From: Peter Xu <peterx@redhat.com>
To: Nadav Amit <nadav.amit@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
LKML <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>,
Andrea Arcangeli <aarcange@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@kernel.org
Subject: Re: [PATCH 1/2] mm/mprotect: use mmu_gather
Date: Wed, 13 Oct 2021 07:20:48 +0800 [thread overview]
Message-ID: <YWYYUOy0W8z5SVKh@t490s> (raw)
In-Reply-To: <2CED2F72-4D1C-4DBC-AC03-4B246E1673C2@gmail.com>
On Tue, Oct 12, 2021 at 10:31:45AM -0700, Nadav Amit wrote:
>
>
> > On Oct 12, 2021, at 3:16 AM, Peter Xu <peterx@redhat.com> wrote:
> >
> > On Sat, Sep 25, 2021 at 01:54:22PM -0700, Nadav Amit wrote:
> >> @@ -338,25 +344,25 @@ static unsigned long change_protection_range(struct vm_area_struct *vma,
> >> struct mm_struct *mm = vma->vm_mm;
> >> pgd_t *pgd;
> >> unsigned long next;
> >> - unsigned long start = addr;
> >> unsigned long pages = 0;
> >> + struct mmu_gather tlb;
> >>
> >> BUG_ON(addr >= end);
> >> pgd = pgd_offset(mm, addr);
> >> flush_cache_range(vma, addr, end);
> >> inc_tlb_flush_pending(mm);
> >> + tlb_gather_mmu(&tlb, mm);
> >> + tlb_start_vma(&tlb, vma);
> >
> > Pure question:
> >
> > I actually have no idea why tlb_start_vma() is needed here, as protection range
> > can be just a single page, but anyway.. I do see that tlb_start_vma() contains
> > a whole-vma flush_cache_range() when the arch needs it, then does it mean that
> > besides the inc_tlb_flush_pending() to be dropped, so as to the other call to
> > flush_cache_range() above?
>
> Good point.
>
> tlb_start_vma() and tlb_end_vma() are required since some archs do not
> batch TLB flushes across VMAs (e.g., ARM).
Sorry I didn't follow here - as change_protection() is per-vma anyway, so I
don't see why it needs to consider vma crossing.
In all cases, it'll be great if you could add some explanation into commit
message on why we need tlb_{start|end}_vma(), as I think it could not be
obvious to all people.
> I am not sure whether that’s the best behavior for all archs, but I do not
> want to change it.
>
> Anyhow, you make a valid point that the flush_cache_range() should be
> dropped as well. I will do so for next version.
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2021-10-12 23:21 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
2021-10-12 10:16 ` Peter Xu
2021-10-12 17:31 ` Nadav Amit
2021-10-12 23:20 ` Peter Xu [this message]
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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