Re: [PATCH v2 1/1] mm/vmscan: avoid split PMD-mapped THP during shrink_folio_list()

[Matthew Wilcox]

On Mon, Apr 22, 2024 at 01:52:13PM +0800, Lance Yang wrote:
> When the user no longer requires the pages, they would use
> madvise(MADV_FREE) to mark the pages as lazy free. IMO, they would not
> typically rewrite to the given range.
> 
> At present, PMD-mapped THPs that are marked as lazyfree during
> shrink_folio_list() are unconditionally split, which may be unnecessary.
> If the THP is clean, its PMD is also clean, and there are no unexpected
> references, then we can attempt to remove the PMD mapping from it. This
> change will improve the efficiency of memory reclamation in this case.

Does this happen outside of benchmarks?  I'm really struggling to see
how we end up in this situation.  We have a clean THP without swap
backing, so it's full of zeroes, but for some reason we haven't used the
shared huge zero page?  What is going on?