linux-kernel - Re: [RFC PATCH 0/2] mm/gup: fix gup_fast with dynamic page table folding

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Date:   Tue, 1 Sep 2020 15:14:12 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        linux-s390@...r.kernel.org, Heiko Carstens <hca@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        John Hubbard <jhubbard@...dia.com>
Subject: Re: [RFC PATCH 0/2] mm/gup: fix gup_fast with dynamic page table
 folding

On Tue, Sep 01, 2020 at 07:40:20PM +0200, Gerald Schaefer wrote:
> +/*
> + * With dynamic page table levels on s390, the static pXd_addr_end() functions
> + * will not return corresponding dynamic boundaries. This is no problem as long
> + * as only pXd pointers are passed down during page table walk, because
> + * pXd_offset() will simply return the given pointer for folded levels, and the
> + * pointer iteration over a range simply happens at the correct page table
> + * level.
> + * It is however a problem with gup_fast, or other places walking the page
> + * tables w/o locks using READ_ONCE(), and passing down the pXd values instead
> + * of pointers. In this case, the pointer given to pXd_offset() is a pointer to
> + * a stack variable, which cannot be used for pointer iteration at the correct
> + * level. Instead, the iteration then has to happen by going up to pgd level
> + * again. To allow this, provide pXd_addr_end_folded() functions with an
> + * additional pXd value parameter, which can be used on s390 to determine the
> + * folding level and return the corresponding boundary.
> + */
> +#ifndef pgd_addr_end_folded
> +#define pgd_addr_end_folded(pgd, addr, end)	pgd_addr_end(addr, end)
> +#endif
> +
> +#ifndef p4d_addr_end_folded
> +#define p4d_addr_end_folded(p4d, addr, end)	p4d_addr_end(addr, end)
> +#endif
> +
> +#ifndef pud_addr_end_folded
> +#define pud_addr_end_folded(pud, addr, end)	pud_addr_end(addr, end)
> +#endif
> +
> +#ifndef pmd_addr_end_folded
> +#define pmd_addr_end_folded(pmd, addr, end)	pmd_addr_end(addr, end)
> +#endif

Feels like it would be cleaner to globally change pmd_addr_end() /etc
to require the extra pmd input rather that introduce this special rule
when *_folded needs to be used? NOP on all arches execpt s390..

There are not so many call sites that it seems too scary, and I
wouldn't be surprised if there are going to be more cases beyond GUP
that *should* be using the READ_ONCE trick.

Jason