Re: [PATCH 1/1] powerpc: mm: Check if serialize_against_pte_lookup() really needs to run

[Leonardo Bras <leonardo@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, 
> 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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