linux-kernel - Re: [PATCH v6 01/23] mm: Introduce PTE

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Message-ID: <Yam9EezLTANRA+Rf@xz-m1.local>
Date:   Fri, 3 Dec 2021 14:45:37 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Hugh Dickins <hughd@...gle.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Jerome Glisse <jglisse@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH v6 01/23] mm: Introduce PTE_MARKER swap entry

On Fri, Dec 03, 2021 at 04:35:38PM +1100, Alistair Popple wrote:
> > > > +static inline pte_marker pte_marker_get(swp_entry_t entry)
> > > > +{
> > > > +	return swp_offset(entry) & PTE_MARKER_MASK;
> > > 
> > > I'm not sure the PTE_MARKER_MASK adds much, especially as we only have one
> > > user. I don't see a problem with open-coding these kind of checks (ie.
> > 
> > It's more or less a safety belt to make sure anything pte_marker_get() returned
> > will be pte_marker defined bits only.
> > 
> > > swp_offset(entry) & PTE_MARKER_UFFD_WP) as you kind of end up doing that anyway.
> > > Alternatively if you want helper functions I think it would be better to define
> > > them for each marker. Eg: is_pte_marker_uffd_wp().
> > 
> > Yes we can have something like is_pte_marker_uffd_wp(), I didn't do that
> > explicitly because I want us to be clear that pte_marker is a bitmask, so
> > calling "is_*" will be slightly opaque - strictly speaking it should be
> > "pte_marker_has_uffd_wp_bit()" if there will be more bits defined, but then the
> > name of the helper will look a bit odd too.  Hence I just keep the only
> > interface to fetch the whole marker and use "&" in the call sites to check.
> 
> Why does a caller need to care if it's a bitmask or not though? Isn't that an
> implementation detail that could be left to the "is_*" functions? I must admit
> I'm still working through the rest of this series though - is it because you
> end up storing some kind of value in the upper bits of the PTE marker?

Nop. I'm just afraid the caller could overlook the fact that it's a bitmask,
then there can be code like:

  if (is_pte_marker_uffd_wp(*ptep) && drop_uffd_wp)
      pte_clear(ptep)

While we should only do:

  if (is_pte_marker_uffd_wp(*ptep) && drop_uffd_wp)
      // remove uffd-wp bit in the pte_marker, keep the reset bitmask

I could be worrying too much, there's no real user of it.  If you prefer the
helper a lot I can add it in the new version.  Thanks,

-- 
Peter Xu