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SubjectRe: [RFC] x86: gup_fast() batch limit (was: DRM lock ordering fix series)
Date
On Saturday 28 March 2009 23:46:14 Peter Zijlstra wrote:
> On Sat, 2009-03-28 at 13:22 +0100, Peter Zijlstra wrote:
> > I'm not really trusting my brain today, but something like the below
> > should work I think.
> >
> > Nick, any thoughts?
> >
> > Not-Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > ---
> > arch/x86/mm/gup.c | 24 +++++++++++++++++++++---
> > 1 files changed, 21 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/mm/gup.c b/arch/x86/mm/gup.c
> > index be54176..4ded5c3 100644
> > --- a/arch/x86/mm/gup.c
> > +++ b/arch/x86/mm/gup.c
> > @@ -11,6 +11,8 @@
> >
> > #include <asm/pgtable.h>
> >
> > +#define GUP_BATCH 32
> > +
> > static inline pte_t gup_get_pte(pte_t *ptep)
> > {
> > #ifndef CONFIG_X86_PAE
> > @@ -91,7 +93,8 @@ static noinline int gup_pte_range(pmd_t pmd, unsigned
> > long addr, get_page(page);
> > pages[*nr] = page;
> > (*nr)++;
> > -
> > + if (*nr > GUP_BATCH)
> > + break;
> > } while (ptep++, addr += PAGE_SIZE, addr != end);
> > pte_unmap(ptep - 1);
> >
> > @@ -157,6 +160,8 @@ static int gup_pmd_range(pud_t pud, unsigned long
> > addr, unsigned long end, if (!gup_pte_range(pmd, addr, next, write,
> > pages, nr))
> > return 0;
> > }
> > + if (*nr > GUP_BATCH)
> > + break;
> > } while (pmdp++, addr = next, addr != end);
> >
> > return 1;
> > @@ -214,6 +219,8 @@ static int gup_pud_range(pgd_t pgd, unsigned long
> > addr, unsigned long end, if (!gup_pmd_range(pud, addr, next, write,
> > pages, nr))
> > return 0;
> > }
> > + if (*nr > GUP_BATCH)
> > + break;
> > } while (pudp++, addr = next, addr != end);
> >
> > return 1;
> > @@ -226,7 +233,7 @@ int get_user_pages_fast(unsigned long start, int
> > nr_pages, int write, unsigned long addr, len, end;
> > unsigned long next;
> > pgd_t *pgdp;
> > - int nr = 0;
> > + int batch = 0, nr = 0;
> >
> > start &= PAGE_MASK;
> > addr = start;
> > @@ -254,6 +261,7 @@ int get_user_pages_fast(unsigned long start, int
> > nr_pages, int write, * (which we do on x86, with the above PAE
> > exception), we can follow the * address down to the the page and take a
> > ref on it.
> > */
> > +again:
> > local_irq_disable();
> > pgdp = pgd_offset(mm, addr);
> > do {
> > @@ -262,11 +270,21 @@ int get_user_pages_fast(unsigned long start, int
> > nr_pages, int write, next = pgd_addr_end(addr, end);
> > if (pgd_none(pgd))
> > goto slow;
> > - if (!gup_pud_range(pgd, addr, next, write, pages, &nr))
> > + if (!gup_pud_range(pgd, addr, next, write, pages, &batch))
> > goto slow;
> > + if (batch > GUP_BATCH) {
> > + local_irq_enable();
> > + addr += batch << PAGE_SHIFT;
> > + nr += batch;
> > + batch = 0;
> > + if (addr != end)
> > + goto again;
> > + }
> > } while (pgdp++, addr = next, addr != end);
> > local_irq_enable();
> >
> > + nr += batch;
> > +
> > VM_BUG_ON(nr != (end - start) >> PAGE_SHIFT);
> > return nr;
>
> Would also need the following bit:
>
> @@ -274,6 +292,7 @@ int get_user_pages_fast(unsigned long start, int
> nr_pages, int write, int ret;
>
> slow:
> + nr += batch;
> local_irq_enable();
> slow_irqon:
> /* Try to get the remaining pages with get_user_pages */


Yeah something like this would be fine (and welcome). And we can
remove the XXX comment in there too. I would suggest 64 being a
reasonable value simply because that's what direct IO does.

Implementation-wise, why not just break "len" into chunks in the
top level function rather than add branches all down the call
chain?



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