The concurrent use of multiple hugetlb page sizes on a single system is becoming more common. One of the reasons is better TLB support for gigantic page sizes on x86 hardware. In addition, hugetlb pages are being used to back VMs in hosting environments. When using hugetlb pages to back VMs in such environments, it is sometimes desirable to preallocate hugetlb pools. This avoids the delay and uncertainty of allocating hugetlb pages at VM startup. In addition, preallocating huge pages minimizes the issue of memory fragmentation that increases the longer the system is up and running. In such environments, a combination of larger and smaller hugetlb pages are preallocated in anticipation of backing VMs of various sizes. Over time, the preallocated pool of smaller hugetlb pages may become depleted while larger hugetlb pages still remain. In such situations, it may be desirable to convert larger hugetlb pages to smaller hugetlb pages. Converting larger to smaller hugetlb pages can be accomplished today by first freeing the larger page to the buddy allocator and then allocating the smaller pages. However, there are two issues with this approach: 1) This process can take quite some time, especially if allocation of the smaller pages is not immediate and requires migration/compaction. 2) There is no guarantee that the total size of smaller pages allocated will match the size of the larger page which was freed. This is because the area freed by the larger page could quickly be fragmented. To address these issues, introduce the concept of hugetlb page demotion. Demotion provides a means of 'in place' splitting a hugetlb page to pages of a smaller size. For example, on x86 one 1G page can be demoted to 512 2M pages. Page demotion is controlled via sysfs files. - demote_size Read only target page size for demotion - demote Writable number of hugetlb pages to be demoted Only hugetlb pages which are free at the time of the request can be demoted. Demotion does not add to the complexity surplus pages. Demotion also honors reserved huge pages. Therefore, when a value is written to the sysfs demote file, that value is only the maximum number of pages which will be demoted. It is possible fewer will actually be demoted. If demote_size is PAGESIZE, demote will simply free pages to the buddy allocator. Real world use cases -------------------- There are groups today using hugetlb pages to back VMs on x86. Their use case is as described above. They have experienced the issues with performance and not necessarily getting the excepted number smaller huge pages after free/allocate cycle. Note to reviewers ----------------- Patches 1-5 provide the basic demote functionality. They are built on next-20210721. Patch 3 deals with this issue of speculative page references as discussed in [1] and [2]. It builds on the ideas used in patches currently in mmotm. There have been few comments on those patches in mmotm, so I do not feel the approach has been well vetted. Patches 6-8 are an optimization to deal with vmemmap optimized pages. This was discussed in the RFC. IMO, the code may not be worth the benefit. They could be dropped with no loss of functionality. In addition, Muchun has recently sent patches to further optimize hugetlb vmemmap reduction by only requiring one vmemmap page per huge page [3]. These patches do not take Muchun's new patches into account. RFC -> v1 - Provides basic support for vmemmap optimized pages - Takes speculative page references into account - Updated Documentation file - Added optimizations for vmemmap optimized pages [1] https://lore.kernel.org/linux-mm/CAG48ez23q0Jy9cuVnwAe7t_fdhMk2S7N5Hdi-GLcCeq5bsfLxw@xxxxxxxxxxxxxx/ [2] https://lore.kernel.org/linux-mm/20210710002441.167759-1-mike.kravetz@xxxxxxxxxx/ [3] https://lore.kernel.org/linux-mm/20210714091800.42645-1-songmuchun@xxxxxxxxxxxxx/ Mike Kravetz (8): hugetlb: add demote hugetlb page sysfs interfaces hugetlb: add HPageCma flag and code to free non-gigantic pages in CMA hugetlb: add demote bool to gigantic page routines hugetlb: add hugetlb demote page support hugetlb: document the demote sysfs interfaces hugetlb: vmemmap optimizations when demoting hugetlb pages hugetlb: prepare destroy and prep routines for vmemmap optimized pages hugetlb: Optimized demote vmemmap optimizatized pages Documentation/admin-guide/mm/hugetlbpage.rst | 29 +- include/linux/hugetlb.h | 8 + include/linux/mm.h | 4 + mm/hugetlb.c | 328 +++++++++++++++++-- mm/hugetlb_vmemmap.c | 72 +++- mm/hugetlb_vmemmap.h | 16 + mm/sparse-vmemmap.c | 123 ++++++- 7 files changed, 538 insertions(+), 42 deletions(-) -- 2.31.1
