linux-kernel - Re: [PATCH v6 02/11] mm/gup: Use functions to track lockless pgtbl walks on gup_pgd

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Date:   Mon, 17 Feb 2020 17:55:49 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@....fr>,
        Steven Price <steven.price@....com>,
        Robin Murphy <robin.murphy@....com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Balbir Singh <bsingharora@...il.com>,
        Reza Arbab <arbab@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Allison Randal <allison@...utok.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Michal Suchanek <msuchanek@...e.de>
Cc:     linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        kvm-ppc@...r.kernel.org
Subject: Re: [PATCH v6 02/11] mm/gup: Use functions to track lockless pgtbl
 walks on gup_pgd_range

Hello John, comments inline;

On Fri, 2020-02-07 at 14:54 -0800, John Hubbard wrote:
> On 2/5/20 7:25 PM, Leonardo Bras wrote:
> > On Thu, 2020-02-06 at 00:08 -0300, Leonardo Bras wrote:
> > >                 gup_pgd_range(addr, end, gup_flags, pages, &nr);
> > > -               local_irq_enable();
> > > +               end_lockless_pgtbl_walk(IRQS_ENABLED);
> > >                 ret = nr;
> > >         }
> > >  
> > 
> > Just noticed IRQS_ENABLED is not available on other archs than ppc64.
> > I will fix this for v7.
> > 
> 
> What's the fix going to look like, approximately?

I am not sure what is the best approach yet. 

1. On irq_mask == 0, always enable irq on end_lockless_pgtbl_walk().
   Not sure how bat would that affect other archs.

2. Add another function like end_lockless_pgtbl_walk_irqen() that
always enables IRQ.

3. Add another parameter in end_lockless_pgtbl_walk(), so that caller
can choose ii IRQ must be enabled.

Also, not sure if internal_get_user_pages_fast() can possibly be called
with IRQ disabled, and then return with it enabled. Maybe just
saving/restoring should be fine.

Other suggestions are welcome.

> 
> 
> thanks,

Best regards,

Leonardo Bras

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