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

[Aneesh Kumar K.V]

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