Re: [PATCH V2 2/4] powerpc/mm/slice: Reduce the stack usage in slice_get_unmapped_area

[Nicholas Piggin <nicholas.piggin@gmail.com>]

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I had a series which goes significantly further with stack reduction. What
do you think about just going with that?

I wonder if we should switch to dynamically allocating the slice stuff on
ppc64

On 27 Feb. 2018 00:28, "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>
wrote:

> This patch kill potential_mask and compat_mask variable and instead use
> tmp_mask
> so that we can reduce the stack usage. This is required so that we can
> increase
> the high_slices bitmap to a larger value.
>
> The patch does result in extra computation in final stage, where it ends up
> recomputing the compat mask again.
>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---
>  arch/powerpc/mm/slice.c | 34 +++++++++++++++++-----------------
>  1 file changed, 17 insertions(+), 17 deletions(-)
>
> diff --git a/arch/powerpc/mm/slice.c b/arch/powerpc/mm/slice.c
> index 259bbda9a222..832c681c341a 100644
> --- a/arch/powerpc/mm/slice.c
> +++ b/arch/powerpc/mm/slice.c
> @@ -413,8 +413,7 @@ unsigned long slice_get_unmapped_area(unsigned long
> addr, unsigned long len,
>  {
>         struct slice_mask mask;
>         struct slice_mask good_mask;
> -       struct slice_mask potential_mask;
> -       struct slice_mask compat_mask;
> +       struct slice_mask tmp_mask;
>         int fixed = (flags & MAP_FIXED);
>         int pshift = max_t(int, mmu_psize_defs[psize].shift, PAGE_SHIFT);
>         unsigned long page_size = 1UL << pshift;
> @@ -449,11 +448,8 @@ unsigned long slice_get_unmapped_area(unsigned long
> addr, unsigned long len,
>         bitmap_zero(mask.high_slices, SLICE_NUM_HIGH);
>
>         /* silence stupid warning */;
> -       potential_mask.low_slices = 0;
> -       bitmap_zero(potential_mask.high_slices, SLICE_NUM_HIGH);
> -
> -       compat_mask.low_slices = 0;
> -       bitmap_zero(compat_mask.high_slices, SLICE_NUM_HIGH);
> +       tmp_mask.low_slices = 0;
> +       bitmap_zero(tmp_mask.high_slices, SLICE_NUM_HIGH);
>
>         /* Sanity checks */
>         BUG_ON(mm->task_size == 0);
> @@ -502,9 +498,11 @@ unsigned long slice_get_unmapped_area(unsigned long
> addr, unsigned long len,
>  #ifdef CONFIG_PPC_64K_PAGES
>         /* If we support combo pages, we can allow 64k pages in 4k slices
> */
>         if (psize == MMU_PAGE_64K) {
> -               slice_mask_for_size(mm, MMU_PAGE_4K, &compat_mask,
> high_limit);
> +               slice_mask_for_size(mm, MMU_PAGE_4K, &tmp_mask,
> high_limit);
>                 if (fixed)
> -                       slice_or_mask(&good_mask, &compat_mask);
> +                       slice_or_mask(&good_mask, &tmp_mask);
> +
> +               slice_print_mask("Mask for compat page size", tmp_mask);
>         }
>  #endif
>         /* First check hint if it's valid or if we have MAP_FIXED */
> @@ -541,11 +539,11 @@ unsigned long slice_get_unmapped_area(unsigned long
> addr, unsigned long len,
>          * We don't fit in the good mask, check what other slices are
>          * empty and thus can be converted
>          */
> -       slice_mask_for_free(mm, &potential_mask, high_limit);
> -       slice_or_mask(&potential_mask, &good_mask);
> -       slice_print_mask(" potential", potential_mask);
> +       slice_mask_for_free(mm, &tmp_mask, high_limit);
> +       slice_or_mask(&tmp_mask, &good_mask);
> +       slice_print_mask("Free area/potential ", tmp_mask);
>
> -       if ((addr != 0 || fixed) && slice_check_fit(mm, mask,
> potential_mask)) {
> +       if ((addr != 0 || fixed) && slice_check_fit(mm, mask, tmp_mask)) {
>                 slice_dbg(" fits potential !\n");
>                 goto convert;
>         }
> @@ -571,7 +569,7 @@ unsigned long slice_get_unmapped_area(unsigned long
> addr, unsigned long len,
>         /* Now let's see if we can find something in the existing slices
>          * for that size plus free slices
>          */
> -       addr = slice_find_area(mm, len, potential_mask,
> +       addr = slice_find_area(mm, len, tmp_mask,
>                                psize, topdown, high_limit);
>
>  #ifdef CONFIG_PPC_64K_PAGES
> @@ -585,9 +583,10 @@ unsigned long slice_get_unmapped_area(unsigned long
> addr, unsigned long len,
>                  * mask variable is free here. Use that for compat
>                  * size mask.
>                  */
> +               slice_mask_for_size(mm, MMU_PAGE_4K, &mask, high_limit);
>                 /* retry the search with 4k-page slices included */
> -               slice_or_mask(&potential_mask, &compat_mask);
> -               addr = slice_find_area(mm, len, potential_mask,
> +               slice_or_mask(&tmp_mask, &mask);
> +               addr = slice_find_area(mm, len, tmp_mask,
>                                        psize, topdown, high_limit);
>         }
>  #endif
> @@ -600,8 +599,9 @@ unsigned long slice_get_unmapped_area(unsigned long
> addr, unsigned long len,
>         slice_print_mask(" mask", mask);
>
>   convert:
> +       slice_mask_for_size(mm, MMU_PAGE_4K, &tmp_mask, high_limit);
>         slice_andnot_mask(&mask, &good_mask);
> -       slice_andnot_mask(&mask, &compat_mask);
> +       slice_andnot_mask(&mask, &tmp_mask);
>         if (mask.low_slices || !bitmap_empty(mask.high_slices,
> SLICE_NUM_HIGH)) {
>                 slice_convert(mm, mask, psize);
>                 if (psize > MMU_PAGE_BASE)
> --
> 2.14.3
>
>

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