Messages in this thread |  | | From | Lance Yang <> | | Subject | Re: [PATCH] mm/khugepaged: clear MMF_VM_HUGEPAGE on mm_slot_alloc() failure | | Date | Fri, 1 May 2026 21:24:38 +0800 |
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On Fri, May 01, 2026 at 04:10:58PM +0530, Dev Jain wrote: > > >On 01/05/26 1:27 pm, Ye Liu wrote: >> From: Ye Liu <liuye@kylinos.cn> >> >> __khugepaged_enter() sets MMF_VM_HUGEPAGE before allocating the >> corresponding mm_slot. If mm_slot_alloc() fails, the function >> returns with the flag set but without inserting the mm into the >> khugepaged tracking structures. >> >> This leaves the mm in an inconsistent state: it is marked as >> registered (MMF_VM_HUGEPAGE set), but will never be scanned by >> khugepaged. Future attempts to register the mm are skipped since >> khugepaged_enter_vma() checks the flag and returns early. >> >> Fix this by clearing MMF_VM_HUGEPAGE when mm_slot_alloc() fails, >> restoring the ability to retry registration later. >> >> Signed-off-by: Ye Liu <liuye@kylinos.cn> >> --- >> mm/khugepaged.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 7d48d4fbd5f3..60ab7c1b61dd 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -559,8 +559,10 @@ void __khugepaged_enter(struct mm_struct *mm) >> return; >> >> slot = mm_slot_alloc(mm_slot_cache); >> - if (!slot) >> + if (!slot) { >> + mm_flags_clear(MMF_VM_HUGEPAGE, mm); >> return; >> + } > >You could just move the test_and_set() after this no?
Yep, that sounds better :)
Just one small thing: if we move the test_and_set(), after mm_slot_alloc(), we need to free the slot when test_and_set() says that the flag was already set. Otherwise the racing caller that loses would leak the slot :)
Something like:
slot = mm_slot_alloc(mm_slot_cache); if (!slot) return;
if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm))) { mm_slot_free(mm_slot_cache, slot); return; }
>So if slot allocation fails then MMF_VM_HUGEPAGE is never set. > >Fixes tag is probably required but not sure about Ccing stable.
Yes, Fixes sounds good. khugepaged collapse is best-effort though, anyway :)
So I'm also not sure about Ccing stable.
>> >> spin_lock(&khugepaged_mm_lock); >> mm_slot_insert(mm_slots_hash, mm, slot);
Cheers, Lance
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