hugetlb: fix huge_pmd_unshare address update

commit 48381273f8734d28ef56a5bdf1966dd8530111bc upstream.

The routine huge_pmd_unshare() is passed a pointer to an address
associated with an area which may be unshared.  If unshare is successful
this address is updated to 'optimize' callers iterating over huge page
addresses.  For the optimization to work correctly, address should be
updated to the last huge page in the unmapped/unshared area.  However, in
the common case where the passed address is PUD_SIZE aligned, the address
is incorrectly updated to the address of the preceding huge page.  That
wastes CPU cycles as the unmapped/unshared range is scanned twice.

Link: https://lkml.kernel.org/r/20220524205003.126184-1-mike.kravetz@oracle.com
Fixes: 39dde65c9940 ("shared page table for hugetlb page")
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
Acked-by: Muchun Song <songmuchun@bytedance.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index c6ae9c9e360d6a9179ab14330865f5ae08eecb24..04ad2bba01ebdc0b2cdc0d730afaefa57687d84c 100644 (file)
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4798,7 +4798,14 @@ int huge_pmd_unshare(struct mm_struct *mm, unsigned long *addr, pte_t *ptep)
        pud_clear(pud);
        put_page(virt_to_page(ptep));
        mm_dec_nr_pmds(mm);
-       *addr = ALIGN(*addr, HPAGE_SIZE * PTRS_PER_PTE) - HPAGE_SIZE;
+       /*
+        * This update of passed address optimizes loops sequentially
+        * processing addresses in increments of huge page size (PMD_SIZE
+        * in this case).  By clearing the pud, a PUD_SIZE area is unmapped.
+        * Update address to the 'last page' in the cleared area so that
+        * calling loop can move to first page past this area.
+        */
+       *addr |= PUD_SIZE - PMD_SIZE;
        return 1;
 }
 #define want_pmd_share()       (1)