Re: [PATCH] mm: vmalloc: Remove double execution of vunmap_page_range

[Chintan Pandya]

On 4/13/2018 5:11 PM, Michal Hocko wrote:
> 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.
Okay.

> > 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.
Here, the trade off is, performance vs catching use-after-free. Original
code is preferring performance gains. At first, it seemed to me that
stability is more important than performance. But giving more thoughts
on this (and reading commit db64fe02258f1507e ("mm: rewrite vmap
layer")), I feel that use-after-free is client side wrong-doing. vmap
layer need not loose its best case settings for potential client side
mistakes. For that, vmap layer can provide debug settings. So, I plan
to do TLB flush conditional on debug settings.

Chintan
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