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SubjectRe: [RFC PATCH] mm/hugetlb: make hugetlb_lock irq safe
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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

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