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

[Andrew Morton]

On Wed, 5 Sep 2018 06:48:48 -0700 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:

> > 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?

Calling put_page() against a huge page from hardirq seems like the
right thing to do - even if it's rare now, it will presumably become
more common as the hugepage virus spreads further across the kernel. 
And the present asymmetry is quite a wart.

That being said, arch/powerpc/mm/mmu_context_iommu.c:mm_iommu_free() is
the only known site which does this (yes?) so perhaps we could put some
stopgap workaround into that site and add a runtime warning into the
put_page() code somewhere to detect puttage of huge pages from hardirq
and softirq contexts.

And attention will need to be paid to -stable backporting.  How long
has mm_iommu_free() existed, and been doing this?