linux-kernel - Re: [RFC 3/3] mm/vma: Introduce some more VMA flag wrappers

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Date:   Tue, 3 Mar 2020 14:43:21 +0530
From:   Anshuman Khandual <anshuman.khandual@....com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     linux-mm@...ck.org, "David S. Miller" <davem@...emloft.net>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.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@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>, sparclinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [RFC 3/3] mm/vma: Introduce some more VMA flag wrappers

On 03/03/2020 12:04 PM, Hugh Dickins wrote:
> On Mon, 2 Mar 2020, Anshuman Khandual wrote:
> 
>> This adds the following new VMA flag wrappers which will replace current
>> open encodings across various places. This should not have any functional
>> implications.
>>
>> vma_is_dontdump()
>> vma_is_noreserve()
>> vma_is_special()
>> vma_is_locked()
>> vma_is_mergeable()
>> vma_is_softdirty()
>> vma_is_thp()
>> vma_is_nothp()
> 
> Why?? Please don't. I am not at all keen on your 1/3 and 2/3 (some
> of us actually like to see what the VM_ flags are where they're used,
> without having to chase through scattered wrappers hiding them),
> but this 3/3 particularly upset me.

Can understand your reservations regarding 3/3. But I had called that out
in the series cover letter that this patch can be dropped if related code
churn is not justified.

But 1/3 does create a default flag combination for VM_DATA_DEFAULT_FLAGS
with a value that is used by multiple platforms at the moment. This is
very similar to the existing VM_STACK_DEFAULT_FLAGS which has a default
value. Then why cannot VM_DATA_DEFAULT_FLAGS have one ? More over this
also saves some code duplication across platforms.

Regarding the patch 2/3, when there are many existing VMA flag overrides
like VM_STACK_FLAGS, VM_STACK_INCOMPLETE_SETUP, VM_INIT_DEF_MASK etc why
cannot a commonly used VMA flag combination with a very specific meaning
(i.e accessibility) get one. Do you have any particular concern here
which I might be missing.

> 
> There is a good reason for the (hideously named) is_vm_hugetlb_page(vma):
> to save "#ifdef CONFIG_HUGETLB_PAGE"s all over (though I suspect the
> same could have been achieved much more nicely by #define VM_HUGETLB 0);
> but hiding all flags in vma_is_whatever()s is counter-productive churn.

Makes sense, I can understand your reservation here.

> 
> Improved readability? Not to my eyes.

As mentioned before, I dont feel strongly about patch 3/3 and will drop.