linux-kernel - Re: [RFC PATCH] mm/hugetlb: make hugetlb

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Message-Id: <19bb32a4-7acc-29ea-c00c-65cd2ebf9878@linux.ibm.com>
Date:   Thu, 6 Sep 2018 09:24:17 +0530
From:   "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] mm/hugetlb: make hugetlb_lock irq safe

On 09/06/2018 01:28 AM, Andrew Morton wrote:
> On Wed, 5 Sep 2018 06:48:48 -0700 Matthew Wilcox <willy@...radead.org> 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?
> 

That is old code that goes back to v4.2 ( 
15b244a88e1b2895605be4300b40b575345bcf50)



-aneesh