linux-kernel - Re: [PATCH] mm: vmalloc: Remove double execution of vunmap_page

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Date:   Fri, 13 Apr 2018 16:15:26 +0530
From:   Chintan Pandya <cpandya@...eaurora.org>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>, vbabka@...e.cz,
        labbott@...hat.com, catalin.marinas@....com, hannes@...xchg.org,
        f.fainelli@...il.com, xieyisheng1@...wei.com,
        ard.biesheuvel@...aro.org, richard.weiyang@...il.com,
        byungchul.park@....com
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: vmalloc: Remove double execution of vunmap_page_range



On 4/13/2018 4:10 PM, Anshuman Khandual wrote:
> On 04/13/2018 03:47 PM, Chintan Pandya wrote:
>>
>>
>> On 4/13/2018 3:29 PM, Anshuman Khandual wrote:
>>> On 04/13/2018 02:46 PM, Chintan Pandya wrote:
>>>> Unmap legs do call vunmap_page_range() irrespective of
>>>> debug_pagealloc_enabled() is enabled or not. So, remove
>>>> redundant check and optional vunmap_page_range() routines.
>>>
>>> vunmap_page_range() tears down the page table entries and does
>>> not really flush related TLB entries normally unless page alloc
>>> debug is enabled where it wants to make sure no stale mapping is
>>> still around for debug purpose. Deferring TLB flush improves
>>> performance. This patch will force TLB flush during each page
>>> table tear down and hence not desirable.
>>>
>> Deferred TLB invalidation will surely improve performance. But force
>> flush can help in detecting invalid access right then and there. I
> 
> Deferred TLB invalidation was a choice made some time ago with the
> commit db64fe02258f1507e ("mm: rewrite vmap layer") as these vmalloc
> mappings wont be used other than inside the kernel and TLB gets
> flushed when they are reused. This way it can still avail the benefit
> of deferred TLB flushing without exposing itself to invalid accesses.
> 
>> chose later. May be I should have clean up the vmap tear down code
>> as well where it actually does the TLB invalidation.
>>
>> Or make TLB invalidation in free_unmap_vmap_area() be dependent upon
>> debug_pagealloc_enabled().
> 
> Immediate TLB invalidation needs to be dependent on debug_pagealloc_
> enabled() and should be done only for debug purpose. Contrary to that
> is not desirable.
> 
Okay. I will raise v2 for that.

Chintan
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