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

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Date:   Mon, 11 Jan 2021 17:47:13 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Will Deacon <will@...nel.org>
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 02:37:42PM +0000, Will Deacon wrote:
> 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?

Unlike vmf->flags or vmf->address, vmf->pte is not going to be reused.
finish_fault() will overwrite it.

Yeah, it's messy.

-- 
 Kirill A. Shutemov