Re: [PATCH v2 4/4] mm/huge_memory: use the large-page PAT encoding for huge pfn mappings

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On 11/12/26 6:14 AM, Eliav Farber wrote:
> insert_pmd() and insert_pud() build the entry from the pgprot that
> vmf_insert_pfn_pmd()/_pud() took from vma->vm_page_prot and passed
> through pfnmap_setup_cachemode_pfn(). That pgprot carries the PTE-level
> cache mode encoding, but x86 keeps the high PAT bit at bit 7 for a PTE
> (_PAGE_BIT_PAT) and at bit 12 for a PMD or PUD (_PAGE_BIT_PAT_LARGE),
> where bit 7 is _PAGE_PSE.
> 
> A memtype that needs that bit, write-through or write-protect, loses it:
> the cache mode index of the huge entry drops from 4-7 to 0-3. With the
> PAT slots Linux programs, write-through (slot 7) becomes UC (slot 3),
> and write-protect (slot 5) becomes write-combining (slot 1), which loses
> the write protection. The other x86 paths that build a large entry from
> a PTE pgprot convert it first, see populate_pmd() and populate_pud().
> 
> Convert in the two huge pfn paths. Add a generic pgprot_4k_2_large()
> that returns its argument unchanged, so that only architectures which
> encode the levels differently have to define it, and x86's existing
> definition is picked up.
> 
> The folio paths keep using vma->vm_page_prot as is. They map
> struct-page-backed memory, which x86 maps write-back.
> 
> Fixes: 5cad465d7fa6 ("mm: add vmf_insert_pfn_pmd()")
> Signed-off-by: Eliav Farber <farbere@amazon.com>

LGTM:

Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

-- 
cheers,
jose a. p-a