linux-kernel - Re: [PATCH] [suggestion] mm/gup: avoid IS_ERR_OR

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Date:   Fri, 19 May 2023 16:30:27 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Ian Rogers <irogers@...gle.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-perf-users@...r.kernel.org
Subject: Re: [PATCH] [suggestion] mm/gup: avoid IS_ERR_OR_NULL

On Fri, May 19, 2023 at 05:09:35PM +0200, Arnd Bergmann wrote:
> On Fri, May 19, 2023, at 16:51, Lorenzo Stoakes wrote:
> > Given you are sharply criticising the code I authored here, is it too much
> > to ask for you to cc- me, the author on commentaries like this? Thanks.
>
> My mistake, I expected this to get added automatically based on
> the "Fixes:" tag, I probably dropped you by accident in the end.
>

OK no worries, it's often the way that something is purely accidental but
seems ruder than intended (or even rude at all) because text is a terrible
format :)

> > On Fri, May 19, 2023 at 11:39:13AM +0200, Arnd Bergmann wrote:
> >> From: Arnd Bergmann <arnd@...db.de>
> >>
> >> While looking at an unused-variable warning, I noticed a new interface coming
> >> in that requires the use of IS_ERR_OR_NULL(), which tends to indicate bad
> >> interface design and is usually surprising to users.
> >
> > I am not sure I understand your reasoning, why does it 'tend to indicate
> > bad interface design'? You say that as if it is an obvious truth. Not
> > obvious to me at all.
> >
> > There are 3 possible outcomes from the function - an error, the function
> > failing to pin a page, or it succeeding in doing so. For some of the
> > callers that results in an error, for others it is not an error.
> >
> > Overloading EIO on the assumption that gup will never, ever return this
> > indicating an error seems to me a worse solution.
>
> The problem is that we have inconsistent error handling in functions
> that return an object, about half of them use NULL to indicate an error,
> and the other half use ERR_PTR(), and users frequently get those
> wrong by picking the wrong one. Functions that can return both make
> this worse because whichever of the two normal ways a user expects,
> they still get it wrong.
>
> > Not a fan at all of this patch, it doesn't achieve anything useful, is in
> > service of some theoretical improvement, and actually introduces a new
> > class of bug (differentiating EIO and failing to pin).
>
> Having another -EIO return code is a problem, so I agree that
> my patch wouldn't be good either. Maybe separating the error return
> from the page pointer by passing a 'struct page **p' argument that
> gets filled would help?

Yeah I see your point, in the majority of cases failing to pin is an error,
I just wonder if something like adding another parameter wouldn't just add
more noise/confusion here than it saves?

Sadly I think aspects of this are C sucking at dealing with multiple return
values sanely, and there probably isn't a totally nice way of dealing with
this.

>
>     Arnd