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






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

Chintan
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