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Date: Sun, 25 Sep 2022 11:55:03 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Hugh Dickins <hughd@google.com>
Cc: Mike Kravetz <mike.kravetz@oracle.com>, Peter Xu <peterx@redhat.com>,
 Axel Rasmussen <axelrasmussen@google.com>, Yang Shi <shy828301@gmail.com>,
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Subject: Re: [syzbot] general protection fault in PageHeadHuge
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On Fri, 23 Sep 2022 17:14:38 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:

> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2060,7 +2060,7 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
>  	struct vm_area_struct *vma = vmf->vma;
>  	struct inode *inode = file_inode(vma->vm_file);
>  	gfp_t gfp = mapping_gfp_mask(inode->i_mapping);
> -	struct folio *folio;
> +	struct folio *folio = NULL;
>  	int err;
>  	vm_fault_t ret = VM_FAULT_LOCKED;
>  
> @@ -2127,7 +2127,8 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
>  				  gfp, vma, vmf, &ret);
>  	if (err)
>  		return vmf_error(err);
> -	vmf->page = folio_file_page(folio, vmf->pgoff);
> +	if (folio)
> +		vmf->page = folio_file_page(folio, vmf->pgoff);
>  	return ret;
>  }

I grabbed this as a fix against
shmem-convert-shmem_fault-to-use-shmem_get_folio_gfp.patch.  Hopefully
someone can send along something more formal when ready.