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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 86C6540009 X-Stat-Signature: harakch5qaf1hinrrisc6rkrf3wpawgg X-Rspam-User: X-HE-Tag: 1738666746-303176 X-HE-Meta: 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 fnOA4AFT MjaRHajRA58kvClcymIozK4eiUfHMcDuNmWoVhX1pevhkz3bWza6a9esxBp5fJQfuLGQNj4kgnQ8gkip7mpRxLZSyuTXZyk5sqGK1Awlb2tWGGIyTLTD4sIwFn+ScbhE/SXFcCIV7MqkQB/vh0WiZvazmKOBqmWo/Ir5m2Dt3mlrvcfV6/ZEF0/YTcnOAohEnVymFTrESZHK6qhmYlaUR84r1h+Hj5IVPd84sV8uHtu63GtizlGVkPkCLHjEGcdLYw6OLAu/n94sd6+LAIzLMgytLFdJmb3DLQzrfEiGKsxGRYoK/CdxHYQHx/LqyDXfeS2A4IomOFxzKGiIjlAQLJuQXfUDEbu/oxDvsLhWxEr9JEZk2TYJov1JNICNA/awEvVmaQxlk+dMyCe9cih1UBUKX5kaTEYevK4AZFRdeqL0gFhkyaT5fgSrAhMTzsRSB3TTp X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 31.01.25 18:05, Simona Vetter wrote: > On Fri, Jan 31, 2025 at 11:55:55AM +0100, David Hildenbrand wrote: >> On 31.01.25 00:06, Alistair Popple wrote: >>> On Thu, Jan 30, 2025 at 02:03:42PM +0100, Simona Vetter wrote: >>>> On Thu, Jan 30, 2025 at 10:58:51AM +0100, David Hildenbrand wrote: >>>>> On 30.01.25 10:51, Simona Vetter wrote: >>>>>> On Wed, Jan 29, 2025 at 12:54:03PM +0100, David Hildenbrand wrote: >>>>>>> Let's do it just like mprotect write-upgrade or during NUMA-hinting >>>>>>> faults on PROT_NONE PTEs: detect if the PTE can be writable by using >>>>>>> can_change_pte_writable(). >>>>>>> >>>>>>> Set the PTE only dirty if the folio is dirty: we might not >>>>>>> necessarily have a write access, and setting the PTE writable doesn't >>>>>>> require setting the PTE dirty. >>>>>> >>>>>> Not sure whether there's much difference in practice, since a device >>>>>> exclusive access means a write, so the folio better be dirty (unless we >>>>>> aborted halfway through). But then I couldn't find the code in nouveau to >>>>>> do that, so now I'm confused. >>>>> >>>>> That confused me as well. Requiring the PTE to be writable does not imply >>>>> that it is dirty. >>>>> >>>>> So something must either set the PTE or the folio dirty. >>>> >>>> Yeah I'm not finding that something. >>>> >>>>> ( In practice, most anonymous folios are dirty most of the time ... ) >>>> >>>> And yup that's why I think it hasn't blown up yet. >>>> >>>>> If we assume that "device-exclusive entries" are always dirty, then it >>>>> doesn't make sense to set the folio dirty when creating device-exclusive >>>>> entries. We'd always have to set the PTE dirty when restoring the exclusive >>>>> pte. >>>> >>>> I do agree with your change, I think it's correct to put this >>>> responsibility onto drivers. It's just that nouveau seems to not be >>>> entirely correct. >>> >>> Yeah, agree it should be a driver responsibility but also can't see how nouveau >>> is correct there either. I might see if I can get it to blow up... >> >> (in context of the rmap walkers) The question is, how do we consider >> device-exclusive entries: >> >> (1) dirty? Not from a CPU perspective. >> (2) referenced? Not from a CPU perspective. >> >> If the answer is always "no" to all questions, then memory notifiers must >> handle it, because we'd be answering the question from the CPU point of >> view. >> >> If the answer is always "yes", there is a problem: we can only make it >> clean/young by converting it to an ordinary PTE first (requiring MMU >> notifiers etc.), which makes it quite nasty. >> >> Mixed answers are not possible, because we don't know just from staring at >> the entry. > > I think it's the gpu's (or whatever is using it) responsibility to update > folio state while it has ptes pointing at memory. Whether that's > device-exclusive system memory or device-private migrated memory. Anything > else doesn't make sense to me conceptually. > > And I don't think we can just blindly assume even for device-exclusive > mappings that they will be dirty when we convert them back to a real pte, > because we might have raced trying to set up the gpu mapping and restarted > before we even put the pte into place. Or maybe someone was real quick at > writing it back after the gpu already dropped it's pte. > > I guess maybe some clear documentation in all these functions > (make_device_exclusive, hmm_range_fault, migration helpers) that it's the > drivers job to dirty pages correctly would help? I'll add a comment to make_device_exclusive(), stating that these entries are considered clean+old from a MM perspective, and that the driver must update the folio when notified by MMU notifiers. We should probably document that somewhere else as well as you suggested separately. -- Cheers, David / dhildenb