linux-kernel - Re: [syzbot] general protection fault in PageHeadHuge

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Date:   Sun, 25 Sep 2022 11:55:03 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Peter Xu <peterx@...hat.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        syzbot <syzbot+152d76c44ba142f8992b@...kaller.appspotmail.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        llvm@...ts.linux.dev, nathan@...nel.org, ndesaulniers@...gle.com,
        songmuchun@...edance.com, syzkaller-bugs@...glegroups.com,
        trix@...hat.com
Subject: Re: [syzbot] general protection fault in PageHeadHuge

On Fri, 23 Sep 2022 17:14:38 -0700 (PDT) Hugh Dickins <hughd@...gle.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.