Re: [PATCH 3/4] mm/huge_memory: don't warn when a page table blocks a huge pfn insert

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

On 11/11/26 3:06 PM, Eliav Farber wrote:
> insert_pmd() and insert_pud() treat a non-empty entry at their own level
> as a leaf and compare its PFN against the one being inserted. A page
> table installed by a concurrent order-0 fault also makes the entry
> non-empty, and pmd_pfn()/pud_pfn() then report the PFN of that table,
> which never matches. insert_pmd() warns unless the entry is the huge
> zero PMD, and insert_pud() warns on any mismatch.
> 
> A VM_PFNMAP mapping with both ->fault and ->huge_fault can produce the
> mixed orders: an order-0 fault installs a page table over an address
> while a concurrent huge fault on the same address reaches insert_pmd().
> Changing an mm's THP policy while another thread faults the same PMD,
> for example with prctl(PR_SET_THP_DISABLE), is enough to do it.
> 
> Compare the PFN only for a leaf entry. A page table is not an error, so
> take the existing out_unlock path, which returns VM_FAULT_NOPAGE and
> lets the access refault onto the smaller entry.
> 
> 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