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

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