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

[Oscar Salvador <osalvador@suse.de>]

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():

 /*
  *
  * Here the check may be racy:
  * it may see total_mapcount > refcount in some cases?
  * But such case is ephemeral we could always retry collapse
  * later.  However it may report false positive if the page
  * has excessive GUP pins (i.e. 512).  Anyway the same check
  * will be done again later the risk seems low.
  */
  if (!is_refcount_suitable(folio)) {
          result = SCAN_PAGE_COUNT;
          goto out_unmap;
  }

So I guess that since we hold an extra refcount from memory-failure
code, this page cannot be part of a THP anymore.
If we confirm that the same goes for slab, I would replace that with
your warn check.
In this way, if we ever trigger that warn path, as least we will know
that that possibility exists.

So it is a good way to get rid of false assumptions and code burden.


-- 
Oscar Salvador
SUSE Labs