On Wed, 17 Apr 2019 09:46:37 +0200 Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote: > On Tue, 16 Apr 2019 09:49:46 -0700 > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Tue, Apr 16, 2019 at 9:16 AM Linus Torvalds > > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > We actually already *have* this function. > > > > > > It's called "gup_fast_permitted()" and it's used by x86-64 to verify > > > the proper address range. Exactly like s390 needs.. > > > > > > Could you please use that instead? > > > > IOW, something like the attached. > > > > Obviously untested. And maybe 'current' isn't declared in > > <asm/pgtable.h>, in which case you'd need to modify it to instead make > > the inline function be "s390_gup_fast_permitted()" that takes a > > pointer to the mm, and do something like > > > > #define gup_fast_permitted(start, pages) \ > > s390_gup_fast_permitted(current->mm, start, pages) > > > > instead. > > > > But I think you get the idea.. > > Nice, I did not realize that gup_fast_permitted is a platform > override-able function. So that part is doable in arch/s390. But I > spoke to soon, I got my first crash and realized that the common gup code > is not usable as it is. The reason is this e.g. this sequence: > > pgdp = pgd_offset(current->mm, addr); > pgd_t pgd = READ_ONCE(*pgdp); > /* some checking on pgd */ > gup_p4d_range(pgd, addr, next, write, pages, nr); > > p4dp = p4d_offset(&pgd, addr); > p4d_t p4d = READ_ONCE(*p4dp); > /* some checking on p4d */ > gup_pud_range(p4d, addr, next, write, pages, nr); > > pudp = pud_offset(&p4d, addr); > pud_t pud = READ_ONCE(*pudp); > /* some checking on pud */ > gup_pmd_range(pud, addr, next, write, pages, nr; > > Each step along the way will read the page table entry and pass the > table entry to the next function. This clashes with the page table > folding on s390. The s390 gup code looks more like this: > > pgdp = pgd_offset(current->mm, addr); > /* some checking on pgd */ > pgd_t pgd = READ_ONCE(*pgdp); > gup_p4d_range(pgdp, pgd, addr, next, write, pages, &nr); > > p4dp = p4d_offset(pgdp, addr); > p4d_t p4d = READ_ONCE(*p4dp); > /* some checking on p4d */ > gup_pud_range(p4dp, p4d, addr, next, write, pages, nr); > > pudp = pud_offset(p4dp, addr); > pud_t pud = READ_ONCE(*pudp); > /* some checking on pud */ > gup_pmd_range(pudp, pud, addr, next, write, pages, nr; > > There are magic dereferences in the s390 versions of p4d_offset, > pud_offset and pmd_offset functions. To make this work the pointer > passed to these functions may not be the local copy of the already > dereferenced table entry. I'll cook up a patch for the common code. Grumpf, that does *not* work. For gup the table entries may be read only once. Now I remember why I open-coded p4d_offset, pud_offset and pmd_offset in arch/s390/mm/gup.c, to avoid to read the table entries twice. It will be hard to use the common gup code after all. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin.
