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Subject: Re: [PATCH v2 1/3] mm, oom: move task_will_free_mem up in the file to
 be used in process_mrelease
To:     Suren Baghdasaryan <surenb@google.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Michal Hocko <mhocko@suse.com>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Roman Gushchin <guro@fb.com>, Rik van Riel <riel@surriel.com>,
        Minchan Kim <minchan@kernel.org>,
        Christian Brauner <christian@brauner.io>,
        Christoph Hellwig <hch@infradead.org>,
        Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
        Shakeel Butt <shakeelb@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Florian Weimer <fweimer@redhat.com>,
        Jan Engelhardt <jengelh@inai.de>,
        Tim Murray <timmurray@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-team <kernel-team@android.com>
References: <20210718214134.2619099-1-surenb@google.com>
 <7eb17da6-03a6-5eaf-16e6-97b53ba163d8@redhat.com>
 <20210720160707.2332738708948f7d865d67c1@linux-foundation.org>
 <ba763ed3-9ad1-9502-1ffc-3175446570d1@redhat.com>
 <CAJuCfpEB994Xj3FcmzyH1p3yOdLVf6EwZaGaRj7NJi_c9hbsRQ@mail.gmail.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
Message-ID: <0ee6775b-589c-3243-1c01-aafad5eecb73@redhat.com>
Date:   Wed, 21 Jul 2021 18:12:56 +0200
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On 21.07.21 17:33, Suren Baghdasaryan wrote:
> On Wed, Jul 21, 2021 at 12:30 AM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 21.07.21 01:07, Andrew Morton wrote:
>>> On Tue, 20 Jul 2021 14:43:52 +0200 David Hildenbrand <david@redhat.com> wrote:
>>>
>>>> On 18.07.21 23:41, Suren Baghdasaryan wrote:
>>>>> process_mrelease needs to be added in the CONFIG_MMU-dependent block which
>>>>> comes before __task_will_free_mem and task_will_free_mem. Move these
>>>>> functions before this block so that new process_mrelease syscall can use
>>>>> them.
>>>>>
>>>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>>>> ---
>>>>> changes in v2:
>>>>> - Fixed build error when CONFIG_MMU=n, reported by kernel test robot. This
>>>>> required moving task_will_free_mem implemented in the first patch
>>>>> - Renamed process_reap to process_mrelease, per majority of votes
>>>>> - Replaced "dying process" with "process which was sent a SIGKILL signal" in
>>>>> the manual page text, per Florian Weimer
>>>>> - Added ERRORS section in the manual page text
>>>>> - Resolved conflicts in syscall numbers caused by the new memfd_secret syscall
>>>>> - Separated boilerplate code wiring-up the new syscall into a separate patch
>>>>> to facilitate the review process
>>>>>
>>>>>     mm/oom_kill.c | 150 +++++++++++++++++++++++++-------------------------
>>>>>     1 file changed, 75 insertions(+), 75 deletions(-)
>>>>
>>>> TBH, I really dislike this move as it makes git blame a lot harder with
>>>> any real benefit.
>>>>
>>>> Can't you just use prototypes to avoid the move for now in patch #2?
>>>>
>>>> static bool task_will_free_mem(struct task_struct *task);
>>>
>>> This change makes the code better - it's silly to be adding forward
>>> declarations just because the functions are in the wrong place.
>>
>> I'd really love to learn what "better" here means and if it's rather
>> subjective. When it comes to navigating the code, we do have established
>> tools for that (ctags), and personally I couldn't care less where
>> exactly in a file the code is located.
>>
>> Sure, ending up with a forward-declaration for every function might not
>> be what we want ;)
>>
>>>
>>> If that messes up git-blame then let's come up with better tooling
>>> rather than suffering poorer kernel code because the tools aren't doing
>>> what we want of them.  Surely?
>>
>> I don't agree that what we get is "poorer kernel code" in this very
>> instance; I can understand that we avoid forward-declarations when
>> moving smallish functions. But moving two functions with 75 LOC is a bit
>> too much for my taste at least -- speaking as someone who cares about
>> easy backports and git-blame.
> 
> There is a third alternative here to have process_mrelease() at the
> end of the file with its own #ifdef CONFIG_MMU block, maybe even
> embedded in the function like this:
> 
>   int process_mrelease(int pidfd, unsigned int flags)
> {
> #ifdef CONFIG_MMU
>          ...
> #else
>          return ENOSYS;
> #endif
> }
> 
> This would not require moving other functions.
> Would that be better than the current approach or the forward declaration?

IMHO that could be an easy, possible alternative.

-- 
Thanks,

David / dhildenb