Re: [RFC PATCH 0/4] Migrate on fault for device pages

[Balbir Singh <balbirs@nvidia.com>]

On 8/14/25 17:19, Mika Penttilä wrote:
> As of this writing, the way device page faulting and migration
> works is not optimal, if you want to do both fault handling
> and migration at once.
> 
> Being able to migrate not present pages (or pages mapped with incorrect
> permissions, eg. COW) to the GPU requires doing either of the following
> sequences:
> 
> 1. hmm_range_fault() - fault in non-present pages with correct
>    permissions,etc.
> 2. migrate_vma_*() - migrate the pages
> 
> Or:
> 
> 1. migrate_vma_*() - migrate present pages
> 2. If non-present pages detected by migrate_vma_*():
>    a) call hmm_range_fault() to fault pages in
>    b) call migrate_vma_*() again to migrate now present pages
> 
> The problem with the first sequence is that you always have to do two
> page walks even when most of the time the pages are present or zero page
> mappings so the common case takes a performance hit.
> 
> The second sequence is better for the common case, but far worse if
> pages aren't present because now you have to walk the page tables three
> times (once to find the page is not present, once so hmm_range_fault()
> can find a non-present page to fault in and once again to setup the
> migration). It also tricky to code correctly.
> 
> We should be able to walk the page table once, faulting
> pages in as required and replacing them with migration entries if
> requested.
> 

The use case makes sense to me, but isn't the sequence always going
to be racy, by the time the pages are faulted in, there could be
others that have been marked non-present or do you intend to lock
all pages during this operation?

Balbir