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

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Date:   Thu, 14 Sep 2023 20:28:23 -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 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?

>
> tl;dr: this appears to be triggerable by userspace.
>
> [ as for the performance of this code, the lack of batching would mean
>   that for multithreaded applications where more than a single page is
>   remapped, performance would suffer ]

Thanks for the note! I'll see if it's possible to implement some
batching mechanism here.
Thanks,
Suren.