linux-kernel - Re: [PATCH 2/2] mm/mprotect: do not flush on permission promotion

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Message-ID: <5485fae5-3cd6-9dc3-0579-dc8aab8a3de1@redhat.com>
Date:   Thu, 7 Oct 2021 14:13:00 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Nadav Amit <nadav.amit@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Linux-MM <linux-mm@...ck.org>,
        Peter Xu <peterx@...hat.com>, Nadav Amit <namit@...are.com>,
        Andrea Arcangeli <aarcange@...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@...nel.org
Subject: Re: [PATCH 2/2] mm/mprotect: do not flush on permission promotion

On 25.09.21 22:54, Nadav Amit wrote:
> From: Nadav Amit <namit@...are.com>
> 
> Currently, using mprotect() to unprotect a memory region or uffd to
> unprotect a memory region causes a TLB flush. At least on x86, as
> protection is promoted, no TLB flush is needed.
> 
> Add an arch-specific pte_may_need_flush() which tells whether a TLB
> flush is needed based on the old PTE and the new one. Implement an x86
> pte_may_need_flush().
> 
> For x86, PTE protection promotion or changes of software bits does
> require a flush, also add logic that considers the dirty-bit. Changes to
> the access-bit do not trigger a TLB flush, although architecturally they
> should, as Linux considers the access-bit as a hint.

Is the added LOC worth the benefit? IOW, do we have some benchmark that 
really benefits from that?


-- 
Thanks,

David / dhildenb