Re: [RFC PATCH 1/1] x86/mm/pat: Clear VM_PAT if copy_p4d_range failed

[Dave Hansen]

On 2/16/23 18:56, Wupeng Ma wrote:
> dup_mm
>   dup_mmap
>     copy_page_range
>       copy_p4d_range
>         copy_pud_range
>           copy_pmd_range
>             pmd_alloc
>               __pmd_alloc
>                 pmd_alloc_one
>                   page = alloc_pages(gfp, 0);
>                     if (!page)
>                       return NULL;
>     mmput
>         exit_mmap
>           unmap_vmas
>             unmap_single_vma
>               untrack_pfn
>                 follow_phys
>                   WARN_ON_ONCE(1);

What's the point of that warning in the first place?  I can certainly
imagine follow_phys() failing for sparse mappings, for instance.  Is
there some requirement that VM_PFNMAP can't be sparse?