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

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Message-Id: <95b9a5d9-1e46-b969-4188-14c0a6efb215@linux.ibm.com>
Date:   Thu, 6 Sep 2018 09:30:02 +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 07:18 PM, Matthew Wilcox wrote:
> On Wed, Sep 05, 2018 at 06:56:19PM +0530, Aneesh Kumar K.V wrote:
>> 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?
> 
> No.  I'm asking "why not disable softirqs in the sysctl handler".  Or
> perhaps equivalently, just replace spin_lock() with spin_lock_bh() in
> set_max_huge_pages().
> 

Disabling only in sysctl handler is not enough right? Every usage of 
locks taken by the page destructor need to be converted to disable 
softirqs right?


>> 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.
> 
> straightforward, sure.  but is it the right thing to do?  do we want to
> be able to put_page() a hugetlb page from hardirq context?
> 

-aneesh