From: Jason Gunthorpe <jgg@nvidia.com>
To: "Kasireddy, Vivek" <vivek.kasireddy@intel.com>
Cc: Alistair Popple <apopple@nvidia.com>,
Gerd Hoffmann <kraxel@redhat.com>,
"Kim, Dongwon" <dongwon.kim@intel.com>,
David Hildenbrand <david@redhat.com>,
"Chang, Junxiao" <junxiao.chang@intel.com>,
Hugh Dickins <hughd@google.com>, Peter Xu <peterx@redhat.com>,
"linux-mm@kvack.org" <linux-mm@kvack.org>,
"dri-devel@lists.freedesktop.org"
<dri-devel@lists.freedesktop.org>,
Mike Kravetz <mike.kravetz@oracle.com>
Subject: Re: [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages)
Date: Sun, 30 Jul 2023 20:09:37 -0300 [thread overview]
Message-ID: <ZMbtsY4O3Ho3B69J@nvidia.com> (raw)
In-Reply-To: <CH3PR11MB71779C83F8A0EC6C2F3F4B0CF807A@CH3PR11MB7177.namprd11.prod.outlook.com>
On Sat, Jul 29, 2023 at 12:46:59AM +0000, Kasireddy, Vivek wrote:
> > Later the importer decides it needs the memory again so it again asks
> > for the dmabuf to be present, which does hmm_range_fault and gets
> > whatever is appropriate at the time.
> Unless I am missing something, I think just doing the above still won't solve
> the problem. Consider this sequence:
> write_to_memfd(addr1, size, 'a');
> buf = create_udmabuf_list(devfd, memfd, size);
> addr2 = mmap_fd(buf, NUM_PAGES * NUM_ENTRIES * getpagesize());
> read(addr2);
> write_to_memfd(addr1, size, 'b');
> punch_hole(memfd, MEMFD_SIZE / 2);
> -> Since we can process the invalidate at this point, as per your suggestion,
> we can trigger dmabuf move to let the importers know that the dmabuf's
> backing memory has changed (or moved).
>
> read(addr2);
> -> Because there is a hole, we can handle the read by either providing the
> old pages or zero pages (if using hmm_range_fault()) to the
> importers.
You never provide the old pages. After trunctate the only correct
value to read is zero.
> Maybe it is against convention, but I think it makes sense to provide old
> pages (that were mapped before the hole punch) because the importers
> have not read the data in these pages ('b' above) yet.
Nope.
> And, another reason to provide old pages is because the data in
> these pages is shown in a window on the Host's screen so it
> doesn't make sense to show zero page data.
So why did you trucate it if you want to keep the data?
> -> write_to_memfd(addr1, size, 'c');
> As the hole gets refilled (with new pages) after the above write, AFAIU, we
> have to tell the importers again that since the backing memory has changed,
> (new pages) they need to recreate their mappings. But herein lies the problem:
> from inside the udmabuf driver, we cannot know when this write occurs, so we
> would not be able to notify the importers of the dmabuf move.
You get another invalidate because the memfd removes the zero pages
that hmm_range_fault installed in the PTEs before replacing them with
actual writable pages. Then you do the move, and another
hmm_range_fault, and basically the whole thing over again. Except this
time instead of returning zero pages it returns actual writable page.
Jason
next prev parent reply other threads:[~2023-07-30 23:09 UTC|newest]
Thread overview: 64+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-07-18 8:28 [RFC v1 0/3] udmabuf: Replace pages when there is FALLOC_FL_PUNCH_HOLE in memfd Vivek Kasireddy
2023-07-18 8:28 ` [RFC v1 1/3] mm/mmu_notifier: Add a new notifier for mapping updates (new pages) Vivek Kasireddy
2023-07-18 15:36 ` Jason Gunthorpe
2023-07-19 0:05 ` Kasireddy, Vivek
2023-07-19 0:24 ` Jason Gunthorpe
2023-07-19 6:19 ` Kasireddy, Vivek
2023-07-19 2:08 ` Alistair Popple
2023-07-20 7:43 ` Kasireddy, Vivek
2023-07-20 9:00 ` Alistair Popple
2023-07-24 7:54 ` Kasireddy, Vivek
2023-07-24 13:35 ` Jason Gunthorpe
2023-07-24 20:32 ` Kasireddy, Vivek
2023-07-25 4:30 ` Hugh Dickins
2023-07-25 22:24 ` Kasireddy, Vivek
2023-07-27 21:43 ` Peter Xu
2023-07-29 0:08 ` Kasireddy, Vivek
2023-07-31 17:05 ` Peter Xu
2023-08-01 7:11 ` Kasireddy, Vivek
2023-08-01 21:57 ` Peter Xu
2023-08-03 8:08 ` Kasireddy, Vivek
2023-08-03 13:02 ` Peter Xu
2023-07-25 12:36 ` Jason Gunthorpe
2023-07-25 22:44 ` Kasireddy, Vivek
2023-07-25 22:53 ` Jason Gunthorpe
2023-07-27 7:34 ` Kasireddy, Vivek
2023-07-27 11:58 ` Jason Gunthorpe
2023-07-29 0:46 ` Kasireddy, Vivek
2023-07-30 23:09 ` Jason Gunthorpe [this message]
2023-08-01 5:32 ` Kasireddy, Vivek
2023-08-01 12:19 ` Jason Gunthorpe
2023-08-01 12:22 ` David Hildenbrand
2023-08-01 12:23 ` Jason Gunthorpe
2023-08-01 12:26 ` David Hildenbrand
2023-08-01 12:26 ` Jason Gunthorpe
2023-08-01 12:28 ` David Hildenbrand
2023-08-01 17:53 ` Kasireddy, Vivek
2023-08-01 18:19 ` Jason Gunthorpe
2023-08-03 7:35 ` Kasireddy, Vivek
2023-08-03 12:14 ` Jason Gunthorpe
2023-08-03 12:32 ` David Hildenbrand
2023-08-04 0:14 ` Alistair Popple
2023-08-04 6:39 ` Kasireddy, Vivek
2023-08-04 7:23 ` David Hildenbrand
2023-08-04 21:53 ` Kasireddy, Vivek
2023-08-04 12:49 ` Jason Gunthorpe
2023-08-08 7:37 ` Kasireddy, Vivek
2023-08-08 12:42 ` Jason Gunthorpe
2023-08-16 6:43 ` Kasireddy, Vivek
2023-08-21 9:02 ` Alistair Popple
2023-08-22 6:14 ` Kasireddy, Vivek
2023-08-22 8:15 ` Alistair Popple
2023-08-24 6:48 ` Kasireddy, Vivek
2023-08-28 4:38 ` Kasireddy, Vivek
2023-08-30 16:02 ` Jason Gunthorpe
2023-07-25 3:38 ` Alistair Popple
2023-07-24 13:36 ` Alistair Popple
2023-07-24 13:37 ` Jason Gunthorpe
2023-07-24 20:42 ` Kasireddy, Vivek
2023-07-25 3:14 ` Alistair Popple
2023-07-18 8:28 ` [RFC v1 2/3] udmabuf: Replace pages when there is FALLOC_FL_PUNCH_HOLE in memfd Vivek Kasireddy
2023-08-02 12:40 ` Daniel Vetter
2023-08-03 8:24 ` Kasireddy, Vivek
2023-08-03 8:32 ` Daniel Vetter
2023-07-18 8:28 ` [RFC v1 3/3] selftests/dma-buf/udmabuf: Add tests for huge pages and FALLOC_FL_PUNCH_HOLE Vivek Kasireddy
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