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Subject: Re: [PATCH 2/2] mm/hugetlb: refactor subpage recording
To:     Jason Gunthorpe <jgg@ziepe.ca>
Cc:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        Joao Martins <joao.m.martins@oracle.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        Dave Hansen <dave.hansen@intel.com>
References: <20210125205744.10203-1-joao.m.martins@oracle.com>
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 <20210127021023.GC4605@ziepe.ca>
From:   Mike Kravetz <mike.kravetz@oracle.com>
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Date:   Sat, 13 Feb 2021 13:12:26 -0800
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On 1/26/21 6:10 PM, Jason Gunthorpe wrote:
> On Tue, Jan 26, 2021 at 05:58:53PM -0800, Mike Kravetz wrote:
> 
>> As pointed out by Joao, you can also see the differences in pfn_to_page
>> for CONFIG_SPARSE_VMEMMAP and CONFIG_SPARSEMEM.  The only time we might
>> have issues is with CONFIG_SPARSEMEM.  I would bet CONFIG_SPARSE_VMEMMAP
>> is far more common.
> 
> I think it is fine to have a different pfn_to_page, it should just be
> illegal to combine pages into a compound if their tail pages are not
> linear in the map.
> 
> Matt's folio work might present an option to audit the whole mm for
> this pattern and provide some folio_next_tail_page() accessor that
> does the fast thing - but I question the value of such a project for a
> 2008 era PPC platform with 16GB pages (seriously?) that may be using
> VMEMMAP today anyhow??
> 
> Maybe others know of more modern use cases
> 
> Jason

When discussing v2 of this patch, Zi confirmed that this issue exists today.
See,
https://lore.kernel.org/linux-mm/3d2acb33-57e2-060d-616f-cce872c77307@oracle.com

I will fix up that unexpected discovery in the hugetlb code.  But, not sure
what approach we want to take elsewhere, such as the GUP code.
-- 
Mike Kravetz