linux-kernel - Re: [PATCH 2/3] mm/mprotect: Use long for page accountings and retval

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Message-ID: <aabd4bb8-560b-10b0-9837-b656e1a0a9e1@redhat.com>
Date:   Thu, 5 Jan 2023 09:44:16 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Peter Xu <peterx@...hat.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Muchun Song <songmuchun@...edance.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        James Houghton <jthoughton@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/3] mm/mprotect: Use long for page accountings and retval

On 04.01.23 23:52, Peter Xu wrote:
> Switch to use type "long" for page accountings and retval across the whole
> procedure of change_protection().
> 
> The change should have shrinked the possible maximum page number to be half
> comparing to previous (ULONG_MAX / 2), but it shouldn't overflow on any
> system either because the maximum possible pages touched by change
> protection should be ULONG_MAX / PAGE_SIZE.

Yeah, highly unlikely.

> 
> Two reasons to switch from "unsigned long" to "long":
> 
>    1. It suites better on count_vm_numa_events(), whose 2nd parameter takes
>       a long type.
> 
>    2. It paves way for returning negative (error) values in the future.
> 
> Currently the only caller that consumes this retval is change_prot_numa(),
> where the unsigned long was converted to an int.  Since at it, touching up
> the numa code to also take a long, so it'll avoid any possible overflow too
> during the int-size convertion.

I'm wondering if we should just return the number of changed pages via a 
separate pointer and later using an int for returning errors -- when 
touching this interface already.

Only who's actually interested in the number of pages would pass a 
pointer to an unsigned long (NUMA).

And code that expects that there never ever are failures (mprotect, 
NUMA) could simply check for WARN_ON_ONCE(ret).

I assume you evaluated that option as well, what was your conclusion?

-- 
Thanks,

David / dhildenb