linux-kernel - Re: [PATCH 2/3] userfaultfd: UFFDIO

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Date:   Thu, 14 Sep 2023 21:15:31 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, viro@...iv.linux.org.uk,
        brauner@...nel.org, shuah@...nel.org,
        Andrea Arcangeli <aarcange@...hat.com>, lokeshgidra@...gle.com,
        Peter Xu <peterx@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, mhocko@...e.com,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Mike Rapoport <rppt@...nel.org>,
        Matthew Wilcox <willy@...radead.org>, Liam.Howlett@...cle.com,
        Jann Horn <jannh@...gle.com>, zhangpeng362@...wei.com,
        bgeffon@...gle.com, kaleshsingh@...gle.com, ngeoffray@...gle.com,
        jdduke@...gle.com, linux-mm <linux-mm@...ck.org>,
        linux-fsdevel@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-kselftest@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 2/3] userfaultfd: UFFDIO_REMAP uABI

On Fri, Sep 15, 2023 at 4:04 AM Nadav Amit <nadav.amit@...il.com> wrote:
>
>
>
> > On Sep 14, 2023, at 8:28 PM, Suren Baghdasaryan <surenb@...gle.com> wrote:
> >
> > On Thu, Sep 14, 2023 at 2:57 PM Nadav Amit <nadav.amit@...il.com> wrote:
> >>
> >>
> >>> On Sep 14, 2023, at 8:26 AM, Suren Baghdasaryan <surenb@...gle.com> wrote:
> >>>
> >>> +     if (!pte_same(ptep_clear_flush(src_vma, src_addr, src_pte),
> >>> +             orig_src_pte))
> >>> +             BUG_ON(1);
> >>
> >> Just a minor detail regarding these few lines:
> >>
> >> Besides the less-than-ideal use of BUG_ON() here, I think that this code
> >> assumes that the PTE cannot change at this point. However, as the PTE was
> >> still mapped at this point, I think the access and dirty bits can be set.
> >
> > At this point we are holding PTLs for both PTEs (see
> > double_pt_lock()).  Can a PTE be modified from under us in this
> > situation?
>
> PTEs has several parts: access-control bits (e.g., writable), physical
> frame number, software-only bits and log-bits. The log-bits, which are
> “access” and “dirty” on x86, track whether the PTE has ever been used
> for translation or write correspondingly.
>
> Without getting into all the subtleties (e.g., “access" can be set
> speculatively even if no actual access take place), as long as the PTE
> is present, it might be used for access (and write if it is writable)
> by other cores. The page-table locks are irrelevant here, because the
> PTE is not updated by software, but it is updated by the CPU itself
> during the page-walk/write.

Ah, I see. I believe Jann also pointed this out in one of his comments
and I didn't realize that. Thanks for the note! I'll see how I can
rework this.

>
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