On Sat, 18 May 2019 07:05:07 +0000 "chenjianhong (A)" <chenjianhong2@xxxxxxxxxx> wrote: > I explain my test code and the problem in detail. This problem is found in > 32-bit user process, because its virtual is limited, 3G or 4G. > > First, I explain the bug I found. Function unmapped_area and > unmapped_area_topdowns adjust search length to account for worst > case alignment overhead, the code is ' length = info->length + info->align_mask; '. > The variable info->length is the length we allocate and the variable > info->align_mask accounts for the alignment, because the gap_start or gap_end > value also should be an alignment address, but we can't know the alignment offset. > So in the current algorithm, it uses the max alignment offset, this value maybe zero > or other(0x1ff000 for shmat function). > Is it reasonable way? The required value is longer than what I allocate. > What's more, why for the first time I can allocate the memory successfully > Via shmat, but after releasing the memory via shmdt and I want to attach > again, it fails. This is not acceptable for many people. > > Second, I explain my test code. The code I have sent an email. The following is > the step. I don't think it's something unusual or unreasonable, because the virtual > memory space is enough, but the process can allocate from it. And we can't pass > explicit addresses to function mmap or shmat, the address is getting from the left > vma gap. > 1, we allocat large virtual memory; > 2, we allocate hugepage memory via shmat, and release one > of the hugepage memory block; > 3, we allocate hugepage memory by shmat again, this will fail. How significant is this problem in real-world use cases? How much trouble is it causing? > Third, I want to introduce my change in the current algorithm. I don't change the > current algorithm. Also, I think there maybe a better way to fix this error. Nowadays, > I can just adjust the gap_start value. Have you looked further into this? Michel is concerned about the performance cost of the current solution.
