Re: (subset) [PATCH 0/2] arm64: hugetlb: Fix page fault loop for sw-dirty/hw-clean contiguous PTEs

[Catalin Marinas]

On Mon, 04 Dec 2023 17:26:44 +0000, James Houghton wrote:
> It is currently possible for a userspace application to enter a page
> fault loop when using HugeTLB pages implemented with contiguous PTEs
> when HAFDBS is not available. This happens because:
> 1. The kernel may sometimes write PTEs that are sw-dirty but hw-clean
>    (PTE_DIRTY | PTE_RDONLY | PTE_WRITE).
> 2. If, during a write, the CPU uses a sw-dirty, hw-clean PTE in handling
>    the memory access on a system without HAFDBS, we will get a page
>    fault.
> 3. HugeTLB will check if it needs to update the dirty bits on the PTE.
>    For contiguous PTEs, it will check to see if the pgprot bits need
>    updating. In this case, HugeTLB wants to write a sequence of
>    sw-dirty, hw-dirty PTEs, but it finds that all the PTEs it is about
>    to overwrite are all pte_dirty() (pte_sw_dirty() => pte_dirty()),
>    so it thinks no update is necessary.
> 
> [...]

Applied to arm64 (for-next/fixes), thanks!

[2/2] arm64: mm: Always make sw-dirty PTEs hw-dirty in pte_modify
      https://git.kernel.org/arm64/c/3c0696076aad

I only picked up the second patch and added the description from the
cover letter into the commit log.

-- 
Catalin