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A3las6fGhx9tKuAInETCN59/Oq+OqNR9mA8jT7nOVG0ZOvChkqiXtfl3QVoyuwknYWZYxUyMdUv6spKcvT7roit9FzXtHTrw4yWaum4c5uwDTv51ya+HgMy20aEWAHKEFe8+yzm+Xbv9IqgzPq1Qmv0wEWGR3NdMFT7veWpj3Wide+c+Vrkp71S7gj2jhoqKxz2CUXolH/Flj47mbB12OvOYJZaq80JPhBhxl4c59yamTjCu7P51mFCH/vodTxGwfOs6BcfYBTKdBfZTRtCQKGGCmUiEgZDDLljffkEXWk4eIeN4ZgA6dex+Z41QkHVjsFkLEz4JodH8bJQx4U/4UpV/a0PFxvsadUXnO1xXeWp1v/jSyJpWGA3+Nqb07TMBv7837ncQY5nn+fsncAuEyiMT9+uqS9jV7ULT0 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 28 Jan 2025, at 13:33, David Hildenbrand wrote: > On 28.01.25 18:07, Juan Yescas wrote: >> On Tue, Jan 28, 2025 at 1:58=E2=80=AFAM David Hildenbrand wrote: >>> >>> On 28.01.25 02:04, Juan Yescas wrote: >>>> Hi LSF organizers, >>>> >>>> I would like to continue discussing this topic with the mm community: >>>> >>>> "CMA reservation optimizations" >>>> >>>> Note: There is already an email in the linux-mm mailing list that is >>>> discussing this issue. The title is: >>>> >>>> "CMA reservations require 32MiB alignment in 16KiB page size kernels >>>> instead of 8MiB in 4KiB page size kernel" >>>> >>>> Background >>>> >>>> When the drivers reserve CMA memory in 16KiB kernels, the minimum >>>> alignment is 32 MiB as per CMA_MIN_ALIGNMENT_BYTES. However, in 4KiB >>>> kernels, the CMA alignment is 4MiB. >>> >>> I'm curious, here you say 4 MiB, above 8 MiB. >>> >> >> My bad, it is a typo. I meant 4 MiB. > > That makes sense. > >> >>> But nowadays it's usually 2 MiB (pageblock size), no? >> >> That's right for the case when THPs are enabled in 4KiB page size config= s. >> >> #define pageblock_order MIN_T(unsigned int, HPAGE_PMD_ORDER, MAX_PAGE_OR= DER) >> https://elixir.bootlin.com/linux/v6.13/source/include/linux/pageblock-fl= ags.h#L50 >> >> This evals to pageblock_order =3D min(21 - 12, 10) =3D 9 >> >> #define CMA_MIN_ALIGNMENT_PAGES pageblock_nr_pages >> #define CMA_MIN_ALIGNMENT_BYTES (PAGE_SIZE * CMA_MIN_ALIGNMENT_PAGES) >> https://elixir.bootlin.com/linux/v6.13/source/include/linux/cma.h#L21 >> >> CMA_MIN_ALIGNMENT_BYTES =3D (4096 * 2 ^ 9) =3D (4096 * 512) =3D 2097152 = =3D 2 MiB >> >> However, when THPs are disabled, we get: >> >> #define pageblock_order MAX_PAGE_ORDER // 10 >> https://elixir.bootlin.com/linux/v6.13/source/arch/arm64/Kconfig#L1630 >> https://elixir.bootlin.com/linux/v6.13/source/include/linux/pageblock-fl= ags.h#L55 >> >> CMA_MIN_ALIGNMENT_BYTES =3D (4096 * 2 ^ 10) =3D (4096 * 1024) =3D 419430= 4 =3D 4 MiB > > Right, and it can depend on ARCH_FORCE_MAX_ORDER. > > I've been wondering for a while if pageblock_order should nowadays defaul= t to HPAGE_PMD_ORDER, with the option to make it smaller/larger (likely sma= ller) -- as discussed. > > As discussed, the topic you are touching on is also relevant for virtio-m= em, which can add/remove memory currently in pageblock granularity: 512 MiB= on arm64 are not particularly helpful. I think we could support adding/rem= oving smaller granularity, but it requires a bit of work, and always isolat= ing 512MiB worth of pages just to effectively allocate e.g., 2 MiB worth of= pages is rather suboptimal. Same applies to CMA I assume. > > So there is more infrastructure that could benefit from pageblocks to rat= her be on the smaller side, even when hugetlb+THP might not be around in a = config. It is related to anti-fragmentation mechanism in the kernel (Mel and Vlasti= ma are cc=E2=80=99d, feel free to add more since I must miss others). If pageblock size is smaller than a PMD THP size, current compaction code will not be able to efficiently defragment memory for PMD THP creation= , since compaction code works at pageblock granularity. This means we need to decouple compaction granularity from pageblock. pageblock uses different migratetypes (UNMOVABLE, MOVABLE, RECLAIMABLE, ...) to help reduce memory fragmentation by grouping pages by mobility. When compaction works on multi= ple pageblocks to generate a PMD THP, it needs all pageblocks within the range share the same migratetype, otherwise, compacting memory within a region wi= th MIGRATE_UNMOVABLE would highly likely result in a waste of time, since unmo= vable pages just prevent a big free page from creation. So the key question is how to prevent MIGRATE_UNMOVABLE fragment a PMD THP size pageblock range, anti-fragmentation for pageblocks? Do we want to have super-pageblock for that? Or we allow sub-pageblock for virtio-mem and CMA reservation? That is probably what we want to discuss on the infrastructure side. Best Regards, Yan, Zi