Re: [PATCH v6 04/23] mm/uffd: PTE_MARKER_UFFD_WP

From: Peter Xu <peterx@redhat.com>
To: Alistair Popple <apopple@nvidia.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Axel Rasmussen <axelrasmussen@google.com>,
	Nadav Amit <nadav.amit@gmail.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Hugh Dickins <hughd@google.com>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	Jerome Glisse <jglisse@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH v6 04/23] mm/uffd: PTE_MARKER_UFFD_WP
Date: Thu, 16 Dec 2021 13:45:28 +0800	[thread overview]
Message-ID: <YbrSeADehDoToV6L@xz-m1.local> (raw)
In-Reply-To: <1966495.R0KUr5dvoW@nvdebian>

On Thu, Dec 16, 2021 at 04:18:50PM +1100, Alistair Popple wrote:
> On Monday, 15 November 2021 6:55:03 PM AEDT Peter Xu wrote:
> 
> [...]
> 
> > +/*
> > + * Returns true if this is a swap pte and was uffd-wp wr-protected in either
> > + * forms (pte marker or a normal swap pte), false otherwise.
> > + */
> > +static inline bool pte_swp_uffd_wp_any(pte_t pte)
> > +{
> > +#ifdef CONFIG_PTE_MARKER_UFFD_WP
> > +	if (!is_swap_pte(pte))
> > +		return false;
> > +
> > +	if (pte_swp_uffd_wp(pte))
> > +		return true;
> 
> If I'm not mistaken normal swap uffd-wp ptes can still exist when
> CONFIG_PTE_MARKER_UFFD_WP=n so shouldn't this be outside the #ifdef protection?
> 
> In fact we could drop the #ifdef entirely here as it is safe to call
> is_pte_marker_uffd_wp() without CONFIG_PTE_MARKER_UFFD_WP.

But if CONFIG_PTE_MARKER_UFFD_WP=n then it means we don't support file-backed
uffd-wp.  The thing is pte_swp_uffd_wp_any() is only needed for file-backed..
Anonymous uffd-wp code never uses it.

In other words, pte_swp_uffd_wp() is indeed still a valid helper, however this
specific function (pte_swp_uffd_wp_any) may not really be necessary anymore.

Keeping the "#ifdef" could still help, imho, to generate clean code for direct
calls to pte_swp_uffd_wp_any() if someone wants to turn pte markers off,
e.g. we could still have chance to optimize below:

        if (pte_swp_uffd_wp_any(pte) &&
                !(zap_flags & ZAP_FLAG_DROP_MARKER))
                set_huge_pte_at(mm, address, ptep,
                                make_pte_marker(PTE_MARKER_UFFD_WP));
        else
                huge_pte_clear(mm, address, ptep, sz);

Into:

        huge_pte_clear(mm, address, ptep, sz);

with any decent compiler.

That's majorly why I still slightly prefer to keep it, and that's also one of
the major reason to have the config knob, imho.

Thanks,

-- 
Peter Xu