From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx2.suse.de ([195.135.220.15]:40408 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751155AbeDDJrB (ORCPT ); Wed, 4 Apr 2018 05:47:01 -0400 Date: Wed, 4 Apr 2018 11:46:56 +0200 From: Jan Kara To: Dan Williams Cc: linux-nvdimm@lists.01.org, Jan Kara , Jeff Moyer , Dave Chinner , Matthew Wilcox , Alexander Viro , "Darrick J. Wong" , Ross Zwisler , Dave Hansen , Andrew Morton , Christoph Hellwig , linux-fsdevel@vger.kernel.org, linux-xfs@vger.kernel.org, linux-kernel@vger.kernel.org, snitzer@redhat.com Subject: Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings Message-ID: <20180404094656.dssixqvvdcp5jff2@quack2.suse.cz> References: <152246892890.36038.18436540150980653229.stgit@dwillia2-desk3.amr.corp.intel.com> <152246901060.36038.4487158506830998280.stgit@dwillia2-desk3.amr.corp.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <152246901060.36038.4487158506830998280.stgit@dwillia2-desk3.amr.corp.intel.com> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: 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. > + while (index < end && pagevec_lookup_entries(&pvec, mapping, index, > + min(end - index, (pgoff_t)PAGEVEC_SIZE), > + indices)) { > + for (i = 0; i < pagevec_count(&pvec); i++) { > + struct page *pvec_ent = pvec.pages[i]; > + void *entry; > + > + index = indices[i]; > + if (index >= end) > + break; > + > + if (!radix_tree_exceptional_entry(pvec_ent)) > + continue; This would be a bug - so WARN_ON_ONCE() here? > + > + spin_lock_irq(&mapping->tree_lock); > + entry = get_unlocked_mapping_entry(mapping, index, NULL); > + if (entry) > + page = dax_busy_page(entry); > + put_unlocked_mapping_entry(mapping, index, entry); > + spin_unlock_irq(&mapping->tree_lock); > + if (page) > + break; > + } > + pagevec_remove_exceptionals(&pvec); > + pagevec_release(&pvec); > + index++; > + > + if (page) > + break; > + } > + return page; > +} > +EXPORT_SYMBOL_GPL(dax_layout_busy_page); > + > static int __dax_invalidate_mapping_entry(struct address_space *mapping, > pgoff_t index, bool trunc) > { Honza -- Jan Kara SUSE Labs, CR