From: Nadav Amit <nadav.amit@gmail.com>
To: Jerome Glisse <jglisse@redhat.com>,
Andrea Arcangeli <aarcange@redhat.com>
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>
Subject: Re: [PATCH 1/2] mm/mprotect: use mmu_gather
Date: Sun, 10 Oct 2021 20:45:17 -0700 [thread overview]
Message-ID: <B8D86FB6-8A9E-4133-9477-0008B3FE46A3@gmail.com> (raw)
In-Reply-To: <20210925205423.168858-2-namit@vmware.com>
> 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
next prev parent reply other threads:[~2021-10-11 3:45 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-25 20:54 [PATCH 0/2] mm/mprotect: avoid unnecessary TLB flushes Nadav Amit
2021-09-25 20:54 ` [PATCH 1/2] mm/mprotect: use mmu_gather Nadav Amit
2021-10-03 12:10 ` Peter Zijlstra
2021-10-04 19:24 ` Nadav Amit
2021-10-05 6:53 ` Peter Zijlstra
2021-10-05 16:34 ` Nadav Amit
2021-10-11 3:45 ` Nadav Amit [this message]
2021-10-12 10:16 ` Peter Xu
2021-10-12 17:31 ` Nadav Amit
2021-10-12 23:20 ` Peter Xu
2021-10-13 15:59 ` Nadav Amit
2021-09-25 20:54 ` [PATCH 2/2] mm/mprotect: do not flush on permission promotion Nadav Amit
2021-10-07 12:13 ` David Hildenbrand
2021-10-07 16:16 ` Nadav Amit
2021-10-07 17:07 ` David Hildenbrand
2021-10-08 6:06 ` Nadav Amit
2021-10-08 7:35 ` David Hildenbrand
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