Anonymous pages have already been supported for multi-size (mTHP) allocation through commit 19eaf44954df, that can allow THP to be configured through the sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'. However, the anonymous shmem will ignore the anonymous mTHP rule configured through the sysfs interface, and can only use the PMD-mapped THP, that is not reasonable. Many implement anonymous page sharing through mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios, therefore, users expect to apply an unified mTHP strategy for anonymous pages, also including the anonymous shared pages, in order to enjoy the benefits of mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc. As discussed in the bi-weekly MM meeting[1], the mTHP controls should control all of shmem, not only anonymous shmem, but support will be added iteratively. Therefore, this patch set starts with support for anonymous shmem. The primary strategy is similar to supporting anonymous mTHP. Introduce a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled', which can have almost the same values as the top-level '/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new additional "inherit" option and dropping the testing options 'force' and 'deny'. By default all sizes will be set to "never" except PMD size, which is set to "inherit". This ensures backward compatibility with the anonymous shmem enabled of the top level, meanwhile also allows independent control of anonymous shmem enabled for each mTHP. Use the page fault latency tool to measure the performance of 1G anonymous shmem with 32 threads on my machine environment with: ARM64 Architecture, 32 cores, 125G memory: base: mm-unstable user-time sys_time faults_per_sec_per_cpu faults_per_sec 0.04s 3.10s 83516.416 2669684.890 mm-unstable + patchset, anon shmem mTHP disabled user-time sys_time faults_per_sec_per_cpu faults_per_sec 0.02s 3.14s 82936.359 2630746.027 mm-unstable + patchset, anon shmem 64K mTHP enabled user-time sys_time faults_per_sec_per_cpu faults_per_sec 0.08s 0.31s 678630.231 17082522.495
