linux-kernel - Re: [PATCH v3] mm/hugetlb: avoid hardcoding while checking if cma is enable

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Date:   Wed, 8 Jul 2020 11:46:15 -0700
From:   Roman Gushchin <guro@...com>
To:     Mike Kravetz <mike.kravetz@...cle.com>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Barry Song <song.bao.hua@...ilicon.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...wei.com>,
        Jonathan Cameron <jonathan.cameron@...wei.com>
Subject: Re: [PATCH v3] mm/hugetlb: avoid hardcoding while checking if cma is
 enable

On Wed, Jul 08, 2020 at 10:45:16AM -0700, Mike Kravetz wrote:
> On 7/7/20 12:56 PM, Andrew Morton wrote:
> > On Tue, 7 Jul 2020 16:02:04 +1200 Barry Song <song.bao.hua@...ilicon.com> wrote:
> > 
> >> hugetlb_cma[0] can be NULL due to various reasons, for example, node0 has
> >> no memory. so NULL hugetlb_cma[0] doesn't necessarily mean cma is not
> >> enabled. gigantic pages might have been reserved on other nodes.
> > 
> > I'm trying to figure out whether this should be backported into 5.7.1,
> > but the changelog doesn't describe any known user-visible effects of
> > the bug.  Are there any?
> 
> Barry must have missed this email.  He reported the issue so I was hoping
> he would reply.
> 
> Based on the code changes, I believe the following could happen:
> - Someone uses 'hugetlb_cma=' kernel command line parameter to reserve
>   CMA for gigantic pages.
> - The system topology is such that no memory is on node 0.  Therefore,
>   no CMA can be reserved for gigantic pages on node 0.  CMA is reserved
>   on other nodes.
> - The user also specifies a number of gigantic pages to pre-allocate on
>   the command line with hugepagesz=<gigantic_page_size> hugepages=<N>
> - The routine which allocates gigantic pages from the bootmem allocator
>   will not detect CMA has been reserved as there is no memory on node 0.
>   Therefore, pages will be pre-allocated from bootmem allocator as well
>   as reserved in CMA.
> 
> This double allocation (bootmem and CMA) is the worst case scenario.  Not
> sure if this is what Barry saw, and I suspect this would rarely happen.
> 
> After writing this, I started to think that perhaps command line parsing
> should be changed.  If hugetlb_cma= is specified, it makes no sense to
> pre-allocate gigantic pages.  Therefore, the hugepages=<N> paramemter
> should be ignored and flagged with a warning if  hugetlb_cma= is specified.
> This could be checked at parsing time and there would be no need for such
> a check in the allocation code (except for sanity cheching).
> 
> Thoughts?  I just cleaned up the parsing code and could make such a change
> quite easily.

I agree. Basically, if hugetlb_cma_size > 0, we should not pre-allocate
gigantic pages. It would be much simpler and more reliable than the existing
code.

Thank you!