Hi all, There are use-cases that need to remove pages from the direct map or at least map them at PTE level. These use-cases include vfree, module loading, ftrace, kprobe, BPF, secretmem and generally any caller of set_memory/set_direct_map APIs. Remapping pages at PTE level causes split of the PUD and PMD sized mappings in the direct map which leads to performance degradation. To reduce the performance hit caused by the fragmentation of the direct map, it makes sense to group and/or cache the base pages removed from the direct map so that the most of base pages created during a split of a large page will be consumed by users requiring PTE level mappings. There were several RFC postings to address this: * grouped page allocations for vmalloc permissions [1] and for PKS protection of page tables [2] * pool of large pages in secretmem [3] * global cache of pte-mapped pages close to the page allocator [4] * a new migrate type to cache pte-mapped pages in free lists [5] I'd like to discuss the direct map fragmentation issue and the possible ways to reduce the fragmentation and performance hit caused by it. [1] https://lore.kernel.org/lkml/20210405203711.1095940-1-rick.p.edgecombe@xxxxxxxxx [2] https://lore.kernel.org/lkml/20210505003032.489164-1-rick.p.edgecombe@xxxxxxxxx [3] https://lore.kernel.org/lkml/20210121122723.3446-8-rppt@xxxxxxxxxx [4] https://lore.kernel.org/all/20210823132513.15836-1-rppt@xxxxxxxxxx [5] https://lore.kernel.org/all/20220127085608.306306-1-rppt@xxxxxxxxxx -- Sincerely yours, Mike.
