On 1/4/21 6:38 PM, Li Xinhai wrote: > > > On 1/5/21 10:10 AM, Li Xinhai wrote: >> >> >> 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). >> >> > sorry, the above example is not correct for vma size, update it as > below: > > (vm_start, vm_end) = (2G - 600M, 2G+800M), 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-600M, 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). Thanks you for the example! This routine is conceptually simple, but for some reason there have been multiple problems in the implementation. -- Mike Kravetz
