linux-kernel - Re: [PATCH] MAINTAINERS: group all VMA-related files into the VMA section

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Message-ID: <c53c8f59-1e5d-40d5-97df-7200c21c43fe@redhat.com>
Date: Mon, 9 Dec 2024 15:09:26 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MAINTAINERS: group all VMA-related files into the VMA
 section

On 09.12.24 11:06, Lorenzo Stoakes wrote:
> On Mon, Dec 09, 2024 at 10:16:21AM +0100, David Hildenbrand wrote:
>> On 06.12.24 20:16, Lorenzo Stoakes wrote:
>>> There are a number of means of interacting with VMA operations within mm,
>>> and we have on occasion not been made aware of impactful changes due to
>>> these sitting in different files, most recently in [0].
>>>
>>> Correct this by bringing all VMA operations under the same section in
>>> MAINTAINERS. Additionally take the opportunity to combine MEMORY MAPPING
>>> with VMA as there needn't be two entries as they amount to the same thing.
>>>
>>> [0]:https://lore.kernel.org/linux-mm/CAG48ez0siYGB8GP5+Szgj2ovBZAkL6Zi4n6GUAjzzjFV9LTkRQ@mail.gmail.com/
>>>
>>> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>> ---
>>>    MAINTAINERS | 19 +++++++------------
>>>    1 file changed, 7 insertions(+), 12 deletions(-)
>>>
>>> diff --git a/MAINTAINERS b/MAINTAINERS
>>> index 1e930c7a58b1..95db20c26f5f 100644
>>> --- a/MAINTAINERS
>>> +++ b/MAINTAINERS
>>> @@ -15060,18 +15060,6 @@ F:	tools/mm/
>>>    F:	tools/testing/selftests/mm/
>>>    N:	include/linux/page[-_]*
>>>
>>> -MEMORY MAPPING
>>> -M:	Andrew Morton <akpm@...ux-foundation.org>
>>> -M:	Liam R. Howlett <Liam.Howlett@...cle.com>
>>> -M:	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
>>> -R:	Vlastimil Babka <vbabka@...e.cz>
>>> -R:	Jann Horn <jannh@...gle.com>
>>> -L:	linux-mm@...ck.org
>>> -S:	Maintained
>>> -W:	http://www.linux-mm.org
>>> -T:	git git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
>>> -F:	mm/mmap.c
>>> -
>>>    MEMORY TECHNOLOGY DEVICES (MTD)
>>>    M:	Miquel Raynal <miquel.raynal@...tlin.com>
>>>    M:	Richard Weinberger <richard@....at>
>>> @@ -25028,6 +25016,13 @@ L:	linux-mm@...ck.org
>>>    S:	Maintained
>>>    W:	https://www.linux-mm.org
>>>    T:	git git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
>>> +F:	mm/madvise.c
>>> +F:	mm/mlock.c
>>> +F:	mm/mmap.c
>>> +F:	mm/mprotect.c
>>> +F:	mm/mremap.c
>>> +F:	mm/mseal.c
>>> +F:	mm/msync.c
>>
>> Not sure about mprotect.c, mlock.c and madvise.c, though. I'd claim that the
>> real "magic" they perform is in page table handling and not primarily VMA
>> handling (yes, both do VMA changes, but they are the "easy" part ;) ).
> 
> And large parts of the VMA logic interface with page tables, see - the entire
> 6.12 cycle - around recent changes in mmap() MAP_FIXED - which... the VMA
> maintainers fixed :)
> 
> And then there were the issues around VMA and mm locking relating to page
> table work which... oh right yeah we had to fix again... :>)
> 
> I mean you can make this argument about probably all of these files (mremap
> has -tons- of page table-specific stuff), and then we get back to not being
> notified about key changes that interface with memory mapping/VMA which we
> end up having to deal with anyway.
> 
> A lot of the reason we have 'magic' in these files anyway is because we
> don't have a decent generic page table handler. Not sure I'd actually use
> the word 'magic' for that though.
> 
> I am planning to make significant changes to mprotect/mlock soon, which
> have some terribly duplicated horrible handling logic, and both are key
> considerations in VMA logic as a whole.
> 
> Anyway, as far as I'm concerned page table manipulation after the point of
> faulting is completely within the purvue of VMA manipulation and a side
> product of it.
> 
> However, can concede mm/madvise.c if you feel strongly about that as that's
> a bit blurry, but of course contains a whole bunch of VMA and... page table
> manipulation :) I mean it still to me seems very pertinent.
> 

And then we have mprotect.c being heavily used by uffd-wp and NUMA 
hinting, which don't perform any VMA modification.

That's why I don't think the change proposed here is really the right step.

 >>>> They have much more in common with memory.c, which I wouldn't want 
to see in
>> here either. Hm.
> 
> No, memory.c is really dedicated to fault handling. This is really
> different from manipulating page tables in specific cases in my opinion.

And fork and such stuff. And if you look into huge_memory.c, we actually 
moved all of the THP logic for mprotect()/madvise()/... in there.

Not sure if something similar should have been done for memory.c, or if 
the THP stuff should actually also have gone into the respective files.

To me it sounds wrong to have VMA maintainers maintain a lot of the code 
in these files code because these files somehow modify VMAs, sorry.

-- 
Cheers,

David / dhildenb