From: Dan Williams <dan.j.williams@intel.com> To: "Jérôme Glisse" <jglisse@redhat.com> Cc: Jan Kara <jack@suse.cz>, John Hubbard <jhubbard@nvidia.com>, Matthew Wilcox <willy@infradead.org>, 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>, Jason Gunthorpe <jgg@ziepe.ca>, Michal Hocko <mhocko@kernel.org>, Mike Marciniszyn <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: Wed, 12 Dec 2018 08:27:35 -0800 Message-ID: <CAPcyv4go0Xzhz8rXdfscWuXDu83BO9v8WD4upDUJWb7gKzX5OQ@mail.gmail.com> (raw) In-Reply-To: <20181212150319.GA3432@redhat.com> On Wed, Dec 12, 2018 at 7:03 AM Jerome Glisse <jglisse@redhat.com> wrote: > > On Mon, Dec 10, 2018 at 11:28:46AM +0100, Jan Kara wrote: > > On Fri 07-12-18 21:24:46, Jerome Glisse wrote: > > > Another crazy idea, why not treating GUP as another mapping of the page > > > and caller of GUP would have to provide either a fake anon_vma struct or > > > a fake vma struct (or both for PRIVATE mapping of a file where you can > > > have a mix of both private and file page thus only if it is a read only > > > GUP) that would get added to the list of existing mapping. > > > > > > So the flow would be: > > > somefunction_thatuse_gup() > > > { > > > ... > > > GUP(_fast)(vma, ..., fake_anon, fake_vma); > > > ... > > > } > > > > > > GUP(vma, ..., fake_anon, fake_vma) > > > { > > > if (vma->flags == ANON) { > > > // Add the fake anon vma to the anon vma chain as a child > > > // of current vma > > > } else { > > > // Add the fake vma to the mapping tree > > > } > > > > > > // The existing GUP except that now it inc mapcount and not > > > // refcount > > > GUP_old(..., &nanonymous, &nfiles); > > > > > > atomic_add(&fake_anon->refcount, nanonymous); > > > atomic_add(&fake_vma->refcount, nfiles); > > > > > > return nanonymous + nfiles; > > > } > > > > Thanks for your idea! This is actually something like I was suggesting back > > at LSF/MM in Deer Valley. There were two downsides to this I remember > > people pointing out: > > > > 1) This cannot really work with __get_user_pages_fast(). You're not allowed > > to get necessary locks to insert new entry into the VMA tree in that > > context. So essentially we'd loose get_user_pages_fast() functionality. > > > > 2) The overhead e.g. for direct IO may be noticeable. You need to allocate > > the fake tracking VMA, get VMA interval tree lock, insert into the tree. > > Then on IO completion you need to queue work to unpin the pages again as you > > cannot remove the fake VMA directly from interrupt context where the IO is > > completed. > > > > You are right that the cost could be amortized if gup() is called for > > multiple consecutive pages however for small IOs there's no help... > > > > So this approach doesn't look like a win to me over using counter in struct > > page and I'd rather try looking into squeezing HMM public page usage of > > struct page so that we can fit that gup counter there as well. I know that > > it may be easier said than done... > > So i want back to the drawing board and first i would like to ascertain > that we all agree on what the objectives are: > > [O1] Avoid write back from a page still being written by either a > device or some direct I/O or any other existing user of GUP. > This would avoid possible file system corruption. > > [O2] Avoid crash when set_page_dirty() is call on a page that is > considered clean by core mm (buffer head have been remove and > with some file system this turns into an ugly mess). > > [O3] DAX and the device block problems, ie with DAX the page map in > userspace is the same as the block (persistent memory) and no > filesystem nor block device understand page as block or pinned > block. > > For [O3] i don't think any pin count would help in anyway. I believe > that the current long term GUP API that does not allow GUP of DAX is > the only sane solution for now. No, that's not a sane solution, it's an emergency hack. > The real fix would be to teach file- > system about DAX/pinned block so that a pinned block is not reuse > by filesystem. We already have taught filesystems about pinned dax pages, see dax_layout_busy_page(). As much as possible I want to eliminate the concept of "dax pages" as a special case that gets sprinkled throughout the mm. > For [O1] and [O2] i believe a solution with mapcount would work. So > no new struct, no fake vma, nothing like that. In GUP for file back > pages With get_user_pages_fast() we don't know that we have a file-backed page, because we don't have a vma.
next prev parent reply index 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 [this message] 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 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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