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Date:   Wed, 23 Dec 2020 21:00:26 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Andy Lutomirski <luto@amacapital.net>
Cc:     Yu Zhao <yuzhao@google.com>, Andy Lutomirski <luto@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Xu <peterx@redhat.com>,
        Nadav Amit <nadav.amit@gmail.com>,
        linux-mm <linux-mm@kvack.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Pavel Emelyanov <xemul@openvz.org>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        stable <stable@vger.kernel.org>,
        Minchan Kim <minchan@kernel.org>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>
Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect
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On Wed, Dec 23, 2020 at 05:21:43PM -0800, Andy Lutomirski wrote:
> I don’t love this as a long term fix. AFAICT we can have mm_tlb_flush_pending set for quite a while — mprotect seems like it can wait in IO while splitting a huge page, for example. That gives us a window in which every write fault turns into a TLB flush.

mprotect can't run concurrently with a page fault in the first place.

One other near zero cost improvement easy to add if this would be "if
(vma->vm_flags & (VM_SOFTDIRTY|VM_UFFD_WP))" and it could be made
conditional to the two config options too.

Still I don't mind doing it in some other way, uffd-wp has much easier
time doing it in another way in fact.

Whatever performs better is fine, but queuing up pending invalidate
ranges don't look very attractive since it'd be a fixed cost that we'd
always have to pay even when there's no fault (and there can't be any
fault at least for mprotect).

Thanks,
Andrea