Migration/remap question for hugee pages of 2MB

Migration/remap question for hugee pages of 2MB

[Mounesh Badiger]

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Hi,
Can linux-mm group confirm, if huge pages of size 2MB cannot be migrated or
remapped when user-space uses them for IO with the bypassing kernel?  We
need to VA->PA intact once it is mapped.

We want to reserve huge pages and map them using hugetlbfs into the
userspace.  Use /proc/<pid>/pagemap  to get VA->PA mapping and use PA for
IO.


-- 
Mounesh Badiger
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Re: Migration/remap question for hugee pages of 2MB

[David Hildenbrand]

On 21.01.25 09:18, Mounesh Badiger wrote:
> Hi,

Hi,

> Can linux-mm group confirm, if huge pages of size 2MB cannot be migrated 
> or remapped when user-space uses them for IO with the bypassing kernel?  
> We need to VA->PA intact once it is mapped.
> 
> We want to reserve huge pages and map them using hugetlbfs into the 
> userspace.  Use /proc/<pid>/pagemap  to get VA->PA mapping and use PA 
> for IO.

A kernel that supports CONFIG_MIGRATION will be able to migrate 2 MiB 
hugetlb folios on most architectures (e.g., x86-64). Such migration 
might be triggered by CMA allocations or memory offlining.

IIRC, ordinary page compaction should not be migrating 2 MB hugetlb 
folios, because they consume a full pageblock and there is, therefore, 
no value in trying to compact such pageblocks.

So short answer: they can get migrated, but migration might not happen 
frequently.

To block migration, you could long-term pin these pages from getting 
migrated either using vmsplice() or iouring fixed buffers. (the latter 
is preferred).

-- 
Cheers,

David / dhildenb