From: Martin Schwidefsky <schwidefsky@de.ibm.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: will.deacon@arm.com, aneesh.kumar@linux.vnet.ibm.com,
akpm@linux-foundation.org, npiggin@gmail.com,
linux-arch@vger.kernel.org, linux-mm@kvack.org,
linux-kernel@vger.kernel.org, linux@armlinux.org.uk,
heiko.carstens@de.ibm.com
Subject: Re: [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment
Date: Thu, 13 Sep 2018 14:18:27 +0200 [thread overview]
Message-ID: <20180913141827.1776985e@mschwideX1> (raw)
In-Reply-To: <20180913105738.GW24124@hirez.programming.kicks-ass.net>
On Thu, 13 Sep 2018 12:57:38 +0200
Peter Zijlstra <peterz@infradead.org> wrote:
> On Thu, Sep 13, 2018 at 12:30:14PM +0200, Martin Schwidefsky wrote:
>
> > > + * The mmu_gather data structure is used by the mm code to implement the
> > > + * correct and efficient ordering of freeing pages and TLB invalidations.
> > > + *
> > > + * This correct ordering is:
> > > + *
> > > + * 1) unhook page
> > > + * 2) TLB invalidate page
> > > + * 3) free page
> > > + *
> > > + * That is, we must never free a page before we have ensured there are no live
> > > + * translations left to it. Otherwise it might be possible to observe (or
> > > + * worse, change) the page content after it has been reused.
> > > + *
> >
> > This first comment already includes the reason why s390 is probably better off
> > with its own mmu-gather implementation. It depends on the situation if we have
> >
> > 1) unhook the page and do a TLB flush at the same time
> > 2) free page
> >
> > or
> >
> > 1) unhook page
> > 2) free page
> > 3) final TLB flush of the whole mm
>
> that's the fullmm case, right?
That includes the fullmm case but we use it for e.g. munmap of a single-threaded
program as well.
> > A variant of the second order we had in the past is to do the mm TLB flush first,
> > then the unhooks and frees of the individual pages. The are some tricky corners
> > switching between the two variants, see finish_arch_post_lock_switch.
> >
> > The point is: we *never* have the order 1) unhook, 2) TLB invalidate, 3) free.
> > If there is concurrency due to a multi-threaded application we have to do the
> > unhook of the page-table entry and the TLB flush with a single instruction.
>
> You can still get the thing you want if for !fullmm you have a no-op
> tlb_flush() implementation, assuming your arch page-table frobbing thing
> has the required TLB flush in.
We have a non-empty tlb_flush_mmu_tlbonly to do a full-mm flush for two cases
1) batches of page-table entries for single-threaded programs
2) flushing of the pages used for the page-table structure itself
In fact only the page-table pages are added to the mmu_gather batch, the target
page of the virtual mapping is always freed immediately.
> Note that that's not utterly unlike how the PowerPC/Sparc hash things
> work, they clear and invalidate entries different from others and don't
> use the mmu_gather tlb-flush.
We may get something working with a common code mmu_gather, but I fear the
day someone makes a "minor" change to that subtly break s390. The debugging of
TLB related problems is just horrible..
--
blue skies,
Martin.
"Reality continues to ruin my life." - Calvin.
next prev parent reply other threads:[~2018-09-13 12:19 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-13 9:21 [RFC][PATCH 00/11] my generic mmu_gather patches Peter Zijlstra
2018-09-13 9:21 ` [RFC][PATCH 01/11] asm-generic/tlb: Provide a comment Peter Zijlstra
2018-09-13 10:30 ` Martin Schwidefsky
2018-09-13 10:57 ` Peter Zijlstra
2018-09-13 12:18 ` Martin Schwidefsky [this message]
2018-09-13 12:39 ` Peter Zijlstra
2018-09-14 10:28 ` Martin Schwidefsky
2018-09-14 13:02 ` Peter Zijlstra
2018-09-14 14:07 ` Martin Schwidefsky
2018-09-14 16:48 ` Will Deacon
2018-09-19 11:33 ` Peter Zijlstra
2018-09-19 11:51 ` Peter Zijlstra
2018-09-19 12:23 ` Will Deacon
2018-09-19 13:12 ` Peter Zijlstra
2018-09-13 9:21 ` [RFC][PATCH 02/11] asm-generic/tlb: Provide HAVE_MMU_GATHER_PAGE_SIZE Peter Zijlstra
2018-09-14 16:56 ` Will Deacon
2018-09-13 9:21 ` [RFC][PATCH 03/11] x86/mm: Page size aware flush_tlb_mm_range() Peter Zijlstra
2018-09-13 17:22 ` Dave Hansen
2018-09-13 18:42 ` Peter Zijlstra
2018-09-13 18:46 ` Peter Zijlstra
2018-09-13 18:48 ` Peter Zijlstra
2018-09-13 18:49 ` Dave Hansen
2018-09-13 18:47 ` Dave Hansen
2018-09-13 9:21 ` [RFC][PATCH 04/11] asm-generic/tlb: Provide generic VIPT cache flush Peter Zijlstra
2018-09-14 16:56 ` Will Deacon
2018-09-13 9:21 ` [RFC][PATCH 05/11] asm-generic/tlb: Provide generic tlb_flush Peter Zijlstra
2018-09-13 13:09 ` Jann Horn
2018-09-13 14:06 ` Peter Zijlstra
2018-09-13 9:21 ` [RFC][PATCH 06/11] asm-generic/tlb: Conditionally provide tlb_migrate_finish() Peter Zijlstra
2018-09-14 16:57 ` Will Deacon
2018-09-13 9:21 ` [RFC][PATCH 07/11] arm/tlb: Convert to generic mmu_gather Peter Zijlstra
2018-09-18 14:10 ` Will Deacon
2018-09-19 11:28 ` Peter Zijlstra
2018-09-19 12:24 ` Will Deacon
2018-09-19 11:30 ` Peter Zijlstra
2018-09-13 9:21 ` [RFC][PATCH 08/11] ia64/tlb: Conver " Peter Zijlstra
2018-09-13 9:21 ` [RFC][PATCH 09/11] sh/tlb: Convert SH " Peter Zijlstra
2018-09-13 9:21 ` [RFC][PATCH 10/11] um/tlb: Convert " Peter Zijlstra
2018-09-13 9:21 ` [RFC][PATCH 11/11] arch/tlb: Clean up simple architectures Peter Zijlstra
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