Re: [QUESTION] Resizing shared mapping without clashing with others

[David Hildenbrand <david@redhat.com>]

On 02.12.24 16:04, Dmitry Dolgov wrote:
>> On Mon, Dec 02, 2024 at 12:07:01PM GMT, David Hildenbrand wrote:
>>
>> Likely it's best to look into reserving a large VMA space using
>> mmap(MAP_PRIVATE|MA_ANON, PROT_NONE); it's reserved but inaccessible, so it
>> cannot get reallocated for different purposes. Then converting pieces of
>> that into actually usable shared anonymous memory (e.g., MAP_FIXED).
> 
> Yes, we've considered this option, preparing an initial "reserving"
> mapping, then take pieces out of it for usage. As I've mentioned to
> Matthew, there seems to be arguments against this approach. In a few
> words:

Note that this approach is extremely common ;)

> 
> * The impact on OOM decisions is not clear (at least to me).

There are none. mmap(MAP_PRIVATE|MAP_ANON, PROT_NONE) will neither 
allocate memory/page tables nor commit memory.

> * In certain cases this will be counted against a reservation limits,
>    although we don't use the memory yet (e.g. with hugetlb insige a
>    cgroup with hugetlb.2MB.rsvd.limit_in_bytes set).


With  mmap(MAP_PRIVATE|MAP_ANON, PROT_NONE)? Unlikely. In any case, you 
might be able to tame reservations using MAP_NORESERVE.


-- 
Cheers,

David / dhildenb