Re: [PATCH v2 3/4] mm/huge_memory: don't warn when a page table blocks a huge pfn insert

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential
issue to consider:
- [Low] mm/huge_memory: inaccurate Fixes trailer for insert_pmd() and
  insert_pud()
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commit b401d2d30ea8d00390aed1659aa84d49600aa4d3
Author: Eliav Farber <farbere@amazon.com>

> mm/huge_memory: don't warn when a page table blocks a huge pfn insert
>
> 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>
> Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

[Severity: Low]
In insert_pmd() and insert_pud(), the pmd_pfn() and pud_pfn() comparisons
and their WARN_ON_ONCE() calls only run when write is true.  Commit
f9e54c3a2f5b ("vfio/pci: implement huge_fault support") always passes
write = false to vmf_insert_pfn_pmd() and vmf_insert_pfn_pud(), so it
cannot reach this warning.

Should the Fixes: tag reference c6f3c5ee40c1 ("mm/huge_memory.c: fix
modifying of page protection by insert_pfn_pmd()"), which added these
checks to mm/huge_memory.c?

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Sashiko AI review · https://sashiko.dev/#/patchset/20261112051440.16140-1-farbere@amazon.com?part=3