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Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <6aaf1556-33e1-c9cd-5f41-11cd96c014f4@oracle.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 1/5/21 2:59 AM, Mike Kravetz wrote: > On 1/3/21 11:10 PM, Li Xinhai wrote: >> >> >> On 1/4/21 11:55 AM, Mike Kravetz wrote: >>> I believe the only case where your patch produced incorrect results is >>> when the range was within a vma that was smaller than PUD_SIZE. Do you >>> agree? >>> >> Not exactly. We need to consider for vma which span at least one >> PUD_SIZE after align its vm_start and vm_end. >> > > I know that I provided an incorrect example again. Sorry (again)! > > Can you provide an example where adding the simple check for vma size less > than PUD_SIZE to your original patch will not work. The logic in your V2 > patch is correct. However, I am having a hard time finding a problem with > this simpler approach. > Thanks for checking. An example like this: (vm_start, vm_end) = (2G - 6M, 2G+8M), so this vma is bigger than PUD_SIZE, and the check for vma size bigger than PUD_SIZE will pass. With (start, end) = (2G-4M, 2G-2M), the previous patch will not adjust start(because adjust it from (2G-4M) to 1G will exceeding vm_start), but it will adjust end from (2G-2M) to 2G (because 2G still below vm_end). The adjustment of end is incorrect, because (2G-6M, 2G) range of vma is not allowed for PMD sharing(i.e., that range do not fully occupy PUD_SIZE). If we make the wrong adjustment, then we will unnecessarily impact to range (2G-2M, 2G).