to > solve it. The race here is caused by block initialisation being > unserialised after a ->get_block call allocates the block (which the > filesystem serialises via internal locking). Hence two simultaneous > ->get_block calls to the same block is guaranteed to have the DAX > block initialisation race with the second ->get_block call that says > the block is already allocated. > > IOWs, the way to handle this is to have the ->get_block call handle > the block zeroing for new blocks instead of doing it after the fact > in the generic DAX code where there is no fine-grained serialisation > object available. By calling dax_clear_blocks() in the ->get_block > callback, the filesystem can ensure that the second racing call will > only make progress once the block has been fully initialised by the > first call. > > IMO the fix is - again - to move the functionality into the > filesystem where we already have the necessary exclusion in place to > avoid this race condition entirely. > Exactly, thanks > Cheers, > > Dave. > -- 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