Message-ID: <387f3516-99f2-41e9-967e-4b051a8d21b8@redhat.com>
Date: Thu, 20 Feb 2025 10:23:57 +0100
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Kalesh Singh <kaleshsingh@...gle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Suren Baghdasaryan <surenb@...gle.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
 "Paul E . McKenney" <paulmck@...nel.org>, Jann Horn <jannh@...gle.com>,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
 linux-api@...r.kernel.org, John Hubbard <jhubbard@...dia.com>,
 Juan Yescas <jyescas@...gle.com>
Subject: Re: [PATCH 0/4] mm: permit guard regions for file-backed/shmem
 mappings

On 20.02.25 10:04, Lorenzo Stoakes wrote:
> On Thu, Feb 20, 2025 at 09:57:37AM +0100, David Hildenbrand wrote:
>> On 20.02.25 09:51, Lorenzo Stoakes wrote:
>>> On Wed, Feb 19, 2025 at 12:56:31PM -0800, Kalesh Singh wrote:
>>>>> We also can't change smaps in the way you want, it _has_ to still give
>>>>> output per VMA information.
>>>>
>>>> Sorry I wasn't suggesting to change the entries in smaps, rather
>>>> agreeing to your marker suggestion. Maybe a set of ranges for each
>>>> smaps entry that has guards? It doesn't solve the use case, but does
>>>> make these regions visible to userspace.
>>>
>>> No, you are not providing a usecase for this. /proc/$pid/pagemaps does not
>>> contaminate the smaps output, mess with efforts to make it RCU readable,
>>> require updating the ioctl interface, etc. so it is clearly the better
>>> choice.
>>>
>>>>
>>>>>
>>>>> The proposed change that would be there would be a flag or something
>>>>> indicating that the VMA has guard regions _SOMEWHERE_ in it.
>>>>>
>>>>> Since this doesn't solve your problem, adds complexity, and nobody else
>>>>> seems to need it, I would suggest this is not worthwhile and I'd rather not
>>>>> do this.
>>>>>
>>>>> Therefore for your needs there are literally only two choices here:
>>>>>
>>>>> 1. Add a bit to /proc/$pid/pagemap OR
>>>>> 2. a new interface.
>>>>>
>>>>> I am not in favour of a new interface here, if we can just extend pagemap.
>>>>>
>>>>> What you'd have to do is:
>>>>>
>>>>> 1. Find virtual ranges via /proc/$pid/maps
>>>>> 2. iterate through /proc/$pid/pagemaps to retrieve state for all ranges.
>>>>>
>>>>
>>>> Could we also consider an smaps field like:
>>>>
>>>> VmGuards: [AAA, BBB), [CCC, DDD), ...
>>>>
>>>> or something of that sort?
>>>
>>> No, absolutely, categorically not. You realise these could be thousands of
>>> characters long right?
>>>
>>> /proc/$pid/pagemaps resolves this without contaminating this output.
>>>
>>>>> Well I'm glad that you guys find it useful for _something_ ;)
>>>>>
>>>>> Again this wasn't written only for you (it is broadly a good feature for
>>>>> upstream), but I did have your use case in mind, so I'm a little
>>>>> disappointed that it doesn't help, as I like to solve problems.
>>>>>
>>>>> But I'm glad it solves at least some for you...
>>>>
>>>> I recall Liam had a proposal to store the guard ranges in the maple tree?
>>>>
>>>> I wonder if that can be used in combination with this approach to have
>>>> a better representation of this?
>>>
>>> This was an alternative proposal made prior to the feature being
>>> implemented (and you and others at Google were welcome to comment and many
>>> were cc'd, etc.).
>>>
>>> There is no 'in combination with'. This feature would take weeks/months to
>>> implement, fundamentally impact the maple tree VMA implementation
>>> and... not actually achieve anything + immediately be redundant.
>>>
>>> Plus it'd likely be slower, have locking implications, would have kernel
>>> memory allocation implications, a lot more complexity and probably other
>>> problems besides (we discussed this at length at the time and a number of
>>> issues came up, I can't recall all of them).
>>>
>>> To be crystal clear - we are empathically NOT changing /proc/$pid/maps to
>>> lie about VMAs regardless of underlying implementation, nor adding
>>> thousands of characters to /proc/$pid/smaps entries.
>>
>> Yes. Calling it a "guard region" might be part of the problem
>> (/"misunderstanding"), because it reminds people of "virtual memory
>> regions".
>>
>> "Guard markers" or similar might have been clearer that these operate on
>> individual PTEs, require page table scanning etc ... which makes them a lot
>> more scalable and fine-grained and provides all these benfits, with the
>> downside being that we don't end up with that many "virtual memory regions"
>> that maps/smaps operate on.
> 
> Honestly David you and the naming... :P
> 
 > I disagree, sorry. Saying 'guard' anything might make people think 
one> thing or another. We can't account for that. I mean don't get me 
started on
> 'pinning' or any of the million other overloaded terms we use...
> 
 > I _hugely_ publicly went out of my way to express the limitations, I 
gave a> talk, we had meetings, I mentioned it in the series.
> 
> Honestly if at that point you still don't realise, that's not a naming
> problem. It's a 'did not participate with upstream' problem.
> 
> I like guard regions, as they're not pages as we previously referred to
> them. People have no idea what a marker is, it doesn't sound like it spans
> ranges, no don't like it sorry.
> 
> And sorry but this naming topic is closed :) I already let you change the
> naming of the MADV_'s, which broke my heart, there will not be a second
> heart breaking...

Lorenzo, I was not pushing for it to be changed *now*, that ship has 
sailed, and personally, I *don't* find it confusing because I know how 
it works under the hood.

I was trying to find a reason *why* people would thing that it would 
show up in smaps in the first place. For example, just when reading the 
MAN page *today*.

Doesn't really matter now, it is named the way it is, and all we can do 
is try making documentation clearer if it keeps confusing people.

Your conclusion is 'did not participate with upstream'; I don't agree 
with that. But maybe you and Kalesh have a history on that that let's 
you react on his questions IMHO more emotionally than it should have been.


> 
>>
>> [...]
>>
>>>
>>> As I said to you earlier, the _best_ we could do in smaps would be to add a
>>> flag like 'Grd' or something to indicate some part of the VMA is
>>> guarded. But I won't do that unless somebody has an -actual use case- for
>>> it.
>>
>> Right, and that would limit where you have to manually scan. Something
>> similar is being done with uffd-wp markers IIRC.
> 
> Yeah that's a good point, but honestly if you're reading smaps that reads
> the page tables, then reading /proc/$pid/pagemaps and reading page tables
> TWICE that seems inefficient vs. just reading /proc/$pid/maps, then reading
> /proc/$pid/pagemaps and reading page tables once.

Right; I recently wished that we would have an interface to obtain more 
VMA flags without having to go through smaps

-- 
Cheers,

David / dhildenb

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