linux-kernel - Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

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Date:   Mon, 23 Sep 2019 14:25:51 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     John Hubbard <jhubbard@...dia.com>, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Linux-MM <linux-mm@...ck.org>
Cc:     Jason Gunthorpe <jgg@...pe.ca>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        YueHaibing <yuehaibing@...wei.com>,
        Keith Busch <keith.busch@...el.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Mahesh Salgaonkar <mahesh@...ux.vnet.ibm.com>,
        Richard Fontana <rfontana@...hat.com>,
        Paul Mackerras <paulus@...ba.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Ganesh Goudar <ganeshgr@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Allison Randal <allison@...utok.net>
Subject: Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting
 method to skip serializing

On Fri, 2019-09-20 at 17:48 -0700, John Hubbard wrote:
> 
[...]
> So it seems that full memory barriers (not just compiler barriers) are required.
> If the irq enable/disable somehow provides that, then your new code just goes
> along for the ride and Just Works. (You don't have any memory barriers in
> start_lockless_pgtbl_walk() / end_lockless_pgtbl_walk(), just the compiler
> barriers provided by the atomic inc/dec.)
> 
> So it's really a pre-existing question about the correctness of the gup_fast()
> irq disabling approach.

I am not experienced in other archs, and I am still pretty new to
Power, but by what I could understand, this behavior is better
explained in serialize_against_pte_lookup. 

What happens here is that, before doing a THP split/collapse, the
function does a update of the pmd and a serialize_against_pte_lookup,
in order do avoid a invalid output on a lockless pagetable walk.

Serialize basically runs a do_nothing in every cpu related to the
process, and wait for it to return. 

This running depends on interrupt being enabled, so disabling it before
gup_pgd_range() and re-enabling after the end, makes the THP
split/collapse wait for gup_pgd_range() completion in every cpu before
continuing. (here happens the lock)

(As told before, every gup_pgd_range() that occurs after it uses a
updated pmd, so no problem.)

I am sure other archs may have a similar mechanism using
local_irq_{disable,enable}.

Did it answer your questions?

Best regards,

Leonardo Bras

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