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

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Message-ID: <617c2e26-8450-4a35-9b47-c0482921f030@lucifer.local>
Date:   Sun, 28 May 2023 16:13:44 +0100
From:   Lorenzo Stoakes <lstoakes@...il.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Arnd Bergmann <arnd@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        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 Sat, May 27, 2023 at 06:52:01AM -0300, Jason Gunthorpe wrote:
> On Sat, May 20, 2023 at 10:12:40AM +0100, Lorenzo Stoakes wrote:
> > > No, this returning 0 on failure is a foot gun. Failing to pin a single
> > > page is always an error, the only question is what sub reason caused
> > > the error to happen. There is no third case where it is not an error.
> >
> > The uprobe path thinks otherwise, but maybe the answer is that we just need
> > to -EFAULT on missing VMA and -EPERM on invalid flags.
>
> I think uprobe is just broken to think there is a third outcome. Let's
> just fix it instead of trying to pretend it makes sense.

Sure, will take a look at that if I get a chance. We can at the very least
adjust get_user_page_vma_remote() with this fixed.

Do you feel that a partially successful pinning for other GUP callers
should equally be treated as an error (and pages unpinned -> return error
code)? In that instance we'd need to audit things somewhat.

I have a couple more GUP cleanups saved up, so could add that to my queue
of things to look at between book work :)

>
> Jason
>
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