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Date: Mon, 07 Jul 2014 12:08:12 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Andrew Morton <akpm@...ux-foundation.org>
CC: Konstantin Khlebnikov <koct9i@...il.com>,
Wu Fengguang <fengguang.wu@...el.com>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
Borislav Petkov <bp@...en8.de>,
"Kirill A. Shutemov" <kirill@...temov.name>,
Johannes Weiner <hannes@...xchg.org>,
Rusty Russell <rusty@...tcorp.com.au>,
David Miller <davem@...emloft.net>,
Andres Freund <andres@...quadrant.com>,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Christoph Hellwig <hch@...radead.org>,
Dave Chinner <david@...morbit.com>,
Michael Kerrisk <mtk.manpages@...il.com>,
Linux API <linux-api@...r.kernel.org>,
Naoya Horiguchi <nao.horiguchi@...il.com>
Subject: Re: [PATCH v3 3/3] man2/fincore.2: document general description about
fincore(2)
On 07/07/2014 11:00 AM, Naoya Horiguchi wrote:
> +.SH RETURN VALUE
> +On success,
> +.BR fincore ()
> +returns 0.
> +On error, \-1 is returned, and
> +.I errno
> +is set appropriately.
Is this accurate? From reading the syscall itself, it looked like it
did this:
> + * Return value is the number of pages whose data is stored in fc->buffer.
> + */
> +static long do_fincore(struct fincore_control *fc, int nr_pages)
and:
> +SYSCALL_DEFINE6(fincore, int, fd, loff_t, start, long, nr_pages,
...
> + while (fc.nr_pages > 0) {
> + memset(fc.buffer, 0, fc.buffer_size);
> + ret = do_fincore(&fc, min(step, fc.nr_pages));
> + /* Reached the end of the file */
> + if (ret == 0)
> + break;
> + if (ret < 0)
> + break;
...
> + }
...
> + return ret;
> +}
Which seems that for a given loop of do_fincore(), you might end up
returning the result of that *single* iteration of do_fincore() instead
of the aggregate of the entire syscall.
So, it can return <0 on failure, 0 on success, or also an essentially
random >0 number on success too.
Why not just use the return value for something useful instead of
hacking in the extras->nr_entries stuff? Oh, and what if that
> + if (extra)
> + __put_user(nr, &extra->nr_entries);
fails? It seems like we might silently forget to tell userspace how
many entries we filled.
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