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

[Matthew Wilcox <willy@infradead.org>]

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().

> 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?