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

[Aneesh Kumar K.V]

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