Re: [PATCH v2 07/11] mm/memory-failure: Convert memory_failure() to use a folio

[Matthew Wilcox <willy@infradead.org>]

On Thu, Apr 11, 2024 at 01:23:43PM +0200, Oscar Salvador wrote:
> On Thu, Apr 11, 2024 at 05:00:33PM +0800, Miaohe Lin wrote:
> > But as code changes, the above page lock shift is gone. And I think below logic can't
> > trigger now. As we hold extra page refcnt so page can't be coallesced into a new THP or Slab page.
> 
> So I was wrong.
> I did not check the slap part, but I did check the code that tries to
> coallesce pages.
> We have the following check in hpage_collapse_scan_pmd():

Even if that check succeeds, collapse_huge_page() works by allocating a
new THP, copying the data into it and freeing the existing pages.  It
doesn't reassemble a THP in-place.  I cannot find anywhere that will
reassemble smaller folios into larger ones (other than the page
allocator, and if you have a refcount on a folio, it can't possibly go
into the page allocator).