Re: [PATCH v4 3/4] cachestat: implement cachestat syscall

Andrew Morton
2022-12-16T21:48:22Z

On Fri, 16 Dec 2022 11:21:48 -0800 Nhat Pham <nphamcs@gmail.com> wrote:

> Implement a new syscall that queries cache state of a file and
> summarizes the number of cached pages, number of dirty pages, number of
> pages marked for writeback, number of (recently) evicted pages, etc. in
> a given range.
>
> NAME
> cachestat - query the page cache status of a file.
>
> SYNOPSIS
> #include <sys/mman.h>
>
> struct cachestat {
> __u64 nr_cache;
> __u64 nr_dirty;
> __u64 nr_writeback;
> __u64 nr_evicted;
> __u64 nr_recently_evicted;
> };
>
> int cachestat(unsigned int fd, off_t off, size_t len,
> size_t cstat_size, struct cachestat *cstat,
> unsigned int flags);
>
> DESCRIPTION
> cachestat() queries the number of cached pages, number of dirty
> pages, number of pages marked for writeback, number of (recently)
> evicted pages, in the bytes range given by `off` and `len`.

I suggest this be spelled out better: "number of evicted and number or
recently evicted pages".

I suggest this clearly tell readers what an "evicted" page is - they
aren't kernel programmers!

What is the benefit of the "recently evicted" pages? "recently" seems
very vague - what use is this to anyone?

> These values are returned in a cachestat struct, whose address is
> given by the `cstat` argument.
>
> The `off` and `len` arguments must be non-negative integers. If
> `len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
> 0, we will query in the range from `off` to the end of the file.
>
> `cstat_size` allows users to obtain partial results. The syscall
> will copy the first `csstat_size` bytes to the specified userspace
> memory. `cstat_size` must be a non-negative value that is no larger
> than the current size of the cachestat struct.
>
> The `flags` argument is unused for now, but is included for future
> extensibility. User should pass 0 (i.e no flag specified).

Why is `flags' here? We could add an unused flags arg to any syscall,
but we don't. What's the plan?

Are there security implications? If I know that some process has a
file open, I can use cachestat() to infer which parts of that file
they're looking at (like mincore(), I guess). And I can infer which
parts they're writing to, unlike mincore().

I suggest the [patch 1/4] fixup be separated from this series.