From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:60377 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753243AbeDIQjN (ORCPT ); Mon, 9 Apr 2018 12:39:13 -0400 Date: Mon, 9 Apr 2018 18:39:10 +0200 From: Jan Kara To: "Paul E. McKenney" Cc: Dan Williams , Jan Kara , linux-nvdimm , Jeff Moyer , Dave Chinner , Matthew Wilcox , Alexander Viro , "Darrick J. Wong" , Ross Zwisler , Dave Hansen , Andrew Morton , Christoph Hellwig , linux-fsdevel , linux-xfs , Linux Kernel Mailing List , Mike Snitzer , Josh Triplett Subject: Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings Message-ID: <20180409163910.2f6t734pwefylak3@quack2.suse.cz> References: <152246892890.36038.18436540150980653229.stgit@dwillia2-desk3.amr.corp.intel.com> <152246901060.36038.4487158506830998280.stgit@dwillia2-desk3.amr.corp.intel.com> <20180404094656.dssixqvvdcp5jff2@quack2.suse.cz> <20180408031113.GO3948@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180408031113.GO3948@linux.vnet.ibm.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Sat 07-04-18 20:11:13, Paul E. McKenney wrote: > On Sat, Apr 07, 2018 at 12:38:24PM -0700, Dan Williams wrote: > > [ adding Paul and Josh ] > > > > On Wed, Apr 4, 2018 at 2:46 AM, Jan Kara wrote: > > > On Fri 30-03-18 21:03:30, Dan Williams wrote: > > >> Background: > > >> > > >> get_user_pages() in the filesystem pins file backed memory pages for > > >> access by devices performing dma. However, it only pins the memory pages > > >> not the page-to-file offset association. If a file is truncated the > > >> pages are mapped out of the file and dma may continue indefinitely into > > >> a page that is owned by a device driver. This breaks coherency of the > > >> file vs dma, but the assumption is that if userspace wants the > > >> file-space truncated it does not matter what data is inbound from the > > >> device, it is not relevant anymore. The only expectation is that dma can > > >> safely continue while the filesystem reallocates the block(s). > > >> > > >> Problem: > > >> > > >> This expectation that dma can safely continue while the filesystem > > >> changes the block map is broken by dax. With dax the target dma page > > >> *is* the filesystem block. The model of leaving the page pinned for dma, > > >> but truncating the file block out of the file, means that the filesytem > > >> is free to reallocate a block under active dma to another file and now > > >> the expected data-incoherency situation has turned into active > > >> data-corruption. > > >> > > >> Solution: > > >> > > >> Defer all filesystem operations (fallocate(), truncate()) on a dax mode > > >> file while any page/block in the file is under active dma. This solution > > >> assumes that dma is transient. Cases where dma operations are known to > > >> not be transient, like RDMA, have been explicitly disabled via > > >> commits like 5f1d43de5416 "IB/core: disable memory registration of > > >> filesystem-dax vmas". > > >> > > >> The dax_layout_busy_page() routine is called by filesystems with a lock > > >> held against mm faults (i_mmap_lock) to find pinned / busy dax pages. > > >> The process of looking up a busy page invalidates all mappings > > >> to trigger any subsequent get_user_pages() to block on i_mmap_lock. > > >> The filesystem continues to call dax_layout_busy_page() until it finally > > >> returns no more active pages. This approach assumes that the page > > >> pinning is transient, if that assumption is violated the system would > > >> have likely hung from the uncompleted I/O. > > >> > > >> Cc: Jan Kara > > >> Cc: Jeff Moyer > > >> Cc: Dave Chinner > > >> Cc: Matthew Wilcox > > >> Cc: Alexander Viro > > >> Cc: "Darrick J. Wong" > > >> Cc: Ross Zwisler > > >> Cc: Dave Hansen > > >> Cc: Andrew Morton > > >> Reported-by: Christoph Hellwig > > >> Reviewed-by: Christoph Hellwig > > >> Signed-off-by: Dan Williams > > >> --- > > >> drivers/dax/super.c | 2 + > > >> fs/dax.c | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++ > > >> include/linux/dax.h | 25 ++++++++++++++ > > >> mm/gup.c | 5 +++ > > >> 4 files changed, 123 insertions(+), 1 deletion(-) > > > > > > ... > > > > > >> +/** > > >> + * dax_layout_busy_page - find first pinned page in @mapping > > >> + * @mapping: address space to scan for a page with ref count > 1 > > >> + * > > >> + * DAX requires ZONE_DEVICE mapped pages. These pages are never > > >> + * 'onlined' to the page allocator so they are considered idle when > > >> + * page->count == 1. A filesystem uses this interface to determine if > > >> + * any page in the mapping is busy, i.e. for DMA, or other > > >> + * get_user_pages() usages. > > >> + * > > >> + * It is expected that the filesystem is holding locks to block the > > >> + * establishment of new mappings in this address_space. I.e. it expects > > >> + * to be able to run unmap_mapping_range() and subsequently not race > > >> + * mapping_mapped() becoming true. It expects that get_user_pages() pte > > >> + * walks are performed under rcu_read_lock(). > > >> + */ > > >> +struct page *dax_layout_busy_page(struct address_space *mapping) > > >> +{ > > >> + pgoff_t indices[PAGEVEC_SIZE]; > > >> + struct page *page = NULL; > > >> + struct pagevec pvec; > > >> + pgoff_t index, end; > > >> + unsigned i; > > >> + > > >> + /* > > >> + * In the 'limited' case get_user_pages() for dax is disabled. > > >> + */ > > >> + if (IS_ENABLED(CONFIG_FS_DAX_LIMITED)) > > >> + return NULL; > > >> + > > >> + if (!dax_mapping(mapping) || !mapping_mapped(mapping)) > > >> + return NULL; > > >> + > > >> + pagevec_init(&pvec); > > >> + index = 0; > > >> + end = -1; > > >> + /* > > >> + * Flush dax_layout_lock() sections to ensure all possible page > > >> + * references have been taken, or otherwise arrange for faults > > >> + * to block on the filesystem lock that is taken for > > >> + * establishing new mappings. > > >> + */ > > >> + unmap_mapping_range(mapping, 0, 0, 1); > > >> + synchronize_rcu(); > > > > > > So I still don't like the use of RCU for this. It just seems as an abuse to > > > use RCU like that. Furthermore it has a hefty latency cost for the truncate > > > path. A trivial test to truncate 100 times the last page of a 16k file that > > > is mmaped (only the first page): > > > > > > DAX+your patches 3.899s > > > non-DAX 0.015s > > > > > > So you can see synchronize_rcu() increased time to run truncate(2) more > > > than 200 times (the process is indeed sitting in __wait_rcu_gp all the > > > time). IMHO that's just too costly. > > > > I wonder if this can be trivially solved by using srcu. I.e. we don't > > need to wait for a global quiescent state, just a > > get_user_pages_fast() quiescent state. ...or is that an abuse of the > > srcu api? > > From what I can see (not that I claim to understand DAX), SRCU > is worth trying. Another thing to try (as a test) is to replace the > synchronize_rcu() above with synchronize_rcu_expedited(), which might > get you an order of magnitude or thereabouts. But having synchronize_rcu_expedited() easily triggerable by userspace (potentially every 100 usec or even less) is not a great thing, right? It would be hogging the system with IPIs... Honza -- Jan Kara SUSE Labs, CR