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

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Message-ID: <2408831.NcqaVN92ti@nvdebian>
Date:   Fri, 4 Jun 2021 16:16:30 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Hugh Dickins <hughd@...gle.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>,
        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 Friday, 4 June 2021 1:14:31 PM AEST Hugh Dickins wrote:
> 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).

Oh my bad, I had somehow missed this was reusing an *existing* arch-dependent 
swap bit (_PAGE_SWP_UFFD_WP, although the same argument could apply) even 
though it's in the commit message. Obviously I should have read that more 
carefully, apologies for the noise but thanks for the clarification.

> 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.)

Agree, that's a somewhat subjective debate. Conceptually I don't think this is 
particularly difficult to understand. It just adds another slightly different 
class of special swap pte's to know about.

 - Alistair

> Hugh