On 09/05/2018 12:58 PM, Andrew Morton wrote: > 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?) IIUC, the powerpc iommu code 'remaps' user allocated hugetlb pages. It is these pages that are of issue at put_page time. I'll admit that code is new to me and I may not fully understand. However, if this is accurate then it makes it really difficult to track down any other similar usage patterns. I can not find a reference to PageHuge in the powerpc iommu code. > 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. I think we would add the warning/etc at free_huge_page. The issue would only apply to hugetlb pages, not THP. But, the more I think about it the more I think Aneesh's patch to do spin_lock/unlock_irqsave is the right way to go. Currently, we only know of one place where a put_page of hugetlb pages is done from softirq context. So, we could take the spin_lock/unlock_bh as Matthew suggested. When the powerpc iommu code was added, I doubt this was taken into account. I would be afraid of someone adding put_page from hardirq context. -- Mike Kravetz > And attention will need to be paid to -stable backporting. How long > has mm_iommu_free() existed, and been doing this?
