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

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Date:   Sun, 28 May 2023 16:07:31 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Lorenzo Stoakes <lstoakes@...il.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 5/28/23 09:22, Jason Gunthorpe wrote:
> On Sun, May 28, 2023 at 04:13:44PM +0100, Lorenzo Stoakes wrote:
>> 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.

Great!

We've had previous discussions about getting rid of this pseudo-tristate
errno in gup, so I just wanted to mention that I'm also glad to see the
movement toward, "return some pages, or else a -errno". That's progress.


>>
>> 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.
> 
> That seems more deeply ingrained at least, I'm not as keen to change
> it as to get rid of the 0 return result.
> 

Yes. It's not just "audit things somewhat", it's more like, "change
quite a few call sites, some of which actually gather sets of partial
results in a retry loop". So some actual coding changes there.


thanks,
-- 
John Hubbard
NVIDIA