Re: [RFC PATCH v2 2/3] mm: make pXd_addr_end() functions page-table entry aware

Jason Gunthorpe
2020-09-08T16:15:12Z

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