From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f65.google.com ([209.85.218.65]:32984 "EHLO mail-oi0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752358AbeDGTiZ (ORCPT ); Sat, 7 Apr 2018 15:38:25 -0400 Received: by mail-oi0-f65.google.com with SMTP id 126-v6so4118754oig.0 for ; Sat, 07 Apr 2018 12:38:25 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <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> <20180404094656.dssixqvvdcp5jff2@quack2.suse.cz> From: Dan Williams Date: Sat, 7 Apr 2018 12:38:24 -0700 Message-ID: Subject: Re: [PATCH v8 15/18] mm, fs, dax: handle layout changes to pinned dax mappings To: Jan Kara Cc: 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 , Paul McKenney , Josh Triplett Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org List-ID: [ 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?