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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: IA0PR11MB7185.namprd11.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: b97fe2b6-a128-4485-e77f-08db8c85235c X-MS-Exchange-CrossTenant-originalarrivaltime: 24 Jul 2023 20:32:45.4661 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 46c98d88-e344-4ed4-8496-4ed7712e255d X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: bvD4eBcgyf5EW53dg78YjoEHO3M5sXRO0GemLRWdzj4ABZIpz8CwoLQ7C/5PEVkCy2XNREsOi0q8PQbt3HYM6wmQfjihEHOH1iXzY3EjViI= X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR11MB7098 X-OriginatorOrg: intel.com X-Rspamd-Queue-Id: 51EA840023 X-Rspam-User: X-Stat-Signature: nk1pujam6tzdwqa9cu7d147mip7z474w X-Rspamd-Server: rspam03 X-HE-Tag: 1690230770-143086 X-HE-Meta: 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 xb73xZKF RJC8BD3/+cCN6SOHuXi/rugdUQmkBQ6ftldncXfqqZ3dHzYAxI+V5eIAn7NMzS/ZcPR2SQ9Tsly7eeA2Xi2qU+kDqyitNl+rx58StvxROP2KcBjv3VRLcD+BIWPh2QqzQfGEdK0SyPJ9DJYu8u8GL3CGKniM5QrtqX2+mPAbCP6sEL3h6eTbWV4I83jgAPrPLyZRczH/nPqN61p8LeRiOaz3fAfInpoPqQppV4jJi931sjamU2VKXYfLUk/5ZKvmhb7bcJRSCEFXQsszMUS/1t5ErDwKjDj82G3HPEFWNjVCVwB+GCMDWhqX8svBhT3gBZaETP3YNV5/qEnB4BqyrcE8bUZ5HKuJEoIgU+1pGtE0Xgr/1sObu4oW/XO1FACxUWbIZ X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: Hi Jason, >=20 > On Mon, Jul 24, 2023 at 07:54:38AM +0000, Kasireddy, Vivek wrote: >=20 > > > I'm not at all familiar with the udmabuf use case but that sounds > > > brittle and effectively makes this notifier udmabuf specific right? > > Oh, Qemu uses the udmabuf driver to provide Host Graphics components > > (such as Spice, Gstreamer, UI, etc) zero-copy access to Guest created > > buffers. In other words, from a core mm standpoint, udmabuf just > > collects a bunch of pages (associated with buffers) scattered inside > > the memfd (Guest ram backed by shmem or hugetlbfs) and wraps > > them in a dmabuf fd. And, since we provide zero-copy access, we > > use DMA fences to ensure that the components on the Host and > > Guest do not access the buffer simultaneously. >=20 > So why do you need to track updates proactively like this? As David noted in the earlier series, if Qemu punches a hole in its memfd that goes through pages that are registered against a udmabuf fd, then udmabuf needs to update its list with new pages when the hole gets filled after (guest) writes. Otherwise, we'd run into the coherency=20 problem (between udmabuf and memfd) as demonstrated in the selftest (patch #3 in this series). >=20 > Trigger a move when the backing memory changes and re-acquire it with AFAICS, without this patch or adding new change_pte calls, there is no way = to get notified when a new page is mapped into the backing memory of a memfd (backed by shmem or hugetlbfs) which happens after a hole punch followed by writes. We can definitely get notified when a hole is punched via the invalidate notifiers though, but as I described earlier this is not very he= lpful for the udmabuf use-case. > hmm_range_fault like everything else does. >=20 > > And replace mmu_notifier_update_mapping(vma->vm_mm, address, > pte_pfn(*ptep)) > > in the current patch with > > mmu_notifier_change_pte(vma->vm_mm, address, ptep, false)); >=20 > It isn't very useful because nothing can do anything meaningful under > the PTLs. Can't allocate memory for instance. Which makes me wonder > what it is udmabuf plans to actually do here. It is useful for udmabuf because it helps ensure coherency with the memfd. If you look at patch #2 in this series particularly the notifier callback (= update_udmabuf), it just updates its list of pages and does not allocate any memory or do an= ything that would cause it to go to sleep other than taking a mutex which I plan t= o drop in v2 as it is not really needed. With that removed, I think it seems ok to= call the notifier callback under the PTL. Thanks, Vivek >=20 > JAson