Message-ID: <83e33641-8c42-4341-8e6e-5c75d00f93b9@redhat.com>
Date: Wed, 8 Oct 2025 17:43:23 +0200
From: David Hildenbrand <david@...hat.com>
To: Michal Hocko <mhocko@...e.com>
Cc: Gregory Price <gourry@...rry.net>, linux-mm@...ck.org, corbet@....net,
 muchun.song@...ux.dev, osalvador@...e.de, akpm@...ux-foundation.org,
 hannes@...xchg.org, laoar.shao@...il.com, brauner@...nel.org,
 mclapinski@...gle.com, joel.granados@...nel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, Mel Gorman <mgorman@...e.de>,
 Alexandru Moise <00moses.alexander00@...il.com>,
 Mike Kravetz <mike.kravetz@...cle.com>, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] Revert "mm, hugetlb: remove hugepages_treat_as_movable
 sysctl"

On 08.10.25 17:23, Michal Hocko wrote:
> On Wed 08-10-25 17:14:26, David Hildenbrand wrote:
>> On 08.10.25 16:59, Michal Hocko wrote:
>>> On Wed 08-10-25 10:58:23, David Hildenbrand wrote:
>>>> On 07.10.25 23:44, Gregory Price wrote:
>>> [...]
>>>>> @@ -926,7 +927,8 @@ static inline gfp_t htlb_alloc_mask(struct hstate *h)
>>>>>     {
>>>>>     	gfp_t gfp = __GFP_COMP | __GFP_NOWARN;
>>>>> -	gfp |= hugepage_movable_supported(h) ? GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
>>>>> +	gfp |= (hugepage_movable_supported(h) || hugepages_treat_as_movable) ?
>>>>> +	       GFP_HIGHUSER_MOVABLE : GFP_HIGHUSER;
>>>>
>>>> I mean, this is as ugly as it gets.
>>>>
>>>> Can't we just let that old approach RIP where it belongs? :)
>>>>
>>>> If something unmovable, it does not belong on ZONE_MOVABLE, as simple as that.
>>>
>>> yes, I do agree. This is just muddying the semantic of the zone.
>>>
>>> Maybe what we really want is to have a configurable zone rather than a
>>> very specific consumer of it instead. What do I mean by that? We clearly
>>> have physically (DMA, DMA32) and usability (NORMAL, MOVABLE) constrained
>>> zones. So rather than having a MOVABLE zone we can have a single zone
>>> $FOO_NAME zone with configurable attributes - like allocation
>>> constrains (kernel, user, movable, etc). Now that we can overlap zones
>>> this should allow for quite a lot flexibility. Implementation wise this
>>> would require some tricks as we have 2 zone types for potentially 3
>>> different major usecases (kernel allocations, userspace reserved ranges
>>> without movability and movable allocations). I haven't thought this
>>> through completely and mostly throwing this as an idea (maybe won't
>>> work). Does that make sense?
>>
>> I suggested something called PREFER_MOVABLE in the past, that would prefer
>> movable allocations but nothing would stop unmovable allocations to end up
>> on it. But only as a last resort or when explicitly requested (e.g.,
>> gigantic pages).
>>
>> Maybe that's similar to what you have in mind?
> 
> Slightly different because what I was thinking about was more towards
> guarantee/predictability. Last resort is quite hard to plan around. It
> might be a peak memory pressure to eat up portion of a memory block and
> then fragmenting it to prevent other use planned for that memroy block.
> That is why I called it user allocations because those are supposed to
> be configured for userspace consumation and planned for that use. So you
> would get pretty much a guarantee that no kernel allocations will fall
> there.

What could end up on it that would not already end up on ZONE_MOVABLE? I 
guess long-term pinned pages, secretmem, guest_memfd, gigantic pages.

Anything else?

I'm not quite clear yet on the use case, though. If all the user 
allocations end up fragmenting the memory, there is also not a lot of 
benefit to be had from that zone long term.

-- 
Cheers

David / dhildenb

Powered by blists - more mailing lists