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

[Chintan Pandya]

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.

Chintan
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