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SubjectRe: [RFC] x86: gup_fast() batch limit (was: DRM lock ordering fix series)
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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 */




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