linux-kernel - Re: [PATCH v2 2/3] mm: Allow architectures to request 'old' entries when prefaulting

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Message-ID: <20210111143742.GD7642@willie-the-truck>
Date:   Mon, 11 Jan 2021 14:37:42 +0000
From:   Will Deacon <will@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org,
        Catalin Marinas <catalin.marinas@....com>,
        Jan Kara <jack@...e.cz>, Minchan Kim <minchan@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Hugh Dickins <hughd@...gle.com>, kernel-team@...roid.com
Subject: Re: [PATCH v2 2/3] mm: Allow architectures to request 'old' entries
 when prefaulting

On Mon, Jan 11, 2021 at 05:25:33PM +0300, Kirill A. Shutemov wrote:
> On Fri, Jan 08, 2021 at 05:15:16PM +0000, Will Deacon wrote:
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index c1f2dc89b8a7..0fb9d1714797 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -3051,14 +3051,18 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
> >  		if (!pte_none(*vmf->pte))
> >  			goto unlock;
> >  
> > +		/* We're about to handle the fault */
> > +		if (vmf->address == address) {
> > +			vmf->flags &= ~FAULT_FLAG_PREFAULT;
> > +			ret = VM_FAULT_NOPAGE;
> > +		} else {
> > +			vmf->flags |= FAULT_FLAG_PREFAULT;
> > +		}
> > +
> 
> Do we need to restore the oririnal status of the bit once we are done?

I can certainly add that, although it doesn't look like we do that for
vmf->pte, so it's hard to tell what the rules are here. It certainly feels
odd to restore some fields but not others, as it looks like vmf->address
will be out-of-whack with vmf->pte when filemap_map_pages() returns. Am I
missing something?

Will