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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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