From 71067a4b74571c15ce65fd6a3bbbf54e2b8f50df Mon Sep 17 00:00:00 2001 From: Li Xinhai Date: Thu, 31 Dec 2020 22:04:09 +0000 Subject: [PATCH] mm/hugetlb.c: fix unnecessary address expansion of pmd sharing The current code would unnecessarily expand the address range. Consider one example, (start, end) = (1G-2M, 3G+2M), and (vm_start, vm_end) = (1G-4M, 3G+4M), the expected adjustment should be keep (1G-2M, 3G+2M) without expand. But the current result will be (1G-4M, 3G+4M). Actually, the range (1G-4M, 1G) and (3G, 3G+4M) would never been involved in pmd sharing. After this patch, if pud aligned *start across vm_start, then we know the *start and vm_start are in same pud_index, and vm_start is not pud aligned, so don't adjust *start. Same logic applied to *end. Mike said: : The 'adjusted range' is used for calls to mmu notifiers and cache(tlb) : flushing. Since the current code unnecessarily expands the range in : some cases, more entries than necessary would be flushed. This : would/could result in performance degradation. However, this is highly : dependent on the user runtime. Is there a combination of vma layout : and calls to actually hit this issue? If the issue is hit, will those : entries unnecessarily flushed be used again and need to be : unnecessarily reloaded? Link: https://lkml.kernel.org/r/20201229042125.2663029-1-lixinhai.lxh@gmail.com Fixes: commit 75802ca66354 ("mm/hugetlb: fix calculation of adjust_range_if_pmd_sharing_possible") Signed-off-by: Li Xinhai Reviewed-by: Mike Kravetz Cc: Peter Xu Signed-off-by: Andrew Morton --- mm/hugetlb.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index a2602969873d..d3caaeab94ed 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5269,11 +5269,16 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma, a_end = ALIGN(*end, PUD_SIZE); /* - * Intersect the range with the vma range, since pmd sharing won't be - * across vma after all + * If the PUD aligned address across vma range, then it means the + * vm_start/vm_end is not PUD aligned. In that case, we must don't + * adjust range because pmd sharing is not possbile at the start and/or + * end part of vma. */ - *start = max(vma->vm_start, a_start); - *end = min(vma->vm_end, a_end); + if (a_start >= vma->vm_start) + *start = a_start; + + if (a_end <= vma->vm_end) + *end = a_end; } /* -- 2.50.1