Re: [PATCH 1/4] mm: drop the huge PMD assertion from walk_to_pmd()

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

On 11/11/26 3:06 PM, Eliav Farber wrote:
> walk_to_pmd() asserts that the PMD it returns is not a huge entry. That
> held when commit c2febafc6773 ("mm: convert generic code to 5-level
> paging") added the assertion: nothing installed a huge PMD over the
> mappings its callers walk.
> 
> ARCH_SUPPORTS_HUGE_PFNMAP changed that. A VM_PFNMAP mapping whose driver
> provides both ->fault and ->huge_fault can have a PMD leaf installed by a
> concurrent huge fault over the same address, so a caller can observe one
> and BUG on CONFIG_DEBUG_VM=y.
> 
> The walk does not need the invariant. pmd_alloc() only allocates when the
> PUD is empty and otherwise returns pmd_offset(), so a leaf is returned
> rather than overwritten. Every caller already copes with one, because
> they all reach pte_offset_map_lock(), which refuses to walk a leaf and
> returns NULL: insert_pages() turns that into -EFAULT, get_locked_pte()
> passes the NULL to its callers, and insert_pfn() returns a fault code.
> 
> Fixes: f9e54c3a2f5b ("vfio/pci: implement huge_fault support")
> Signed-off-by: Eliav Farber <farbere@amazon.com>

LGTM:

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

-- 
cheers,
jose a. p-a