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Message-ID: <bd6a2e6bc8981ced1b6693fdcfa1e111b1c609b7.camel@linux.ibm.com>
Subject: Re: [PATCH 1/1] powerpc: mm: Check if
 serialize_against_pte_lookup() really needs to run
From:   Leonardo Bras <leonardo@linux.ibm.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Cc:     Christoph Lameter <cl@linux.com>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Jann Horn <jannh@google.com>,
        Jesper Dangaard Brouer <brouer@redhat.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Ralph Campbell <rcampbell@nvidia.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Paul Mackerras <paulus@samba.org>,
        "Tobin C. Harding" <tobin@kernel.org>,
        Andrey Ryabinin <aryabinin@virtuozzo.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Vlastimil Babka <vbabka@suse.cz>,
        Souptick Joarder <jrdr.linux@gmail.com>
Date:   Tue, 17 Sep 2019 18:21:22 -0300
In-Reply-To: <c1ff17e2-902d-87e6-3c1d-fc5db2428b69@linux.ibm.com>
References: <20190916205527.1859-1-leonardo@linux.ibm.com>
         <c1ff17e2-902d-87e6-3c1d-fc5db2428b69@linux.ibm.com>
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Hello Aneesh, thanks for the feedback!

On Tue, 2019-09-17 at 08:26 +0530, Aneesh Kumar K.V wrote:
> We could possibly avoid that serialize for a full task exit unmap. ie,=
=20
> when tlb->fullmm =3D=3D 1 . But that won't help the Qemu case because it=
=20
> does an umap of the guest ram range for which tlb->fullmm !=3D 1.
Yes, in qemu we can hot-add memory to guests using memory allocated by
qemu using mmap(). If we want/need to free this memory to other
processes (or guests), we remove it from this guest and do a common
munmap(). This happens without exiting qemu, or pausing the guest.


> That is not correct. We need to keep the count updated  across the=20
> local_irq_disable()/local_irq_enable().=20
So, by that you mean incrementing the counter before
local_irq_{disable,save} and decrementing only after
local_irq_{enable,restore}?

> That is we mostly should have a variant like=20
> start_lockless_pgtbl_walk()/end_lockless_pgtbl_walk().=20
And with that, you mean replacing the current local_irq_{disable,save}
with a start_lockless_pgtbl_walk() (that contains increment + irq
disable)? (and doing similarly with local_irq_{enable,restore} and
end_lockless_pgtbl_walk())=20

> Also there is hash_page_mm which we need to make sure we are protected=
=20
> against w.r.t collapse pmd.
I did not get what I need to do here.=20
Is it a new thing to protect?=20
If so, I need to better understand how to protect it.
If not, I would like to understand how it's protected by current
behavior.=20

> Move that to ppc64 specific mm_context_t.
Ok, fixed! I will send that on v2.

Best regards,
Leonardo Bras

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