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njR2Ds4AFd6yo09PRJEjBPmT8bbFTX+ma7BhpSE/kWRX7ICH581/CyoxXWpNhpzAaewoXfezrEPOqGwyPhugI09Rcl3CmV/tfTimt6gUUXp0Y2E8bmU8Tp1hPum+gkPP+0eVT5BVck0tYyJpBAHXoYZXNldsmT06kMF3VcSvIOWBAHii22gVwn1+4AiCZcRnIn8v+slHX0fTnjYavLe7fcooiLuan9AsezMyRvwIqE0TAeU3NSornrVfPLUSY++v0Uq2eSVoBsANOmGSU+82i9GEYXS81NanG3zrzLO0hKj+jVyF2SfAE6fNcXesoFAoCoYj3WU+iJIABVEy3rKmO3ioqK3Iyh4DGDNopoaMwkzWJAZc= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000001, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Thomas Hellstr=C3=B6m writes: > On Tue, 2024-10-15 at 10:02 -0300, Jason Gunthorpe wrote: >> On Tue, Oct 15, 2024 at 02:41:24PM +0200, Thomas Hellstr=C3=B6m wrote: >> > > It has nothing to do with kernel P2P, you are just allowing more >> > > selective filtering of dev_private_owner. You should focus on >> > > that in >> > > the naming, not p2p. ie allow_dev_private() >> > >=20 >> > > P2P is stuff that is dealing with MEMORY_DEVICE_PCI_P2PDMA. >> >=20 >> > Yes, although the intention was to incorporate also other fast >> > interconnects in "P2P", not just "PCIe P2P", but I'll definitely >> > take a >> > look at the naming. >>=20 >> It has nothing to do with that, you are just filtering the device >> private pages differently than default. >>=20 >> Your end use might be P2P, but at this API level it certainly is not. > > Sure. Will find something more suitable. > >>=20 >> > > This is just allowing more instances of the same driver to co- >> > > ordinate >> > > their device private memory handle, for whatever purpose. >> >=20 >> > Exactly, or theoretically even cross-driver. >>=20 >> I don't want to see things like drivers changing their pgmap handles >> privately somehow. If we are going to make it cross driver then it >> needs to be generalized alot more. > > Cross-driver is initially not a thing, so let's worry about that later. > My impression though is that this is the only change required for > hmm_range_fault() and that infrastructure for opt-in and dma-mapping > would need to be provided elsewhere? Cross-driver is tricky because the device-private pages have no meaning outside of the driver which owns/allocates them. One option is to have a callback which returns P2PDMA pages which can then be dma-mapped. See https://lore.kernel.org/linux-mm/20241015152348.3055360-1-ymaman@nvidia.com= / for an example of that. >>=20 >> > >=20 >> > > Otherwise I don't see a particular problem, though we have talked >> > > about widening the matching for device_private more broadly using >> > > some >> > > kind of grouping tag or something like that instead of a >> > > callback. >> > > You >> > > may consider that as an alternative >> >=20 >> > Yes. Looked at that, but (if I understand you correctly) that would >> > be >> > the case mentioned in the commit message where the group would be >> > set >> > up statically at dev_pagemap creation time? >>=20 >> Not necessarily statically, but the membership would be stored in the >> pagemap and by updated during hotplug/etc >>=20 >> If this is for P2P then the dynamic behavior is pretty limited, some >> kind of NxN bitmap. >>=20 >> > > hmm_range struct inside a caller private data struct and use that >> > > instead if inventing a whole new struct and pointer. >> >=20 >> > Our first attempt was based on that but then that wouldn't be >> > reusable >> > in the migrate_device.c code. Hence the extra indirection. >>=20 >> It is performance path, you should prefer duplication rather than >> slowing it down.. > > OK. Will look at duplicating. > > Thanks, > Thomas > > >>=20 >> Jason