Hey, This small series, attempts at minimizing 'struct page' overhead by pursuing a similar approach as Muchun Song series "Free some vmemmap pages of hugetlb page"[0] but applied to devmap/ZONE_DEVICE. [0] https://lore.kernel.org/linux-mm/20201130151838.11208-1-songmuchun@xxxxxxxxxxxxx/ The link above describes it quite nicely, but the idea is to reuse tail page vmemmap areas, particular the area which only describes tail pages. So a vmemmap page describes 64 struct pages, and the first page for a given ZONE_DEVICE vmemmap would contain the head page and 63 tail pages. The second vmemmap page would contain only tail pages, and that's what gets reused across the rest of the subsection/section. The bigger the page size, the bigger the savings (2M hpage -> save 6 vmemmap pages; 1G hpage -> save 4094 vmemmap pages). In terms of savings, per 1Tb of memory, the struct page cost would go down with compound pagemap: * with 2M pages we lose 4G instead of 16G (0.39% instead of 1.5% of total memory) * with 1G pages we lose 8MB instead of 16G (0.0007% instead of 1.5% of total memory) Along the way I've extended it past 'struct page' overhead *trying* to address a few performance issues we knew about for pmem, specifically on the {pin,get}_user_pages* function family with device-dax vmas which are really slow even of the fast variants. THP is great on -fast variants but all except hugetlbfs perform rather poorly on non-fast gup. So to summarize what the series does: Patches 1-5: Much like Muchun series, we reuse tail page areas across a given page size (namely @align was referred by remaining memremap/dax code) and enabling of memremap to initialize the ZONE_DEVICE pages as compound pages or a given @align order. The main difference though, is that contrary to the hugetlbfs series, there's no vmemmap for the area, because we are onlining it. IOW no freeing of pages of already initialized vmemmap like the case for hugetlbfs, which simplifies the logic (besides not being arch-specific). After these, there's quite visible region bootstrap of pmem memmap given that we would initialize fewer struct pages depending on the page size. NVDIMM namespace bootstrap improves from ~750ms to ~190ms/<=1ms on emulated NVDIMMs with 2M and 1G respectivally. The net gain in improvement is similarly observed in proportion when running on actual NVDIMMs. Patch 6 - 8: Optimize grabbing/release a page refcount changes given that we are working with compound pages i.e. we do 1 increment/decrement to the head page for a given set of N subpages compared as opposed to N individual writes. {get,pin}_user_pages_fast() for zone_device with compound pagemap consequently improves considerably, and unpin_user_pages() improves as well when passed a set of consecutive pages: before after (get_user_pages_fast 1G;2M page size) ~75k us -> ~3.2k ; ~5.2k us (pin_user_pages_fast 1G;2M page size) ~125k us -> ~3.4k ; ~5.5k us The RDMA patch (patch 8/9) is to demonstrate the improvement for an existing user. For unpin_user_pages() we have an additional test to demonstrate the improvement. The test performs MR reg/unreg continuously and measuring its rate for a given period. So essentially ib_mem_get and ib_mem_release being stress tested which at the end of day means: pin_user_pages_longterm() and unpin_user_pages() for a scatterlist: Before: 159 rounds in 5.027 sec: 31617.923 usec / round (device-dax) 466 rounds in 5.009 sec: 10748.456 usec / round (hugetlbfs) After: 305 rounds in 5.010 sec: 16426.047 usec / round (device-dax) 1073 rounds in 5.004 sec: 4663.622 usec / round (hugetlbfs) Patch 9: Improves {pin,get}_user_pages() and its longterm counterpart. It is very experimental, and I imported most of follow_hugetlb_page(), except that we do the same trick as gup-fast. In doing the patch I feel this batching should live in follow_page_mask() and having that being changed to return a set of pages/something-else when walking over PMD/PUDs for THP / devmap pages. This patch then brings the previous test of mr reg/unreg (above) on parity between device-dax and hugetlbfs. Some of the patches are a little fresh/WIP (specially patch 3 and 9) and we are still running tests. Hence the RFC, asking for comments and general direction of the work before continuing. Patches apply on top of linux-next tag next-20201208 (commit a9e26cb5f261). Comments and suggestions very much appreciated! Thanks, Joao Joao Martins (9): memremap: add ZONE_DEVICE support for compound pages sparse-vmemmap: Consolidate arguments in vmemmap section populate sparse-vmemmap: Reuse vmemmap areas for a given page size mm/page_alloc: Reuse tail struct pages for compound pagemaps device-dax: Compound pagemap support mm/gup: Grab head page refcount once for group of subpages mm/gup: Decrement head page once for group of subpages RDMA/umem: batch page unpin in __ib_mem_release() mm: Add follow_devmap_page() for devdax vmas drivers/dax/device.c | 54 ++++++--- drivers/infiniband/core/umem.c | 25 +++- include/linux/huge_mm.h | 4 + include/linux/memory_hotplug.h | 16 ++- include/linux/memremap.h | 2 + include/linux/mm.h | 6 +- mm/gup.c | 130 ++++++++++++++++----- mm/huge_memory.c | 202 +++++++++++++++++++++++++++++++++ mm/memory_hotplug.c | 13 ++- mm/memremap.c | 13 ++- mm/page_alloc.c | 28 ++++- mm/sparse-vmemmap.c | 97 +++++++++++++--- mm/sparse.c | 16 +-- 13 files changed, 531 insertions(+), 75 deletions(-) -- 2.17.1
