linux-kernel - Re: [PATCH v3 04/27] mm/userfaultfd: Introduce special pte for unmapped file-backed mem

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Date:   Thu, 3 Jun 2021 20:14:31 -0700 (PDT)
From:   Hugh Dickins <hughd@...gle.com>
To:     Alistair Popple <apopple@...dia.com>
cc:     Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, Axel Rasmussen <axelrasmussen@...gle.com>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v3 04/27] mm/userfaultfd: Introduce special pte for
 unmapped file-backed mem

On Fri, 4 Jun 2021, Alistair Popple wrote:
> 
> The detail which is perhaps less important is whether to implement this using 
> a new swap entry type or arch-specific swap bit. The argument for using a swap 
> type is it will work across architectures due to the use of pte_to_swp_entry() 
> and swp_entry_to_pte() to convert to and from the arch-dependent and 
> independent representations.
> 
> The argument against seems to have been that it is wasting a swap type. 
> However if I'm understanding correctly that's not true for all architectures, 
> and needing to reserve a bit is more wasteful than using a swap type.

I'm on the outside, not paying much attention here,
but thought Peter would have cleared this up already.

My understanding is that it does *not* use an additional arch-dependent
bit, but puts the _PAGE_UFFD_WP bit (already set aside by any architecture
implementing UFFD WP) to an additional use.  That's why I called this
design (from Andrea) more elegant than mine (swap type business).

If I've got that wrong, and yet another arch-dependent bit is needed,
then I very much agree with you: finding arch-dependent pte bits is a
much tougher job than another play with swap type.

(And "more elegant" might not be "easier to understand": you decide.)

Hugh