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Subject: Re: [PATCH 1/2] mm/mprotect: use mmu_gather
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <20210925205423.168858-2-namit@vmware.com>
Date:   Sun, 10 Oct 2021 20:45:17 -0700
Cc:     LKML <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>,
        Peter Xu <peterx@redhat.com>,
        Andrew Cooper <andrew.cooper3@citrix.com>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Yu Zhao <yuzhao@google.com>,
        Nick Piggin <npiggin@gmail.com>, X86 ML <x86@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Hildenbrand <david@redhat.com>
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References: <20210925205423.168858-1-namit@vmware.com>
 <20210925205423.168858-2-namit@vmware.com>
To:     Jerome Glisse <jglisse@redhat.com>,
        Andrea Arcangeli <aarcange@redhat.com>
Precedence: bulk


> On Sep 25, 2021, at 1:54 PM, Nadav Amit <nadav.amit@gmail.com> wrote:
> 
> From: Nadav Amit <namit@vmware.com>
> 
> change_pXX_range() currently does not use mmu_gather, but instead
> implements its own deferred TLB flushes scheme. This both complicates
> the code, as developers need to be aware of different invalidation
> schemes, and prevents opportunities to avoid TLB flushes or perform them
> in finer granularity.
> 
> Use mmu_gather in change_pXX_range(). As the pages are not released,
> only record the flushed range using tlb_flush_pXX_range().

Andrea pointed out that I do not take care of THP. Actually, there is
indeed a missing TLB flush on THP, but it is not required due to the
pmdp_invalidate(). Anyhow, the patch needs to address it cleanly, and
to try to avoid the flush on pmdp_invalidate(), which at least on x86
does not appear to be necessary.

There is an additional bug, as tlb_change_page_size() needs to be
called.

-- Jerome,

While I am reviewing my (bad) code, I wanted to understand whether
update of migration entries requires a TLB flush, because I do not
think I got that right either.

I thought they should not, but I now am not very sure. I am very
confused by the following code in migrate_vma_collect_pmd():

        pte_unmap_unlock(ptep - 1, ptl);

        /* Only flush the TLB if we actually modified any entries */
        if (unmapped)
                flush_tlb_range(walk->vma, start, end);


According to this code flush_tlb_range() is called without the ptl.
So theoretically there is a possible race:


	CPU0				CPU1
	----				----
	migrate_vma_collect_pmd()
	 set_pte_at() [ present->
			non-present]

	 pte_unmap_unlock()

					madvise(MADV_DONTNEED)
					 zap_pte_range()

					[ PTE non-present =>
					  no flush ]

So my questions:

1. Is there a reason the above scenario is invalid?
2. Does one need to flush a migration entry he updates it?

Thanks,
Nadav