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Subject: Re: LTP hugemmap05 test case failure on arm64 with linux-next
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From:   Mike Kravetz <mike.kravetz@oracle.com>
To:     Qian Cai <cai@lca.pw>, Will Deacon <will@kernel.org>
Cc:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org
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Date:   Thu, 27 Jun 2019 11:09:16 -0700
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On 6/24/19 2:53 PM, Mike Kravetz wrote:
> On 6/24/19 2:30 PM, Qian Cai wrote:
>> So the problem is that ipcget_public() has held the semaphore "ids->rwsem" for
>> too long seems unnecessarily and then goes to sleep sometimes due to direct
>> reclaim (other times LTP hugemmap05 [1] has hugetlb_file_setup() returns
>> -ENOMEM),
> 
> Thanks for looking into this!  I noticed that recent kernels could take a
> VERY long time trying to do high order allocations.  In my case it was trying
> to do dynamic hugetlb page allocations as well [1].  But, IMO this is more
> of a general direct reclaim/compation issue than something hugetlb specific.
> 

<snip>

>> Ideally, it seems only ipc_findkey() and newseg() in this path needs to hold the
>> semaphore to protect concurrency access, so it could just be converted to a
>> spinlock instead.
> 
> I do not have enough experience with this ipc code to comment on your proposed
> change.  But, I will look into it.
> 
> [1] https://lkml.org/lkml/2019/4/23/2

I only took a quick look at the ipc code, but there does not appear to be
a quick/easy change to make.  The issue is that shared memory creation could
take a long time.  With issue [1] above unresolved, creation of hugetlb backed
shared memory segments could take a VERY long time.

I do not believe the test failure is arm specific.  Most likely, it is just
because testing was done on a system with memory size to trigger this issue?

My plan is to focus on [1].  When that is resolved, this issue should go away.
-- 
Mike Kravetz