linux-kernel - Re: [PATCH 06/17] arm: mmu

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Date:	Wed, 02 Mar 2011 11:54:03 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Metcalf <cmetcalf@...era.com>
Cc:	Russell King <rmk@....linux.org.uk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	linux-mm@...ck.org,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...nel.dk>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Yanmin Zhang <yanmin_zhang@...ux.intel.com>,
	"Luck,Tony" <tony.luck@...el.com>, PaulMundt <lethal@...ux-sh.org>
Subject: Re: [PATCH 06/17] arm: mmu_gather rework

On Tue, 2011-03-01 at 17:05 -0500, Chris Metcalf wrote:

> For Tile, the concern is that we want to make sure to invalidate the
> i-cache.  The I-TLB is handled by the regular TLB flush just fine, like the
> other architectures.  So our concern is that once we have cleared the page
> table entries and invalidated the TLBs, we still have to deal with i-cache
> lines in any core that may have run code from that page.  The risk is that
> the kernel might free, reallocate, and then run code from one of those
> pages, all before the stale i-cache lines happened to be evicted.

>From reading Documentation/cachetlb.txt, update_mmu_cache() can be used
to flush i-cache whenever you install a pte with executable permissions,
and covers the particular case you mention above.

DaveM any comment? You seem to be the one who wrote that document :-)

> The current Tile code flushes the icache explicitly at two different times:
> 
> 1. Whenever we flush the TLB, since this is one time when we know who might
> currently be using the page (via cpu_vm_mask) and we can flush all of them
> easily, piggybacking on the infrastructure we use to flush remote TLBs.
> 
> 2. Whenever we context switch, to handle the case where cpu 1 is running
> process A, then switches to B, but another cpu still running process A
> unmaps an executable page that was in cpu 1's icache.  This way when cpu 1
> switches back to A, it doesn't have to worry about any unmaps that occurred
> while it was switched out.
> 
> 
> > I'm not sure what we can do about TILE's VM_HUGETLB usage though, if it
> > needs explicit flushes for huge ptes it might just have to issue
> > multiple tlb invalidates and do them from tlb_start_vma()/tlb_end_vma().
> 
> I'm not too concerned about this.  We can make the flush code check both
> page sizes at a small cost in efficiency, relative to the overall cost of
> global TLB invalidation.

OK, that's basically what I made it do now:

Index: linux-2.6/arch/tile/kernel/tlb.c
===================================================================
--- linux-2.6.orig/arch/tile/kernel/tlb.c
+++ linux-2.6/arch/tile/kernel/tlb.c
@@ -64,14 +64,13 @@ void flush_tlb_page(const struct vm_area
 }
 EXPORT_SYMBOL(flush_tlb_page);

-void flush_tlb_range(const struct vm_area_struct *vma,
+void flush_tlb_range(const struct mm_struct *mm,
                     unsigned long start, unsigned long end)
 {
-       unsigned long size = hv_page_size(vma);
-       struct mm_struct *mm = vma->vm_mm;
-       int cache = (vma->vm_flags & VM_EXEC) ? HV_FLUSH_EVICT_L1I : 0;
-       flush_remote(0, cache, &mm->cpu_vm_mask, start, end - start, size,
-                    &mm->cpu_vm_mask, NULL, 0);
+       flush_remote(0, HV_FLUSH_EVICT_L1I, &mm->cpu_vm_mask,
+                    start, end - start, PAGE_SIZE, &mm->cpu_vm_mask, NULL, 0);
+       flush_remote(0, 0, &mm->cpu_vm_mask,
+                    start, end - start, HPAGE_SIZE, &mm->cpu_vm_mask, NULL, 0);
 }

And I guess that if the update_mmu_cache() thing works out we can remove
the HV_FLUSH_EVICT_L1I thing.



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