linux-kernel - Re: [PATCH RFC v8 18/56] x86/fault: fix handle_split_page

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Date:   Mon, 20 Feb 2023 11:57:59 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     Michael Roth <michael.roth@....com>
cc:     kvm@...r.kernel.org, linux-coco@...ts.linux.dev,
        linux-mm@...ck.org, linux-crypto@...r.kernel.org, x86@...nel.org,
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        dgilbert@...hat.com, jarkko@...nel.org, ashish.kalra@....com,
        nikunj.dadhania@....com, Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH RFC v8 18/56] x86/fault: fix handle_split_page_fault()
 to work with memfd backed pages

On Mon, 20 Feb 2023, Michael Roth wrote:

> From: Hugh Dickins <hughd@...gle.com>

No.

> 
> When the address is backed by a memfd, the code to split the page does
> nothing more than remove the PMD from the page tables. So immediately
> install a PTE to ensure that any other pages in that 2MB region are
> brought back as in 4K pages.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>

No.  Suggested-by would be okay.

> Cc: Hugh Dickins <hughd@...gle.com>

Thanks.  I'm really sorry to be such a jobsworth,
and have nothing more constructive to say than I did before in
https://lore.kernel.org/linux-mm/7f2228c4-1586-2934-7b92-1a9d23b6046@google.com/
(please re-read), but adding a Signed-off-by where none was given is wrong;
and I'm not ever going to comprehend enough of the context to give it.

Best of luck for the series,
Hugh

> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> Signed-off-by: Michael Roth <michael.roth@....com>
> ---
>  mm/memory.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index e68da7e403c6..33c9020ba1f8 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4999,6 +4999,11 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>  static int handle_split_page_fault(struct vm_fault *vmf)
>  {
>  	__split_huge_pmd(vmf->vma, vmf->pmd, vmf->address, false, NULL);
> +	/*
> +	 * Install a PTE immediately to ensure that any other pages in
> +	 * this 2MB region are brought back in as 4K pages.
> +	 */
> +	__pte_alloc(vmf->vma->vm_mm, vmf->pmd);
>  	return 0;
>  }
>  
> -- 
> 2.25.1