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

From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Will Deacon <will@kernel.org>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org,
	linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Jan Kara <jack@suse.cz>, Minchan Kim <minchan@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Vinayak Menon <vinmenon@codeaurora.org>,
	Hugh Dickins <hughd@google.com>,
	kernel-team@android.com
Subject: Re: [PATCH v2 2/3] mm: Allow architectures to request 'old' entries when prefaulting
Date: Mon, 11 Jan 2021 17:47:13 +0300	[thread overview]
Message-ID: <20210111144713.3akhv75xzk7u6rgu@box> (raw)
In-Reply-To: <20210111143742.GD7642@willie-the-truck>

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