From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com [148.163.156.1]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ml01.01.org (Postfix) with ESMTPS id 1340A226516D8 for ; Sat, 7 Apr 2018 20:10:20 -0700 (PDT) Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.22/8.16.0.22) with SMTP id w3839Nr7138560 for ; Sat, 7 Apr 2018 23:10:20 -0400 Received: from e13.ny.us.ibm.com (e13.ny.us.ibm.com [129.33.205.203]) by mx0a-001b2d01.pphosted.com with ESMTP id 2h6t2yb0dw-1 (version=TLSv1.2 cipher=AES256-SHA256 bits=256 verify=NOT) for ; Sat, 07 Apr 2018 23:10:19 -0400 Received: from localhost by e13.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Sat, 7 Apr 2018 23:10:18 -0400 Date: Sat, 7 Apr 2018 20:11:13 -0700 From: "Paul E. McKenney" Subject: Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings 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> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: Message-Id: <20180408031113.GO3948@linux.vnet.ibm.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: paulmck@linux.vnet.ibm.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-nvdimm-bounces@lists.01.org Sender: "Linux-nvdimm" To: Dan Williams Cc: Dave Hansen , Jan Kara , Andrew Morton , Mike Snitzer , Matthew Wilcox , linux-nvdimm , Dave Chinner , Linux Kernel Mailing List , Christoph Hellwig , linux-xfs , Alexander Viro , linux-fsdevel , Josh Triplett , "Darrick J. Wong" List-ID: 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. Thanx, Paul _______________________________________________ Linux-nvdimm mailing list Linux-nvdimm@lists.01.org https://lists.01.org/mailman/listinfo/linux-nvdimm