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

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Date:   Mon, 20 Feb 2023 14:31:27 -0600
From:   Michael Roth <michael.roth@....com>
To:     Hugh Dickins <hughd@...gle.com>
CC:     <kvm@...r.kernel.org>, <linux-coco@...ts.linux.dev>,
        <linux-mm@...ck.org>, <linux-crypto@...r.kernel.org>,
        <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <tglx@...utronix.de>, <mingo@...hat.com>, <jroedel@...e.de>,
        <thomas.lendacky@....com>, <hpa@...or.com>, <ardb@...nel.org>,
        <pbonzini@...hat.com>, <seanjc@...gle.com>, <vkuznets@...hat.com>,
        <jmattson@...gle.com>, <luto@...nel.org>,
        <dave.hansen@...ux.intel.com>, <slp@...hat.com>,
        <pgonda@...gle.com>, <peterz@...radead.org>,
        <srinivas.pandruvada@...ux.intel.com>, <rientjes@...gle.com>,
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        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        <alpergun@...gle.com>, <dgilbert@...hat.com>, <jarkko@...nel.org>,
        <ashish.kalra@....com>, <nikunj.dadhania@....com>
Subject: Re: [PATCH RFC v8 18/56] x86/fault: fix handle_split_page_fault() to
 work with memfd backed pages

On Mon, Feb 20, 2023 at 11:57:59AM -0800, Hugh Dickins wrote:
> 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.

Hi Hugh,

Sorry for the mix-up, I'd intended to remove this patch before submitting. I'll
make sure to remove it from future postings.

> 
> Best of luck for the series,

Thank you!

-Mike

> 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