linux-kernel - Re: [PATCH 10/12] selftests/mm: move uffd* routines from vm

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Message-ID: <b67dec68-6648-f73a-cda7-31e3c5fcd65b@nvidia.com>
Date:   Fri, 2 Jun 2023 18:39:24 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Peter Xu <peterx@...hat.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Shuah Khan <shuah@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>, <linux-mm@...ck.org>,
        <linux-kselftest@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/12] selftests/mm: move uffd* routines from vm_util.c to
 uffd-common.c

On 6/2/23 18:18, Peter Xu wrote:
...
>> ...whoops, correction, our very own David Hildenbrand recently made
>> changes that contradict the claim that "ksm and uffd selftests are
>> independent". In fact, ksm now *intentionally* depends upon uffd, as of
>> commit 93fb70aa5904c ("selftests/vm: add KSM unmerge tests"), aha!
>>
>> That added commit added a call to test_unmerge_uffd_wp(), to
>> ksm_functional_tests.c .
>>
>> So this needs to stay approximately as-is, it seems.
> 
> So I think it depends on what is "as-is" to me in the above sentence. :)
> 
> test_unmerge_uffd_wp() impled its own uffd ioctls, and it still doesn't use
> any of uffd-common.h of now (e.g. uffd_test_ops).
> 
> IMHO if we want we can let test_unmerge_uffd_wp() reuse either
> uffd_get_features(), uffd_open(), uffd_register() etc., but still all of
> them are provided by vm_util.h not uffd-common.h for now, and that's
> intended (vm_util.h can contain uffd helpers, or whatever helpers as long
> as generic mm/ unit tests need).

ksm_functional_tests.c calls uffd_register(). That's about as clear
as it gets: this file distinctly depends upon uffd test functionality.

The goal here is to put uffd*() routines into uffd-common.[ch], and
everything else into vm_utils.[ch]. Because that's what you do, when you
have such named files.

Putting uffd*() routines somewhere other than uffd-common.* requires
some...reason. And all I've heard so far is, "it was already
scrambled--as intended, don't mess with it!" :)

> 
> We can even move wp_range() from uffd-common.[ch] into vm_utils.[ch], then
> it can also share that (need to replace err(), that's uffd-common
> specific).  Not necessary anything must be done in this series, though.
> 

But wp_range(), despite its generic-sounding name, is another example of
something that remains tightly coupled to the uffd code: it uses
UFFDIO_WRITEPROTECT to get its work done.

So I'd recommend leaving this one in uffd-common.c.


thanks,
-- 
John Hubbard
NVIDIA