Re: [PATCH] mm/mmap: fix the adjusted length error

[Andrew Morton]

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.