Re: [PATCH v2] mm: fix finish_fault() handling for large folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/2/27 00:23, Brian Geffon wrote:
> When handling faults for anon shmem finish_fault() will attempt to install
> ptes for the entire folio. Unfortunately if it encounters a single
> non-pte_none entry in that range it will bail, even if the pte that
> triggered the fault is still pte_none. When this situation happens the
> fault will be retried endlessly never making forward progress.
> 
> This patch fixes this behavior and if it detects that a pte in the range
> is not pte_none it will fall back to setting a single pte.
> 
> Cc: stable@vger.kernel.org
> Cc: Hugh Dickins <hughd@google.com>
> Fixes: 43e027e41423 ("mm: memory: extend finish_fault() to support large folio")
> Suggested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Reported-by: Marek Maslanka <mmaslanka@google.com>
> Signed-off-by: Brian Geffon <bgeffon@google.com>
> ---
>   mm/memory.c | 15 ++++++++++-----
>   1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index b4d3d4893267..b6c467fdbfa4 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5183,7 +5183,11 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   	bool is_cow = (vmf->flags & FAULT_FLAG_WRITE) &&
>   		      !(vma->vm_flags & VM_SHARED);
>   	int type, nr_pages;
> -	unsigned long addr = vmf->address;
> +	unsigned long addr;
> +	bool needs_fallback = false;
> +
> +fallback:
> +	addr = vmf->address;
>   
>   	/* Did we COW the page? */
>   	if (is_cow)
> @@ -5222,7 +5226,8 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   	 * approach also applies to non-anonymous-shmem faults to avoid
>   	 * inflating the RSS of the process.
>   	 */
> -	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma))) {
> +	if (!vma_is_anon_shmem(vma) || unlikely(userfaultfd_armed(vma)) ||
> +			unlikely(needs_fallback)) {

Nit: can you align the code? Otherwise look good to me.

>   		nr_pages = 1;
>   	} else if (nr_pages > 1) {
>   		pgoff_t idx = folio_page_idx(folio, page);
> @@ -5258,9 +5263,9 @@ vm_fault_t finish_fault(struct vm_fault *vmf)
>   		ret = VM_FAULT_NOPAGE;
>   		goto unlock;
>   	} else if (nr_pages > 1 && !pte_range_none(vmf->pte, nr_pages)) {
> -		update_mmu_tlb_range(vma, addr, vmf->pte, nr_pages);
> -		ret = VM_FAULT_NOPAGE;
> -		goto unlock;
> +		needs_fallback = true;
> +		pte_unmap_unlock(vmf->pte, vmf->ptl);
> +		goto fallback;
>   	}
>   
>   	folio_ref_add(folio, nr_pages - 1);