linux-kernel - Re: [PATCH v8 2/3] cachestat: implement cachestat syscall

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Message-Id: <54c8ecbd-1d6e-40f1-af30-7efd04c63a7e@app.fastmail.com>
Date:   Fri, 27 Jan 2023 16:46:38 +0100
From:   "Arnd Bergmann" <arnd@...db.de>
To:     "Nhat Pham" <nphamcs@...il.com>,
        "Andrew Morton" <akpm@...ux-foundation.org>
Cc:     "Johannes Weiner" <hannes@...xchg.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, bfoster@...hat.com,
        "Matthew Wilcox" <willy@...radead.org>, linux-api@...r.kernel.org,
        kernel-team@...a.com
Subject: Re: [PATCH v8 2/3] cachestat: implement cachestat syscall

On Thu, Jan 26, 2023, at 18:53, Nhat Pham wrote:
>
> 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,
>           unsigned int cstat_version, struct cachestat *cstat,
>           unsigned int flags);

Is this "off_t off" argument intentionally limited to the old
32-bit type on 32-bit architectures? Unfortunately I fear 
there are no good options to pass an offset here:

- if you make it a 32-bit type, this breaks calling it from
  normal userspace that defines off_t as a 64-bit type

- if you change it to a 64-bit loff_t, there are three
  separate calling conventions for 64-bit, 32-bit with
  aligned register pairs and other 32-bit, plus you
  exceed the usual limit of six system call arguments

A separate problem may be the cstat_version argument, usually
we don't use interface versions but instead use a new
system call number if something changes in an incompatible
way.

       Arnd