[PATCH v4 0/4] cachestat: a new syscall for page cache state of files

Nhat Pham
2022-12-16T19:22:02Z

Changelog:
v4:
* Refactor cachestat and move it to mm/filemap.c (patch 3)
(suggested by Brian Foster)
* Remove redundant checks (!folio, access_ok)
(patch 3) (suggested by Matthew Wilcox and Al Viro)
* Fix a bug in handling multipages folio.
(patch 3) (suggested by Matthew Wilcox)
* Add a selftest for shmem files, which can be used to test huge
pages (patch 4) (suggested by Johannes Weiner)
v3:
* Fix some minor formatting issues and build errors.
* Add the new syscall entry to missing architecture syscall tables.
(patch 3).
* Add flags argument for the syscall. (patch 3).
* Clean up the recency refactoring (patch 2) (suggested by Yu Zhao)
* Add the new Kconfig (CONFIG_CACHESTAT) to disable the syscall.
(patch 3) (suggested by Josh Triplett)
v2:
* len == 0 means query to EOF. len < 0 is invalid.
(patch 3) (suggested by Brian Foster)
* Make cachestat extensible by adding the `cstat_size` argument in the
syscall (patch 3)

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

This series consist of 4 patches:

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 | 7 +
arch/alpha/kernel/syscalls/syscall.tbl | 1 +
arch/arm/tools/syscall.tbl | 1 +
arch/ia64/kernel/syscalls/syscall.tbl | 1 +
arch/m68k/kernel/syscalls/syscall.tbl | 1 +
arch/microblaze/kernel/syscalls/syscall.tbl | 1 +
arch/parisc/kernel/syscalls/syscall.tbl | 1 +
arch/powerpc/kernel/syscalls/syscall.tbl | 1 +
arch/s390/kernel/syscalls/syscall.tbl | 1 +
arch/sh/kernel/syscalls/syscall.tbl | 1 +
arch/sparc/kernel/syscalls/syscall.tbl | 1 +
arch/x86/entry/syscalls/syscall_32.tbl | 1 +
arch/x86/entry/syscalls/syscall_64.tbl | 1 +
arch/xtensa/kernel/syscalls/syscall.tbl | 1 +
include/linux/fs.h | 3 +
include/linux/swap.h | 1 +
include/linux/syscalls.h | 3 +
include/uapi/asm-generic/unistd.h | 5 +-
include/uapi/linux/mman.h | 9 +
init/Kconfig | 10 +
kernel/sys_ni.c | 1 +
mm/filemap.c | 137 +++++++++
mm/workingset.c | 130 ++++++---
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/cachestat/.gitignore | 2 +
tools/testing/selftests/cachestat/Makefile | 8 +
.../selftests/cachestat/test_cachestat.c | 259 ++++++++++++++++++
27 files changed, 550 insertions(+), 39 deletions(-)
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

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2.30.2