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Wed, 04 Feb 2026 22:08:10 -0800 (PST) Message-ID: <075feda4-86ee-4521-8b92-914d24aee582@gmail.com> Date: Wed, 4 Feb 2026 22:08:10 -0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC 00/12] mm: PUD (1GB) THP implementation Content-Language: en-GB To: Lorenzo Stoakes Cc: ziy@nvidia.com, Andrew Morton , David Hildenbrand , linux-mm@kvack.org, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, npache@redhat.com, Liam.Howlett@oracle.com, ryan.roberts@arm.com, vbabka@suse.cz, lance.yang@linux.dev, linux-kernel@vger.kernel.org, kernel-team@meta.com References: <20260202005451.774496-1-usamaarif642@gmail.com> <2efaa5ed-bd09-41f0-9c07-5cd6cccc4595@gmail.com> From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 04/02/2026 03:08, Lorenzo Stoakes wrote: > On Tue, Feb 03, 2026 at 05:00:10PM -0800, Usama Arif wrote: >> >> >> On 02/02/2026 03:20, Lorenzo Stoakes wrote: >>> OK so this is somewhat unexpected :) >>> >>> It would have been nice to discuss it in the THP cabal or at a conference >>> etc. so we could discuss approaches ahead of time. Communication is important, >>> especially with major changes like this. >> >> Makes sense! >> >>> >>> And PUD THP is especially problematic in that it requires pages that the page >>> allocator can't give us, presumably you're doing something with CMA and... it's >>> a whole kettle of fish. >> >> So we dont need CMA. It helps ofcourse, but we don't *need* it. >> Its summarized in the first reply I gave to Zi in [1]: >> >>> >>> It's also complicated by the fact we _already_ support it in the DAX, VFIO cases >>> but it's kinda a weird sorta special case that we need to keep supporting. >>> >>> There's questions about how this will interact with khugepaged, MADV_COLLAPSE, >>> mTHP (and really I want to see Nico's series land before we really consider >>> this). >> >> >> So I have numbers and experiments for page faults which are in the cover letter, >> but not for khugepaged. I would be very surprised (although pleasently :)) if >> khugepaged by some magic finds 262144 pages that meets all the khugepaged requirements >> to collapse the page. In the basic infrastructure support which this series is adding, >> I want to keep khugepaged collapse disabled for 1G pages. This is also the initial >> approach that was taken in other mTHP sizes. We should go slow with 1G THPs. > > Yes we definitely want to limit to page faults for now. > > But keep in mind for that to be viable you'd surely need to update who gets > appropriate alignment in __get_unmapped_area()... not read through series far > enough to see so not sure if you update that though! > > I guess that'd be the sanest place to start, if an allocation _size_ is aligned > 1 GB, then align the unmapped area _address_ to 1 GB for maximum chance of 1 GB > fault-in. Yeah this was definitely missing. I was manually aligning the fault address in selftest and benchmarks with the trick used in other selftests (((unsigned long)addr + PUD_SIZE - 1) & ~(PUD_SIZE - 1)) Thanks for pointing this out! This is basically what I wanted with the RFC, to find out what I am missing and not testing. Will look into VFIO and DAX as you mentioned as well. > > Oh by the way I made some rough THP notes at > https://publish.obsidian.md/mm/Transparent+Huge+Pages+(THP) which are helpful > for reminding me about what does what where, useful for a top-down view of how > things are now. > Thanks! >> >>> >>> So overall, I want to be very cautious and SLOW here. So let's please not drop >>> the RFC tag until David and I are ok with that? >>> >>> Also the THP code base is in _dire_ need of rework, and I don't really want to >>> add major new features without us paying down some technical debt, to be honest. >>> >>> So let's proceed with caution, and treat this as a very early bit of >>> experimental code. >>> >>> Thanks, Lorenzo >> >> Ack, yeah so this is mainly an RFC to discuss what the major design choices will be. >> I got a kernel with selftests for allocation, memory integrity, fork, partial munmap, >> mprotect, reclaim and migration passing and am running them with DEBUG_VM to make sure >> we dont get the VM bugs/warnings and the numbers are good, so just wanted to share it >> upstream and get your opinions! Basically try and trigger a discussion similar to what >> Zi asked in [2]! And also if someone could point out if there is something fundamental >> we are missing in this series. > > Well that's fair enough :) > > But do come to a THP cabal so we can chat, face-to-face (ok, digital face to > digital face ;). It's usually a force-multiplier I find, esp. if multiple people > have input which I think is the case here. We're friendly :) Yes, Thanks for this! It would be really helpful to discuss in a call. I didn't know there was a meeting but have requested details (date/time) in another thread. > > In any case, conversations are already kicking off so that's definitely positive! > > I think we will definitely get there with this at _some point_ but I would urge > patience and also I really want to underline my desire for us in THP to start > paying down some of this technical debt. > > I know people are already making efforts (Vernon, Luiz), and sorry that I've not > been great at review recently (should be gradually increasing over time), but I > feel that for large features to be added like this now we really do require some > refactoring work before we take it. > Yes agreed! I will definitely need your and others guidance on what needs to be properly refractored so that this fits well with the current code. > We definitely need to rebase this once Nico's series lands (should do next > cycle) and think about how it plays with this, I'm not sure if arm64 supports > mTHP between PMD and PUD size (Dev? Do you know?) so maybe that one is moot, but > in general want to make sure it plays nice. > Will do! >> >> Thanks for the reviews! Really do apprecaite it! > > No worries! :) > >> >> [1] https://lore.kernel.org/all/20f92576-e932-435f-bb7b-de49eb84b012@gmail.com/#t >> [2] https://lore.kernel.org/all/3561FD10-664D-42AA-8351-DE7D8D49D42E@nvidia.com/ > > Cheers, Lorenzo