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
