linux-kernel - Re: [RFC PATCH] mm/hugetlb: make hugetlb

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Message-Id: <d76771e6-1664-5d38-a5a0-e98f1120494c@linux.ibm.com>
Date:   Wed, 5 Sep 2018 18:56:19 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     akpm@...ux-foundation.org, Mike Kravetz <mike.kravetz@...cle.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm/hugetlb: make hugetlb_lock irq safe

On 09/05/2018 06:34 PM, Matthew Wilcox wrote:
> On Wed, Sep 05, 2018 at 04:53:41PM +0530, Aneesh Kumar K.V wrote:
>>   inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
> 
> How do you go from "can be taken in softirq context" problem report to
> "must disable hard interrupts" solution?  Please explain why spin_lock_bh()
> is not a sufficient fix.
> 
>>   swapper/68/0 [HC0[0]:SC1[1]:HE1:SE0] takes:
>>   0000000052a030a7 (hugetlb_lock){+.?.}, at: free_huge_page+0x9c/0x340
>>   {SOFTIRQ-ON-W} state was registered at:
>>     lock_acquire+0xd4/0x230
>>     _raw_spin_lock+0x44/0x70
>>     set_max_huge_pages+0x4c/0x360
>>     hugetlb_sysctl_handler_common+0x108/0x160
>>     proc_sys_call_handler+0x134/0x190
>>     __vfs_write+0x3c/0x1f0
>>     vfs_write+0xd8/0x220
> 
> Also, this only seems to trigger here.  Is it possible we _already_
> have softirqs disabled through every other code path, and it's just this
> one sysctl handler that needs to disable softirqs?  Rather than every
> lock access?

Are you asking whether I looked at moving that put_page to a worker 
thread? I didn't. The reason I looked at current patch is to enable the 
usage of put_page() from irq context. We do allow that for non hugetlb 
pages. So was not sure adding that additional restriction for hugetlb
is really needed. Further the conversion to irqsave/irqrestore was
straightforward.

Now with respect to making sure we don't have irq already disabled in 
those code paths, I did check that. But let me know if you find anything 
I missed.

> I'm not seeing any analysis in this patch description, just a kneejerk
> "lockdep complained, must disable interrupts".
> 

-aneesh