From: Mike Kravetz <mike.kravetz@xxxxxxxxxx> 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@xxxxxxxxxx Fixes: 39dde65c9940 ("shared page table for hugetlb page") Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> Acked-by: Muchun Song <songmuchun@xxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> --- mm/hugetlb.c | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4798,7 +4798,14 @@ int huge_pmd_unshare(struct mm_struct *m 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)
