[RFC PATCH 2/2] arm64: Implement vmalloc based thread_info allocator
jungseoklee85 at gmail.com
Tue May 26 04:29:59 PDT 2015
On May 25, 2015, at 11:40 PM, Minchan Kim wrote:
> Hello Jungseok,
> On Mon, May 25, 2015 at 01:02:20AM +0900, Jungseok Lee wrote:
>> Fork-routine sometimes fails to get a physically contiguous region for
>> thread_info on 4KB page system although free memory is enough. That is,
>> a physically contiguous region, which is currently 16KB, is not available
>> since system memory is fragmented.
> Order less than PAGE_ALLOC_COSTLY_ORDER should not fail in current
> mm implementation. If you saw the order-2,3 high-order allocation fail
> maybe your application received SIGKILL by someone. LMK?
Exactly right. The allocation is failed via the following path.
if (test_thread_flag(TIF_MEMDIE) && !(gfp_mask & __GFP_NOFAIL))
IMHO, a reclaim operation would be not needed in this context if memory is
allocated from vmalloc space. It means there is no need to traverse shrinker list.
>> This patch tries to solve the problem as allocating thread_info memory
>> from vmalloc space, not 1:1 mapping one. The downside is one additional
>> page allocation in case of vmalloc. However, vmalloc space is large enough,
> The size you want to allocate is 16KB in here but additional 4K?
> It increases 25% memory footprint, which is huge downside.
I agree with the point, and most people who try to use vmalloc might know the number.
However, an interoperation on the number depends on a point of view.
Vmalloc is large enough and not fully utilized in case of ARM64.
With the considerations, there is a room to do math as follows.
4KB / 240GB = 1.5e-8 (4KB page + 3 level combo)
It would be not a huge downside if fork-routine is not damaged due to fragmentation.
However, this is one of reasons to add "RFC" prefix in the patch set. How is the
additional 4KB interpreted and considered?
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