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h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=B6yi5fEWqy8MWcTv8zi+90Kv9d8NeRJLHFRGZcn11QlHv0JTPwF7zuCmlgFNxV9Re fR8jHQQP4pY3+/KE43KPG5+7k68DhFSiazqBxXQhdRy5GTed23z2GCMdHO+jMqzv1g uEZhHCkbt1y1PqmxzH/ftuncw0x5YGcLU38lnq8I= Message-ID: <41676526-ff0f-4172-8871-0a825ab524d0@arm.com> Date: Mon, 11 May 2026 09:30:42 +0530 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] mm/khugepaged: clear MMF_VM_HUGEPAGE on mm_slot_alloc() failure To: "David Hildenbrand (Arm)" , Lance Yang Cc: ye.liu@linux.dev, akpm@linux-foundation.org, ljs@kernel.org, xhao@linux.alibaba.com, liuye@kylinos.cn, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, akpm@linux-foudation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <3fb8c00d-d60d-4fec-81b1-e7633384a52d@arm.com> <20260506105150.21504-1-lance.yang@linux.dev> <6b6b094b-8dcb-423b-bb86-ef1439887eed@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: <6b6b094b-8dcb-423b-bb86-ef1439887eed@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: E42A4C0007 X-Stat-Signature: i7knm5jb3apefhnw99ghhipbrgdtfrhn X-HE-Tag: 1778472070-482471 X-HE-Meta: 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 kxw54w2T oPuPDwBvkxk+zDYs8CNvVIoCJne70soXS/sUm0pzMHCun1kFFz/TtxALuUfn+1pCVfbAuIjFF09vzU8Cy/hdsKZ8o0Td64QuJM/0O/WDfCF8sX92KJgLYxVuwwFfMy9jm9Ykq+xQhCkuvirEjOUa1MPhpNqgymr6n5MgKfXpjvC/GaHWUWJ/AfEc6FtUjUshW5LJMs/kOZpEENOCY2KM7xuqB6Gb5iXHCwcVuWN3/odZ/SvLfVYJ4GUQjhg== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 09/05/26 3:11 am, David Hildenbrand (Arm) wrote: > On 5/6/26 12:51, Lance Yang wrote: >> >> On Wed, May 06, 2026 at 12:16:35PM +0530, Dev Jain wrote: >>> >>> >>> On 06/05/26 6:51 am, Ye Liu wrote: >>>> From: Ye Liu >>>> >>>> __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. >>>> >>>> Fixes: 16618670276a ("mm: khugepaged: avoid pointless allocation for struct mm_slot") >>>> Signed-off-by: Ye Liu >>>> --- >>>> Changes since v1: >>>> - Add Fixes tag as suggested by Dev Jain and Lance Yang >>>> >>>> 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; >>>> + } >>> >>> Note that, a racing khugepaged_enter_vma() may back off >>> when it sees that MMF_VM_HUGEPAGE is set, but then the above >>> clears the flag after slot alloc failure. So we end up not >>> registering the mm with khugepaged. But I am sure no one >>> cares, we are in much big trouble if slot alloc is failing. >> >> Right. A racing khugepaged_enter_vma() can see MMF_VM_HUGEPAGE is set >> and return, then !slot clears it again. If there is no later >> khugepaged_enter_vma(), the mm still wouldn't get registered :) > > So why not > > diff --git a/mm/khugepaged.c b/mm/khugepaged.c > index 5f4e009593e0..78735f34250a 100644 > --- a/mm/khugepaged.c > +++ b/mm/khugepaged.c > @@ -437,13 +437,16 @@ void __khugepaged_enter(struct mm_struct *mm) > > /* __khugepaged_exit() must not run from under us */ > VM_BUG_ON_MM(collapse_test_exit(mm), mm); > - if (unlikely(mm_flags_test_and_set(MMF_VM_HUGEPAGE, mm))) > - return; > > 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; > + } > + > spin_lock(&khugepaged_mm_lock); > mm_slot_insert(mm_slots_hash, mm, slot); > /* > > > Arguably, on the race described above, likely the thread seeing the > MMF_VM_HUGEPAGE would likely similarly have failed the allocation. > > I'm fine with either, just wanted to raise the (cleaner looking?) alternative > where we just properly back off? Yes this is also fine - I am overthinking but I wasn't going this way because ... A process doing THP allocations will fail on the mm_flags_test_and_set everytime after the first time. So we will have an unconditional overhead of mm_slot_alloc in the fault handler. Again I am overthinking : ) >