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Date:   Fri, 13 Apr 2018 13:41:33 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Chintan Pandya <cpandya@codeaurora.org>
Cc:     Anshuman Khandual <khandual@linux.vnet.ibm.com>, vbabka@suse.cz,
        labbott@redhat.com, catalin.marinas@arm.com, hannes@cmpxchg.org,
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        ard.biesheuvel@linaro.org, richard.weiyang@gmail.com,
        byungchul.park@lge.com, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: vmalloc: Remove double execution of vunmap_page_range
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On Fri 13-04-18 16:57:06, Chintan Pandya wrote:
> 
> 
> 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.

Do not forget to add some numbers presenting the benefits when
resubmitting.

> More than that, I see it
> as a clean up. Sure, I will try to address *why* in next change log.

As Anshuman pointed out the current code layout is deliberate. If you
believe that reasons mentioned previously are not valid then dispute
them and provide your arguments in the changelog.
-- 
Michal Hocko
SUSE Labs