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

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Date:   Fri, 13 Apr 2018 13:09:49 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Chintan Pandya <cpandya@...eaurora.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 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.
-- 
Michal Hocko
SUSE Labs