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Date:   Fri, 8 Jan 2021 19:12:28 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Will Deacon <will@kernel.org>, Linux-MM <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Mike Kravetz <mike.kravetz@oracle.com>,
        Mike Rapoport <rppt@linux.vnet.ibm.com>,
        Minchan Kim <minchan@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Hugh Dickins <hughd@google.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
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        Kirill Tkhai <ktkhai@virtuozzo.com>,
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        Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 2/2] mm: soft_dirty: userfaultfd: introduce
 wrprotect_tlb_flush_pending
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On Fri, Jan 08, 2021 at 11:25:21AM -0800, Linus Torvalds wrote:
> On Fri, Jan 8, 2021 at 9:53 AM Andrea Arcangeli <aarcange@redhat.com> wrote:
> >
> > Do you intend to eventually fix the zygote vmsplice case or not?
> > Because in current upstream it's not fixed currently using the
> > enterprise default config.
> 
> Is this the hugepage case? Neither of your patches actually touched
> that, so I've forgotten the details.

The two patches only fixed the TLB flushing deferral in clear_refs and
uffd-wp.

So I didn't actually try to fix the hugepage case by adding the
page_count checks there too. I could try to do that at least it'd be
consistent but I still would try to find an alternate solution later.

> > Irrelevant special case as in: long term GUP pin on the memory?
> 
> Irrelevant special case in that
> 
>  (a) an extra COW shouldn't be a correctness issue unless somebody
> does something horribly wrong (and obviously the code that hasn't
> taken the mmap_lock for writing are then examples of that)
> 
> and
> 
>  (b) it's not a performance issue either unless you can find a real
> load that does it.
> 
> Hmm?

For b) I don't have an hard time to imagine `ps` hanging for seconds,
if clear_refs is touched on a 4T mm, but b) is not the main
concern.

Having to rely on a) is the main concern and it's not about tlb
flushes but the long term GUP pins.

Thanks,
Andrea