musl - musl: about malloc 'expand heap' issue

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Message-ID: <D53ADE21CD59B0478DFBD751B7B6F4711F235E57@dggeml530-mbx.china.huawei.com>
Date: Tue, 30 Oct 2018 11:11:07 +0000
From: "zhangwentao (M)" <zhangwentao234@...wei.com>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
CC: "Huangqiang (H)" <h.huangqiang@...wei.com>, "Jianing (OS-LAB)"
	<ning.jia@...wei.com>, leijitang <leijitang@...wei.com>, wanghaozhan
	<wanghaozhan2@...wei.com>
Subject: musl: about malloc 'expand heap' issue

Hi all,
  I am using musl in my project and I found an issue about the malloc function in musl:

Issue Description:
*             When in muti-threads environment, malloc/free are called in high concurrency<http://dict.cn/high%20concurrency>.

Malloc:
  Will find 'struct bin' from bitmap(without lock), and allocate memory from the bin (with lock).

Free:
 Will merge the chunk together if the free memory is 'connected' to the existing chunk.

? It will remove the old chunk first then combine the chunk to a larger one.

? After merge operation done, insert the chunk to the bin list.

? Each of the chunk operation is locked while merging, but the whole steps aren't within a lock.

So here is the issue:

1.      There is only one chunk in largest bin list, and Free is on process, just remove the largest bins chunk from bin, the bitmap(mal.binmap) on that bit will be zero.

2.      A malloc comes, the bitmap is zero, and goes to expand heap. (Actually there is enough memories in process)

3.      Free operation goes on, and put the merged big chunk to bins.

But in operation 2, the process has expand heap.

If we have a loop on step 1-3, the process will expand heap frequently.
So it will cost more Virtual Memory  (of course, physical memory would be freed by calling '__madvise' if the chunk is big enough)

In my environment , we do not have that much virtual memory. I think stop expand heap would a better choice.

Do you have plan to fix it ??

THANKS
Best Regard

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