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Date:   Tue, 8 Sep 2020 10:26:10 -0300
From:   Jason Gunthorpe <jgg@ziepe.ca>
To:     Gerald Schaefer <gerald.schaefer@linux.ibm.com>
Cc:     John Hubbard <jhubbard@nvidia.com>,
        LKML <linux-kernel@vger.kernel.org>,
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        Mike Rapoport <rppt@kernel.org>,
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        Alexander Gordeev <agordeev@linux.ibm.com>,
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        Christian Borntraeger <borntraeger@de.ibm.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>
Subject: Re: [RFC PATCH v2 2/3] mm: make pXd_addr_end() functions page-table
 entry aware
Message-ID: <20200908132610.GB87483@ziepe.ca>
References: <20200907180058.64880-1-gerald.schaefer@linux.ibm.com>
 <20200907180058.64880-3-gerald.schaefer@linux.ibm.com>
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On Mon, Sep 07, 2020 at 08:00:57PM +0200, Gerald Schaefer wrote:
> From: Alexander Gordeev <agordeev@linux.ibm.com>
> 
> Unlike all other page-table abstractions pXd_addr_end() do not take
> into account a particular table entry in which context the functions
> are called. On architectures with dynamic page-tables folding that
> might lead to lack of necessary information that is difficult to
> obtain other than from the table entry itself. That already led to
> a subtle memory corruption issue on s390.
> 
> By letting pXd_addr_end() functions know about the page-table entry
> we allow archs not only make extra checks, but also optimizations.
> 
> As result of this change the pXd_addr_end_folded() functions used
> in gup_fast traversal code become unnecessary and get replaced with
> universal pXd_addr_end() variants.
> 
> The arch-specific updates not only add dereferencing of page-table
> entry pointers, but also small changes to the code flow to make those
> dereferences possible, at least for x86 and powerpc. Also for arm64,
> but in way that should not have any impact.
> 
> So, even though the dereferenced page-table entries are not used on
> archs other than s390, and are optimized out by the compiler, there
> is a small change in kernel size and this is what bloat-o-meter reports:

This looks pretty clean and straightfoward, only
__munlock_pagevec_fill() had any real increased complexity.

Thanks,
Jason