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

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Date:   Fri, 13 Apr 2018 16:57:06 +0530
From:   Chintan Pandya <cpandya@...eaurora.org>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     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, 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 4:39 PM, Michal Hocko wrote:
> On Fri 13-04-18 16:15:26, Chintan Pandya wrote:
>>
>>
>> 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.
> 
> More importantly. Your changelog absolutely lacks the _why_ part. It
> just states what the code does which is not all that hard to read from
> the diff. It is usually much more important to present _why_ the patch
> is an improvement and worth merging.
> 

It is improving performance in debug scenario. More than that, I see it
as a clean up. Sure, I will try to address *why* in next change log.

Chintan
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