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, f.fainelli@gmail.com, xieyisheng1@huawei.com,
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
Date: Fri, 13 Apr 2018 13:09:49 +0200 [thread overview]
Message-ID: <20180413110949.GA17670@dhcp22.suse.cz> (raw)
In-Reply-To: <fa104cc6-c32a-9081-280f-2e03e4279f65@codeaurora.org>
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
next prev parent reply other threads:[~2018-04-13 11:09 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-04-13 9:16 [PATCH] mm: vmalloc: Remove double execution of vunmap_page_range Chintan Pandya
2018-04-13 9:59 ` Anshuman Khandual
2018-04-13 10:17 ` Chintan Pandya
2018-04-13 10:40 ` Anshuman Khandual
2018-04-13 10:45 ` Chintan Pandya
2018-04-13 11:09 ` Michal Hocko [this message]
2018-04-13 11:27 ` Chintan Pandya
2018-04-13 11:41 ` Michal Hocko
2018-04-13 12:12 ` Chintan Pandya
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