On Wed, Sep 23, 2026 at 10:24:36AM +0800, Lance Yang wrote:
> > mm/khugepaged.c | 32 ++++++++++++++++++++++++++++++--
> > 1 file changed, 30 insertions(+), 2 deletions(-)
> >
> >diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> >index f49a6710933b..dab421f8233e 100644
> >--- a/mm/khugepaged.c
> >+++ b/mm/khugepaged.c
> >@@ -1278,6 +1278,23 @@ static enum scan_result alloc_charge_folio(struct
> >folio **foliop, struct mm_stru
> > return SCAN_SUCCEED;
> > }
> >
> >+static pgtable_t alloc_deposit_pte(struct mm_struct *mm)
> >+{
> >+ /*
> >+ * khugepaged is run from a kernel thread, so need to manually set the
> >+ * correct memcg so the allocation gets charged correctly.
> >+ */
> >+ struct mem_cgroup *memcg = get_mem_cgroup_from_mm(mm);
> >+ struct mem_cgroup *old_memcg = set_active_memcg(memcg);
> >+ pgtable_t pgtable;
> >+
> >+ pgtable = pte_alloc_one(mm);
> >+
> >+ set_active_memcg(old_memcg);
> >+ mem_cgroup_put(memcg);
> >+ return pgtable;
> >+}
> >+
> > /*
> > * collapse_huge_page() expects the mmap_lock to be unlocked before
> > entering and
> > * will always return with the lock unlocked, to avoid holding the mmap_lock
> >@@ -1293,7 +1310,7 @@ static enum scan_result collapse_huge_page(struct
> >mm_struct *mm, unsigned long s
> > LIST_HEAD(compound_pagelist);
> > pmd_t *pmd, _pmd;
> > pte_t *pte = NULL;
> >- pgtable_t pgtable;
> >+ pgtable_t pgtable = NULL;
> > struct folio *folio;
> > spinlock_t *pmd_ptl, *pte_ptl;
> > enum scan_result result = SCAN_FAIL;
> >@@ -1310,6 +1327,12 @@ static enum scan_result collapse_huge_page(struct
> >mm_struct *mm, unsigned long s
> > goto out_nolock;
> > }
> >
> >+ if (is_pmd_order(order)) {
> >+ pgtable = alloc_deposit_pte(mm);
> >+ if (!pgtable)
> >+ goto out_nolock;
>
> One small thing: result is still SCAN_SUCCEED after alloc_charge_folio(),
>
> result = alloc_charge_folio(&folio, mm, cc, order);
> if (result != SCAN_SUCCEED)
> goto out_nolock;
>
> if (folio_memcg_alloc_deferred(folio)) {
> result = SCAN_ALLOC_HUGE_PAGE_FAIL;
> goto out_nolock;
> }
>
> if (is_pmd_order(order)) {
> pgtable = alloc_deposit_pte(mm);
> if (!pgtable)
> goto out_nolock;
> }
>
> so if alloc_deposit_pte() fails, collapse_huge_page() returns success
> without installing a PMD.
>
> I see sashiko pointed that out too :)
>
> Cheers, Lance
Sigh ack yeah oops :) I think SCAN_ALLOC_HUGE_PAGE_FAIL is the closest though
it's not allocating a huge page so much as the page table, but gets the correct
MADV_COLLAPSE -ENOMEM back.
(Another case of local AI review not catchign something btw! Sashiko does lean
towards over-reporting/hallucinating but catches more)
Will ask Andrew to update in reply to this patch as a quick thing rather than
respin so quick, and fix up on any respin.
--
Cheers, Lorenzo
