Re: [PATCH] mm: Reduce memory bloat with THP

[Kirill A. Shutemov]

On Thu, Dec 14, 2017 at 05:28:52PM -0800, Nitin Gupta wrote:
> Currently, if the THP enabled policy is "always", or the mode
> is "madvise" and a region is marked as MADV_HUGEPAGE, a hugepage
> is allocated on a page fault if the pud or pmd is empty.  This
> yields the best VA translation performance, but increases memory
> consumption if some small page ranges within the huge page are
> never accessed.
> 
> An alternate behavior for such page faults is to install a
> hugepage only when a region is actually found to be (almost)
> fully mapped and active.  This is a compromise between
> translation performance and memory consumption.  Currently there
> is no way for an application to choose this compromise for the
> page fault conditions above.
> 
> With this change, when an application issues MADV_DONTNEED on a
> memory region, the region is marked as "space-efficient". For
> such regions, a hugepage is not immediately allocated on first
> write.  Instead, it is left to the khugepaged thread to do
> delayed hugepage promotion depending on whether the region is
> actually mapped and active. When application issues
> MADV_HUGEPAGE, the region is marked again as non-space-efficient
> wherein hugepage is allocated on first touch.

I think this would be NAK. At least in this form.

What performance testing have you done? Any numbers?

Making whole vma "space_efficient" just because somebody freed one page
from it is just wrong. And there's no way back after this.

> 
> Orabug: 26910556

Wat?

-- 
 Kirill A. Shutemov