From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f176.google.com (mail-pd0-f176.google.com [209.85.192.176]) by kanga.kvack.org (Postfix) with ESMTP id 13B256B0031 for ; Fri, 4 Oct 2013 16:41:47 -0400 (EDT) Received: by mail-pd0-f176.google.com with SMTP id q10so4507928pdj.21 for ; Fri, 04 Oct 2013 13:41:46 -0700 (PDT) Message-ID: <524F282B.2080809@gmail.com> Date: Fri, 04 Oct 2013 16:42:19 -0400 From: KOSAKI Motohiro MIME-Version: 1.0 Subject: Re: [PATCH 0/26] get_user_pages() cleanup References: <1380724087-13927-1-git-send-email-jack@suse.cz> <20131002162009.GA5778@infradead.org> <20131002202941.GF16998@quack.suse.cz> <524F2592.1040305@gmail.com> In-Reply-To: <524F2592.1040305@gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Jan Kara Cc: Christoph Hellwig , LKML , linux-mm@kvack.org, Alexander Viro , Andreas Dilger , Andy Walls , Arnd Bergmann , Benjamin LaHaise , ceph-devel@vger.kernel.org, Dan Williams , David Airlie , dri-devel@lists.freedesktop.org, Gleb Natapov , Greg Kroah-Hartman , hpdd-discuss@ml01.01.org, Jarod Wilson , Jayant Mangalampalli , Jean-Christophe Plagniol-Villard , Jesper Nilsson , Kai Makisara , kvm@vger.kernel.org, Laurent Pinchart , linux-aio@kvack.org, linux-cris-kernel@axis.com, linux-fbdev@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-media@vger.kernel.org, linux-nfs@vger.kernel.org, linux-rdma@vger.kernel.org, linux-scsi@vger.kernel.org, Manu Abraham , Mark Allyn , Mikael Starvik , Mike Marciniszyn , Naren Sankar , Paolo Bonzini , Peng Tao , Roland Dreier , Sage Weil , Scott Davilla , Timur Tabi , Tomi Valkeinen , Tony Luck , Trond Myklebust (10/4/13 4:31 PM), KOSAKI Motohiro wrote: > (10/2/13 4:29 PM), Jan Kara wrote: >> On Wed 02-10-13 09:20:09, Christoph Hellwig wrote: >>> On Wed, Oct 02, 2013 at 04:27:41PM +0200, Jan Kara wrote: >>>> Hello, >>>> >>>> In my quest for changing locking around page faults to make things easier for >>>> filesystems I found out get_user_pages() users could use a cleanup. The >>>> knowledge about necessary locking for get_user_pages() is in tons of places in >>>> drivers and quite a few of them actually get it wrong (don't have mmap_sem when >>>> calling get_user_pages() or hold mmap_sem when calling copy_from_user() in the >>>> surrounding code). Rather often this actually doesn't seem necessary. This >>>> patch series converts lots of places to use either get_user_pages_fast() >>>> or a new simple wrapper get_user_pages_unlocked() to remove the knowledge >>>> of mmap_sem from the drivers. I'm still looking into converting a few remaining >>>> drivers (most notably v4l2) which are more complex. >>> >>> Even looking over the kerneldoc comment next to it I still fail to >>> understand when you'd want to use get_user_pages_fast and when not. >> AFAIU get_user_pages_fast() should be used >> 1) if you don't need any special get_user_pages() arguments (like calling >> it for mm of a different process, forcing COW, or similar). >> 2) you don't expect pages to be unmapped (then get_user_pages_fast() is >> actually somewhat slower because it walks page tables twice). > > If target page point to anon or private mapping pages, get_user_pages_fast() > is fork unsafe. O_DIRECT man pages describe a bit about this. > > > see http://man7.org/linux/man-pages/man2/open.2.html > >> O_DIRECT I/Os should never be run concurrently with the fork(2) >> system call, if the memory buffer is a private mapping (i.e., any >> mapping created with the mmap(2) MAP_PRIVATE flag; this includes >> memory allocated on the heap and statically allocated buffers). Any >> such I/Os, whether submitted via an asynchronous I/O interface or >> from another thread in the process, should be completed before >> fork(2) is called. Failure to do so can result in data corruption >> and undefined behavior in parent and child processes. This >> restriction does not apply when the memory buffer for the O_DIRECT >> I/Os was created using shmat(2) or mmap(2) with the MAP_SHARED flag. >> Nor does this restriction apply when the memory buffer has been >> advised as MADV_DONTFORK with madvise(2), ensuring that it will not >> be available to the child after fork(2). IMHO, get_user_pages_fast() should be renamed to get_user_pages_quirk(). Its semantics is not equal to get_user_pages(). When someone simply substitute get_user_pages() to get_user_pages_fast(), they might see huge trouble. -- 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