From: Jan Kara <jack@suse.cz> To: Dave Chinner <david@fromorbit.com> Cc: Jan Kara <jack@suse.cz>, Jason Gunthorpe <jgg@ziepe.ca>, Jerome Glisse <jglisse@redhat.com>, John Hubbard <jhubbard@nvidia.com>, Matthew Wilcox <willy@infradead.org>, Dan Williams <dan.j.williams@intel.com>, John Hubbard <john.hubbard@gmail.com>, Andrew Morton <akpm@linux-foundation.org>, Linux MM <linux-mm@kvack.org>, tom@talpey.com, Al Viro <viro@zeniv.linux.org.uk>, benve@cisco.com, Christoph Hellwig <hch@infradead.org>, Christopher Lameter <cl@linux.com>, "Dalessandro, Dennis" <dennis.dalessandro@intel.com>, Doug Ledford <dledford@redhat.com>, Michal Hocko <mhocko@kernel.org>, mike.marciniszyn@intel.com, rcampbell@nvidia.com, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org> Subject: Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions Date: Thu, 20 Dec 2018 10:07:53 +0100 [thread overview] Message-ID: <20181220090753.GB27845@quack2.suse.cz> (raw) In-Reply-To: <20181219223312.GP6311@dastard> On Thu 20-12-18 09:33:12, Dave Chinner wrote: > On Wed, Dec 19, 2018 at 12:35:40PM +0100, Jan Kara wrote: > > On Wed 19-12-18 21:28:25, Dave Chinner wrote: > > > On Tue, Dec 18, 2018 at 08:03:29PM -0700, Jason Gunthorpe wrote: > > > > On Wed, Dec 19, 2018 at 10:42:54AM +1100, Dave Chinner wrote: > > > > > > > > > Essentially, what we are talking about is how to handle broken > > > > > hardware. I say we should just brun it with napalm and thermite > > > > > (i.e. taint the kernel with "unsupportable hardware") and force > > > > > wait_for_stable_page() to trigger when there are GUP mappings if > > > > > the underlying storage doesn't already require it. > > > > > > > > If you want to ban O_DIRECT/etc from writing to file backed pages, > > > > then just do it. > > > > > > O_DIRECT IO *isn't the problem*. > > > > That is not true. O_DIRECT IO is a problem. In some aspects it is easier > > than the problem with RDMA but currently O_DIRECT IO can crash your machine > > or corrupt data the same way RDMA can. > > It's not O_DIRECT - it's a ""transient page pin". Yes, there are > problems with that right now, but as we've discussed the issues can > be avoided by: > > a) stable pages always blocking in ->page_mkwrite; > b) blocking in write_cache_pages() on an elevated map count > when WB_SYNC_ALL is set; and > c) blocking in truncate_pagecache() on an elevated map > count. > > That prevents: > a) gup pinning a page that is currently under writeback and > modifying it while IO is in flight; > b) a dirty page being written back while it is pinned by > GUP, thereby turning it clean before the gup reference calls > set_page_dirty() on DMA completion; and This is not prevented by what you wrote above as currently GUP does not increase page->_mapcount. Currently, there's no way to distinguish GUP page reference from any other page reference - GUP simply does get_page() - and big part of this thread as I see it is exactly about how to introduce this distinction and how to convert all GUP users to the new convention safely (as currently they just pass struct page * pointers around and eventually do put_page() on them). Increasing page->_mapcount in GUP and trying to deduce the pin count from that is one option Jerome suggested. At this point I'm not 100% sure this is going to work but we'll see. > c) truncate/hole punch for pulling the page out from under > the gup operation that is ongoing. > > This is an adequate solution for a short term transient pins. It > doesn't break fsync(), it doesn't change how truncate works and it > fixes the problem where a mapped file is the buffer for an O_DIRECT > IO rather than the open fd and that buffer file gets truncated. > IOWs, transient pins (and hence O_DIRECT) is not really the problem > here. For now let's assume that the mechanism how to detect page pinned by GUP is actually somehow solved and we have already page_pinned() implemented. Then what you suggest can actually create a deadlock AFAICS: Process 1: Process 2: fsync("file") /* Evil memory buffer with page order reversed */ addr1 = mmap(NULL, 4096, PROT_WRITE, MAP_SHARED, "file", 4096); addr2 = mmap(addr1+4096, 4096, PROT_WRITE, MAP_SHARED, "file", 0); /* Fault in pages */ *addr1 = 0; *addr2 = 0; adds page with index 0 to bio fd = open("file2", O_RDWR | O_DIRECT); read(fd, addr1, 8192) -> eventually gets to iov_iter_get_pages() and then to get_user_pages_fast(). -> pins "file" page with index 1 blocks on pin for page with index 1 -> blocks in PageWriteback for page with index 0 Possibility of deadlocks like this is why I've decided it will be easier to just bounce the page for writeback we cannot avoid rather than block the writeback... > The problem with this is that blocking on elevated map count does > not work for long term pins (i.e. gup_longterm()) which are defined > as: > > * "longterm" == userspace controlled elevated page count lifetime. > * Contrast this to iov_iter_get_pages() usages which are transient. > > It's the "userspace controlled" part of the long term gup pin that > is the problem we need to solve. If we treat them the same as a > transient pin, then this leads to fsync() and truncate either > blocking for a long time waiting for userspace to drop it's gup > reference, or having to be failed with something like EBUSY or > EAGAIN. I agree. "userspace controlled" pins are another big problem to solve. > This is the problem revokable file layout leases solve. The NFS > server is already using this for revoking delegations from remote > clients. Userspace holding long term GUP references is essentially > the same thing - it's a delegation of file ownership to userspace > that the filesystem must be able to revoke when it needs to run > internal and/or 3rd-party requested operations on that delegated > file. > > If the hardware supports page faults, then we can further optimise > the long term pin case to relax stable page requirements and allow > page cleaning to occur while there are long term pins. In this case, > the hardware will write-fault the clean pages appropriately before > DMA is initiated, and hence avoid the need for data integrity > operations like fsync() to trigger lease revocation. However, > truncate/hole punch still requires lease revocation to work sanely, > especially when we consider DAX *must* ensure there are no remaining > references to the physical pmem page after the space has been freed. > > i.e. conflating the transient and long term gup pins as the same > problem doesn't help anyone. If we fix the short term pin problems, > then the long term pin problem become tractable by adding a layer > over the top (i.e. hardware page fault capability and/or file lease > requirements). Existing apps and hardware will continue to work - > external operations on the pinned file will simply hang rather than > causing corruption or kernel crashes. New (or updated) applications > will play nicely with lease revocation and at that point the "long > term pin" basically becomes a transient pin where the unpin latency > is determined by how quickly the app responds to the lease > revocation. And page fault capable hardware will reduce the > occurrence of lease revocations due to data writeback/integrity > operations and behave almost identically to cpu-based mmap accesses > to file backed pages. Agreed. I think we are on the same page wrt this. Just at this point I'm trying to solve the "transient pin" problem... Honza -- Jan Kara <jack@suse.com> SUSE Labs, CR
next prev parent reply other threads:[~2018-12-20 9:07 UTC|newest] Thread overview: 213+ messages / expand[flat|nested] mbox.gz Atom feed top 2018-12-04 0:17 [PATCH 0/2] put_user_page*(): start converting the call sites john.hubbard 2018-12-04 0:17 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard 2018-12-04 7:53 ` Mike Rapoport 2018-12-05 1:40 ` John Hubbard 2018-12-04 20:28 ` Dan Williams 2018-12-04 21:56 ` John Hubbard 2018-12-04 23:03 ` Dan Williams 2018-12-05 0:36 ` Jerome Glisse 2018-12-05 0:40 ` Dan Williams 2018-12-05 0:59 ` John Hubbard 2018-12-05 0:58 ` John Hubbard 2018-12-05 1:00 ` Dan Williams 2018-12-05 1:15 ` Matthew Wilcox 2018-12-05 1:44 ` Jerome Glisse 2018-12-05 1:57 ` John Hubbard 2018-12-07 2:45 ` John Hubbard 2018-12-07 19:16 ` Jerome Glisse 2018-12-07 19:26 ` Dan Williams 2018-12-07 19:40 ` Jerome Glisse 2018-12-08 0:52 ` John Hubbard 2018-12-08 2:24 ` Jerome Glisse 2018-12-10 10:28 ` Jan Kara 2018-12-12 15:03 ` Jerome Glisse 2018-12-12 16:27 ` Dan Williams 2018-12-12 17:02 ` Jerome Glisse 2018-12-12 17:49 ` Dan Williams 2018-12-12 19:07 ` John Hubbard 2018-12-12 21:30 ` Jerome Glisse 2018-12-12 21:40 ` Dan Williams 2018-12-12 21:53 ` Jerome Glisse 2018-12-12 22:11 ` Matthew Wilcox 2018-12-12 22:16 ` Jerome Glisse 2018-12-12 23:37 ` Jason Gunthorpe 2018-12-12 23:46 ` John Hubbard 2018-12-12 23:54 ` Dan Williams 2018-12-13 0:01 ` Jerome Glisse 2018-12-13 0:18 ` Dan Williams 2018-12-13 0:44 ` Jerome Glisse 2018-12-13 3:26 ` Jason Gunthorpe 2018-12-13 3:20 ` Jason Gunthorpe 2018-12-13 12:43 ` Jerome Glisse 2018-12-13 13:40 ` Tom Talpey 2018-12-13 14:18 ` Jerome Glisse 2018-12-13 14:51 ` Tom Talpey 2018-12-13 15:18 ` Jerome Glisse 2018-12-13 18:12 ` Tom Talpey 2018-12-13 19:18 ` Jerome Glisse 2018-12-14 10:41 ` Jan Kara 2018-12-14 15:25 ` Jerome Glisse 2018-12-12 21:56 ` John Hubbard 2018-12-12 22:04 ` Jerome Glisse 2018-12-12 22:11 ` John Hubbard 2018-12-12 22:14 ` Jerome Glisse 2018-12-12 22:17 ` John Hubbard 2018-12-12 21:46 ` Dave Chinner 2018-12-12 21:59 ` Jerome Glisse 2018-12-13 0:51 ` Dave Chinner 2018-12-13 2:02 ` Jerome Glisse 2018-12-13 15:56 ` Christopher Lameter 2018-12-13 16:02 ` Jerome Glisse 2018-12-14 6:00 ` Dave Chinner 2018-12-14 15:13 ` Jerome Glisse 2018-12-14 3:52 ` John Hubbard 2018-12-14 5:21 ` Dan Williams 2018-12-14 6:11 ` John Hubbard 2018-12-14 15:20 ` Jerome Glisse 2018-12-14 19:38 ` Dan Williams 2018-12-14 19:48 ` Matthew Wilcox 2018-12-14 19:53 ` Dave Hansen 2018-12-14 20:03 ` Matthew Wilcox 2018-12-14 20:17 ` Dan Williams 2018-12-14 20:29 ` Matthew Wilcox 2018-12-15 0:41 ` John Hubbard 2018-12-17 8:56 ` Jan Kara 2018-12-17 18:28 ` Dan Williams 2018-12-14 15:43 ` Jan Kara 2018-12-16 21:58 ` Dave Chinner 2018-12-17 18:11 ` Jerome Glisse 2018-12-17 18:34 ` Matthew Wilcox 2018-12-17 19:48 ` Jerome Glisse 2018-12-17 19:51 ` Matthew Wilcox 2018-12-17 19:54 ` Jerome Glisse 2018-12-17 19:59 ` Matthew Wilcox 2018-12-17 20:55 ` Jerome Glisse 2018-12-17 21:03 ` Matthew Wilcox 2018-12-17 21:15 ` Jerome Glisse 2018-12-18 1:09 ` Dave Chinner 2018-12-18 6:12 ` Darrick J. Wong 2018-12-18 9:30 ` Jan Kara 2018-12-18 23:29 ` John Hubbard 2018-12-19 2:07 ` Jerome Glisse 2018-12-19 11:08 ` Jan Kara 2018-12-20 10:54 ` John Hubbard 2018-12-20 16:50 ` Jerome Glisse 2018-12-20 16:57 ` Dan Williams 2018-12-20 16:49 ` Jerome Glisse 2019-01-03 1:55 ` Jerome Glisse 2019-01-03 3:27 ` John Hubbard 2019-01-03 14:57 ` Jerome Glisse 2019-01-03 9:26 ` Jan Kara 2019-01-03 14:44 ` Jerome Glisse 2019-01-11 2:59 ` John Hubbard 2019-01-11 2:59 ` John Hubbard 2019-01-11 16:51 ` Jerome Glisse 2019-01-11 16:51 ` Jerome Glisse 2019-01-12 1:04 ` John Hubbard 2019-01-12 1:04 ` John Hubbard 2019-01-12 2:02 ` Jerome Glisse 2019-01-12 2:02 ` Jerome Glisse 2019-01-12 2:38 ` John Hubbard 2019-01-12 2:38 ` John Hubbard 2019-01-12 2:46 ` Jerome Glisse 2019-01-12 2:46 ` Jerome Glisse 2019-01-12 3:06 ` John Hubbard 2019-01-12 3:06 ` John Hubbard 2019-01-12 3:25 ` Jerome Glisse 2019-01-12 3:25 ` Jerome Glisse 2019-01-12 20:46 ` John Hubbard 2019-01-12 20:46 ` John Hubbard 2019-01-14 14:54 ` Jan Kara 2019-01-14 14:54 ` Jan Kara 2019-01-14 17:21 ` Jerome Glisse 2019-01-14 17:21 ` Jerome Glisse 2019-01-14 19:09 ` John Hubbard 2019-01-14 19:09 ` John Hubbard 2019-01-15 8:34 ` Jan Kara 2019-01-15 8:34 ` Jan Kara 2019-01-15 21:39 ` John Hubbard 2019-01-15 21:39 ` John Hubbard 2019-01-15 8:07 ` Jan Kara 2019-01-15 8:07 ` Jan Kara 2019-01-15 17:15 ` Jerome Glisse 2019-01-15 17:15 ` Jerome Glisse 2019-01-15 21:56 ` John Hubbard 2019-01-15 21:56 ` John Hubbard 2019-01-15 22:12 ` Jerome Glisse 2019-01-15 22:12 ` Jerome Glisse 2019-01-16 0:44 ` John Hubbard 2019-01-16 0:44 ` John Hubbard 2019-01-16 1:56 ` Jerome Glisse 2019-01-16 1:56 ` Jerome Glisse 2019-01-16 2:01 ` Dan Williams 2019-01-16 2:01 ` Dan Williams 2019-01-16 2:23 ` Jerome Glisse 2019-01-16 2:23 ` Jerome Glisse 2019-01-16 4:34 ` Dave Chinner 2019-01-16 4:34 ` Dave Chinner 2019-01-16 14:50 ` Jerome Glisse 2019-01-16 14:50 ` Jerome Glisse 2019-01-16 22:51 ` Dave Chinner 2019-01-16 22:51 ` Dave Chinner 2019-01-16 11:38 ` Jan Kara 2019-01-16 11:38 ` Jan Kara 2019-01-16 13:08 ` Jerome Glisse 2019-01-16 13:08 ` Jerome Glisse 2019-01-17 5:42 ` John Hubbard 2019-01-17 5:42 ` John Hubbard 2019-01-17 15:21 ` Jerome Glisse 2019-01-17 15:21 ` Jerome Glisse 2019-01-18 0:16 ` Dave Chinner 2019-01-18 1:59 ` Jerome Glisse 2019-01-17 9:30 ` Jan Kara 2019-01-17 9:30 ` Jan Kara 2019-01-17 15:17 ` Jerome Glisse 2019-01-17 15:17 ` Jerome Glisse 2019-01-22 15:24 ` Jan Kara 2019-01-22 16:46 ` Jerome Glisse 2019-01-23 18:02 ` Jan Kara 2019-01-23 19:04 ` Jerome Glisse 2019-01-29 0:22 ` John Hubbard 2019-01-29 1:23 ` Jerome Glisse 2019-01-29 6:41 ` John Hubbard 2019-01-29 10:12 ` Jan Kara 2019-01-30 2:21 ` John Hubbard 2019-01-17 5:25 ` John Hubbard 2019-01-17 5:25 ` John Hubbard 2019-01-17 9:04 ` Jan Kara 2019-01-17 9:04 ` Jan Kara 2019-01-12 3:14 ` Jerome Glisse 2019-01-12 3:14 ` Jerome Glisse 2018-12-18 10:33 ` Jan Kara 2018-12-18 23:42 ` Dave Chinner 2018-12-19 3:03 ` Jason Gunthorpe 2018-12-19 5:26 ` Dan Williams 2018-12-19 11:19 ` Jan Kara 2018-12-19 10:28 ` Dave Chinner 2018-12-19 11:35 ` Jan Kara 2018-12-19 16:56 ` Jason Gunthorpe 2018-12-19 22:33 ` Dave Chinner 2018-12-20 9:07 ` Jan Kara [this message] 2018-12-20 16:54 ` Jerome Glisse 2018-12-19 13:24 ` Jan Kara 2018-12-08 5:18 ` Matthew Wilcox 2018-12-12 19:13 ` John Hubbard 2018-12-08 7:16 ` Dan Williams 2018-12-08 16:33 ` Jerome Glisse 2018-12-08 16:48 ` Christoph Hellwig 2018-12-08 17:47 ` Jerome Glisse 2018-12-08 18:26 ` Christoph Hellwig 2018-12-08 18:45 ` Jerome Glisse 2018-12-08 18:09 ` Dan Williams 2018-12-08 18:12 ` Christoph Hellwig 2018-12-11 6:18 ` Dave Chinner 2018-12-05 5:52 ` Dan Williams 2018-12-05 11:16 ` Jan Kara 2018-12-04 0:17 ` [PATCH 2/2] infiniband/mm: convert put_page() to put_user_page*() john.hubbard 2018-12-04 17:10 ` [PATCH 0/2] put_user_page*(): start converting the call sites David Laight 2018-12-05 1:05 ` John Hubbard 2018-12-05 14:08 ` David Laight 2018-12-28 8:37 ` Pavel Machek 2019-02-08 7:56 [PATCH 0/2] mm: put_user_page() call site conversion first john.hubbard 2019-02-08 7:56 ` [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions john.hubbard 2019-02-08 10:32 ` Mike Rapoport 2019-02-08 20:44 ` John Hubbard
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