Re: [PATCH 1/1] mm: thp: Redefine default THP defrag behaviour disable it by default

[Kirill A. Shutemov]

On Thu, Feb 25, 2016 at 08:01:44PM +0100, Andrea Arcangeli wrote:
> Another problem is that khugepaged isn't able to collapse shared
> readonly anon pages, mostly because of the rmap complexities.  I agree
> with Kirill we should be looking into how make this work, although I
> doubt the simpler refcounting is going to help much in this regard as
> the problem is in dealing with rmap, not so much with refcounts.

Could you elaborate on problems with rmap? I have looked into this deeply
yet.

Do you see anything what would prevent following basic scheme:

 - Identify series of small pages as candidate for collapsing into
   a compound page. Not sure how difficult it would be. I guess it can be
   done by looking for adjacent pages which belong to the same anon_vma.

 - Setup migration entries for pte which maps these pages.

 - Collapse small pages into compound page. IIUC, it only will be possible
   if these pages are not pinned.

 - Replace migration entries with ptes which point to subpages of the new
   compound page.

 - Scan over all vmas mapping this compound page, looking for VMA suitable
   for huge page. We cannot collapse it right away due lock inversion of
   anon_vma->rwsem vs. mmap_sem.

 - For found VMAs, collapse page table into PMD one VMA a time under
   down_write(mmap_sem).

Even if would fail to create any PMDs, we would reduce LRU pressure by
collapsing small pages into compound one.

-- 
 Kirill A. Shutemov