linux-kernel - Re: [PATCH 1/1] powerpc: mm: Check if serialize_against_pte

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Message-ID: <bd6a2e6bc8981ced1b6693fdcfa1e111b1c609b7.camel@linux.ibm.com>
Date:   Tue, 17 Sep 2019 18:21:22 -0300
From:   Leonardo Bras <leonardo@...ux.ibm.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Cc:     Christoph Lameter <cl@...ux.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Jann Horn <jannh@...gle.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Paul Mackerras <paulus@...ba.org>,
        "Tobin C. Harding" <tobin@...nel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vlastimil Babka <vbabka@...e.cz>,
        Souptick Joarder <jrdr.linux@...il.com>
Subject: Re: [PATCH 1/1] powerpc: mm: Check if
 serialize_against_pte_lookup() really needs to run

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, 
> when tlb->fullmm == 1 . But that won't help the Qemu case because it 
> does an umap of the guest ram range for which tlb->fullmm != 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 
> local_irq_disable()/local_irq_enable(). 
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 
> start_lockless_pgtbl_walk()/end_lockless_pgtbl_walk(). 
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()) 

> Also there is hash_page_mm which we need to make sure we are protected 
> against w.r.t collapse pmd.
I did not get what I need to do here. 
Is it a new thing to protect? 
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. 

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

Best regards,
Leonardo Bras

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