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[2003:d8:2f18:f900:e0ae:65d5:8bf8:8cfd]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e94163c03dsm6934860f8f.63.2025.09.15.00.48.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 15 Sep 2025 00:48:56 -0700 (PDT) Message-ID: <107d29f2-9474-4e58-8dd1-a1e8b552f4f7@redhat.com> Date: Mon, 15 Sep 2025 09:48:55 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm/mm_slot: make sure slot is the first element of its wrapper structure To: Wei Yang Cc: Lance Yang , linux-mm@kvack.org, chengming.zhou@linux.dev, ryan.roberts@arm.com, baohua@kernel.org, xu.xin16@zte.com.cn, dev.jain@arm.com, npache@redhat.com, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, akpm@linux-foundation.org, ziy@nvidia.com, baolin.wang@linux.alibaba.com References: <20250914000026.17986-1-richard.weiyang@gmail.com> <3a1adf4c-9931-4cdb-a024-37c856b1702b@linux.dev> <20250915014738.slkw4nwihb4aavas@master> <20250915074231.xnjzoqozqihqetwm@master> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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>>>>> mm_slot = mm_slot_entry(slot, struct ksm_mm_slot, slot); >>>>> if (mm_slot && ..) { >>>>> } >>>>> >>>>> This is only valid when mm_slot is the first element of its wrapper >>>>> structure, otherwise a NULL slot would converted to a mm_slot with >>>>> negative value. And current code thinks it is valid and continue. >>>>> >>>>> Current code works since mm_slot is the first element, but make sure it >>>>> won't be disturbed. >>>> >>>> Good catch! That's indeed quite brittle ;) >>>> >>>> Just one nit below. >>>> >>>>> >>>>> Signed-off-by: Wei Yang >>>>> --- >>>>> mm/khugepaged.c | 5 ++++- >>>>> mm/ksm.c | 5 ++++- >>>>> 2 files changed, 8 insertions(+), 2 deletions(-) >>>>> >>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>> index af5f5c80fe4e..668e74ad33b7 100644 >>>>> --- a/mm/khugepaged.c >>>>> +++ b/mm/khugepaged.c >>>>> @@ -108,7 +108,7 @@ struct collapse_control { >>>>> * @slot: hash lookup from mm to mm_slot >>>>> */ >>>>> struct khugepaged_mm_slot { >>>>> - struct mm_slot slot; >>>>> + struct mm_slot slot; /* keep it the first element */ >>>>> }; >>>>> /** >>>>> @@ -2382,6 +2382,9 @@ static unsigned int khugepaged_scan_mm_slot(unsigned int pages, int *result, >>>>> struct vm_area_struct *vma; >>>>> int progress = 0; >>>>> + BUILD_BUG_ON_MSG(mm_slot_entry(NULL, struct khugepaged_mm_slot, slot), >>>>> + "slot should be the first element"); >>>>> + >>>> >>>> I wonder if this BUILD_BUG_ON_MSG() in khugepaged_scan_mm_slot() would >>>> be better placed in khugepaged_init(), like you did in ksm_init()? >>> >>> khugepaged_mm_slot is defined in khugepaged.c, maybe we don't want to export >>> it. >>> >> >> We should probably just use a static_assert right nxt to the struct and make >> sure that offsetof(struct khugepaged_mm_slot, slot) == 0 ? >> > > I am ok with this, but Dev suggest to fix in the code. > > Hmm... both works to me. Right, if it can just be made working without requiring it to be the first entry, we should do that instead. -- Cheers David / dhildenb