linux-kernel - Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to monitor lockless pgtable walks

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Date:   Fri, 4 Oct 2019 13:28:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Leonardo Bras <leonardo@...ux.ibm.com>
Cc:     Song Liu <songliubraving@...com>, Michal Hocko <mhocko@...e.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        Keith Busch <keith.busch@...el.com>, linux-mm@...ck.org,
        Paul Mackerras <paulus@...ba.org>,
        Christoph Lameter <cl@...ux.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        linux-arch@...r.kernel.org, Santosh Sivaraj <santosh@...six.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Ingo Molnar <mingo@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Arnd Bergmann <arnd@...db.de>, Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        kvm-ppc@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Reza Arbab <arbab@...ux.ibm.com>,
        Allison Randal <allison@...utok.net>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Logan Gunthorpe <logang@...tatee.com>,
        Souptick Joarder <jrdr.linux@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linuxppc-dev@...ts.ozlabs.org, Roman Gushchin <guro@...com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v5 01/11] asm-generic/pgtable: Adds generic functions to
 monitor lockless pgtable walks

On Thu, Oct 03, 2019 at 06:24:07PM -0300, Leonardo Bras wrote:
> Hello Peter, thanks for the feedback!
> 
> On Thu, 2019-10-03 at 13:51 +0200, Peter Zijlstra wrote:
> > On Thu, Oct 03, 2019 at 09:11:45AM +0200, Peter Zijlstra wrote:
> > > On Wed, Oct 02, 2019 at 10:33:15PM -0300, Leonardo Bras wrote:
> > > > diff --git a/include/asm-generic/pgtable.h b/include/asm-generic/pgtable.h
> > > > index 818691846c90..3043ea9812d5 100644
> > > > --- a/include/asm-generic/pgtable.h
> > > > +++ b/include/asm-generic/pgtable.h
> > > > @@ -1171,6 +1171,64 @@ static inline bool arch_has_pfn_modify_check(void)
> > > >  #endif
> > > >  #endif
> > > >  
> > > > +#ifndef __HAVE_ARCH_LOCKLESS_PGTBL_WALK_CONTROL
> > > > +static inline unsigned long begin_lockless_pgtbl_walk(struct mm_struct *mm)
> > > > +{
> > > > +	unsigned long irq_mask;
> > > > +
> > > > +	if (IS_ENABLED(CONFIG_LOCKLESS_PAGE_TABLE_WALK_TRACKING))
> > > > +		atomic_inc(&mm->lockless_pgtbl_walkers);
> > > 
> > > This will not work for file backed THP. Also, this is a fairly serious
> > > contention point all on its own.
> > 
> > Kiryl says we have tmpfs-thp, this would be broken vs that, as would
> > your (PowerPC) use of mm_cpumask() for that IPI.
> 
> Could you please explain it?
> I mean, why this breaks tmpfs-thp?
> Also, why mm_cpumask() is also broken?

Because shared pages are not bound by a mm; or does it not share the thp
state between mappings?

> > And I still think all that wrong, you really shouldn't need to wait on
> > munmap().
> 
> That is something I need to better understand. I mean, before coming
> with this patch, I thought exactly this: not serialize when on munmap. 
> 
> But on the way I was convinced it would not work on munmap. I need to
> recall why, and if it was false to assume this, re-think the whole
> solution.

And once you (re)figure it out, please write it down. It is a crucial
bit of the puzzle and needs to be part of the Changelogs.