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

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Date:   Thu, 14 Sep 2023 14:57:04 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Suren Baghdasaryan <surenb@...gle.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 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.

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 ]