Re: [PATCH mm-unstable v18 11/14] mm/khugepaged: Introduce mTHP collapse support

On Sun, May 31, 2026 at 2:48 AM Lance Yang [off-list ref] wrote:

On 2026/5/31 15:18, Lance Yang wrote:

quoted

On Fri, May 22, 2026 at 09:00:06AM -0600, Nico Pache wrote:
[...]

quoted

@@ -1587,10 +1749,11 @@ static enum scan_result collapse_scan_pmd(struct mm_struct *mm,
     if (result == SCAN_SUCCEED) {
             /* collapse_huge_page expects the lock to be dropped before calling */
             mmap_read_unlock(mm);
-            result = collapse_huge_page(mm, start_addr, referenced,
-                                        unmapped, cc, HPAGE_PMD_ORDER);
-            /* collapse_huge_page will return with the mmap_lock released */
+            nr_collapsed = mthp_collapse(mm, vma, start_addr, referenced,
+                                         unmapped, cc, enabled_orders);
+            /* mmap_lock was released above, set lock_dropped */
             *lock_dropped = true;
+            result = nr_collapsed ? SCAN_SUCCEED : SCAN_FAIL;

Hmm ... don't we lose the allocation-failure result here?

Previously collapse_scan_pmd() propagated SCAN_ALLOC_HUGE_PAGE_FAIL from
collapse_huge_page(), so khugepaged would call khugepaged_alloc_sleep()
in khugepaged_do_scan().

Now if allocation fails and nr_collapsed stays 0, we just return
SCAN_FAIL. So we won't back off via khugepaged_alloc_sleep() anymore?

Looks like this is a more general issue with mthp_collapse() only
returning nr_collapsed.

For example, SCAN_PMD_MAPPED used to be propagated too, and
madvise_collapse() treats that as success. With the new code, if
nothing was collapsed by this call, that can also become SCAN_FAIL ...

So I think we should keep both.

Yeah I thought about this before, but more regarding the "incorrect"
propagation of errors; I didn't consider that those results were
actually being considered.

I actually had a patch to track the last_failure (with some
prioritization on certain results). I think that would solve this
issue.

Thanks for reminding me to improve this.

Depending on how the rest of the reviews go, I can either send up a
follow up series to do some more cleanups and improvements of the
current approach or we can send out a v19.

Cheers,
-- Nico

Cheers, Lance