linux-kernel - Re: [PATCH 1/2] mm/mprotect: use mmu

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Message-Id: <B8D86FB6-8A9E-4133-9477-0008B3FE46A3@gmail.com>
Date:   Sun, 10 Oct 2021 20:45:17 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Jerome Glisse <jglisse@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Peter Xu <peterx@...hat.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Nick Piggin <npiggin@...il.com>, X86 ML <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/2] mm/mprotect: use mmu_gather

> On Sep 25, 2021, at 1:54 PM, Nadav Amit <nadav.amit@...il.com> wrote:
> 
> From: Nadav Amit <namit@...are.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