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

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 <redacted>
---
 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