Re: Consolidate calls to unmap_mapping_range in collapse_shmem

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Tue, Mar 28, 2017 at 07:15:03PM -0700, Matthew Wilcox wrote:
> 
> Is there a reason we call unmap_mapping_range() for a single page at a
> time instead of the entire hugepage?  This is surely more efficient ...
> but does it do something like increase the refcount on the page?

Yes, mapcount holds refcount on the page. So page_ref_freeze() will fail
with proposed change.

> I suppose we might be able to skip all the calls to unmap_mapping_range()
> if none of the pages are mapped, but surely anonymous pages are usually
> mapped?

The valid optimization I *think* would be to call unmap_mapping_range()
for whole huge page range before iterating over individual pages. This way
we would only hit page_mapped() case if there was race with page fault.

P.S. proper To/Cc would helped me to notice it. :)

-- 
 Kirill A. Shutemov

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