linux-kernel - Re: [PATCH v3 3/3] man2/fincore.2: document general description about fincore(2)

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Date:	Tue, 8 Jul 2014 11:43:03 -0400
From:	Naoya Horiguchi <n-horiguchi@...jp.nec.com>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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 Mon, Jul 07, 2014 at 03:34:06PM -0700, Dave Hansen wrote:
> On 07/07/2014 01:59 PM, Naoya Horiguchi wrote:
> > On Mon, Jul 07, 2014 at 12:08:12PM -0700, Dave Hansen wrote:
> >> 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.
> > 
> > We don't break this while loop if do_fincore() returned a positive value
> > unless copy_to_user() fails. And in that case ret is set to -EFAULT.
> > So I think sys_fincore() never returns a positive value.
> 
> OK, that makes sense as I'm reading it again.
> 
> >> Why not just use the return value for something useful instead of
> >> hacking in the extras->nr_entries stuff?
> > 
> > Hmm, I got the opposite complaint previously, where we shouldn't
> > interpret the return value differently depending on the flag.
> > And I'd like to keep the extra argument for future extensibility.
> > For example, if we want to collect pages only with a specific
> > set of page flags, this extra argument will be necessary.
> 
> Couldn't it simply be the number of elements that it wrote in to the
> buffer, or even the number of bytes?

Yes, returning the number of elements looks clearer to me.
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