There is currently no good way to query the page cache state of large file sets and directory trees. There is mincore(), but it scales poorly: the kernel writes out a lot of bitmap data that userspace has to aggregate, when the user really doesn not care about per-page information in that case. The user also needs to mmap and unmap each file as it goes along, which can be quite slow as well. This series of patches introduces a new system call, cachestat, that summarizes the page cache statistics (number of cached pages, dirty pages, pages marked for writeback, evicted pages etc.) of a file, in a specified range of bytes. It also include a selftest suite that tests some typical usage This interface is inspired by past discussion and concerns with fincore, which has a similar design (and as a result, issues) as mincore. Relevant links: https://lkml.indiana.edu/hypermail/linux/kernel/1302.1/04207.html https://lkml.indiana.edu/hypermail/linux/kernel/1302.1/04209.html For comparison with mincore, I ran both syscalls on a 2TB sparse file: Using mincore: real 0m37.510s user 0m2.934s sys 0m34.558s Using cachestat: real 0m0.009s user 0m0.000s sys 0m0.009s Mincore takes about 4000 times longer to obtain less aggregated information! Some open questions: * What other fields might be useful? * Huge pages: another useful stat to include is the number of huge pages cached. However, as the size of a huge page can vary, having just a single field is not very meaningful. * An alternative would be to have one field for each possible size - but this is not future-proof, as bigger sizes might be introduced later. Johannes Weiner (1): workingset: fix confusion around eviction vs refault container Nhat Pham (3): workingset: refactor LRU refault to expose refault recency check cachestat: implement cachestat syscall selftests: Add selftests for cachestat MAINTAINERS | 8 + arch/x86/entry/syscalls/syscall_32.tbl | 1 + arch/x86/entry/syscalls/syscall_64.tbl | 1 + include/linux/swap.h | 1 + include/linux/syscalls.h | 2 + include/uapi/asm-generic/unistd.h | 5 +- include/uapi/linux/mman.h | 8 + kernel/sys_ni.c | 1 + mm/Makefile | 2 +- mm/cachestat.c | 109 +++++++++++ mm/workingset.c | 142 +++++++++----- tools/testing/selftests/Makefile | 1 + tools/testing/selftests/cachestat/.gitignore | 2 + tools/testing/selftests/cachestat/Makefile | 9 + .../selftests/cachestat/test_cachestat.c | 184 ++++++++++++++++++ 15 files changed, 430 insertions(+), 46 deletions(-) create mode 100644 mm/cachestat.c create mode 100644 tools/testing/selftests/cachestat/.gitignore create mode 100644 tools/testing/selftests/cachestat/Makefile create mode 100644 tools/testing/selftests/cachestat/test_cachestat.c -- 2.30.2
