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Sun, 03 Jan 2021 23:10:42 -0800 (PST) Subject: Re: [PATCH] mm/hugetlb.c: fix unnecessary address expansion of pmd sharing To: Mike Kravetz , linux-mm@kvack.org Cc: akpm@linux-foundation.org, Peter Xu References: <20201229042125.2663029-1-lixinhai.lxh@gmail.com> <2b6534f5-2ffa-6325-6af1-8c067ba9c0bc@oracle.com> <2d8a7726-b7fb-3dc8-e7bc-54a67d28db24@oracle.com> <2a95c320-4408-2603-f266-8e111a2f1620@gmail.com> <049fb8f0-73c8-b296-7670-7b60fdcc092a@oracle.com> From: Li Xinhai Message-ID: <1617a471-b837-a868-67a0-0b0f17cb3748@gmail.com> Date: Mon, 4 Jan 2021 15:10:33 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: <049fb8f0-73c8-b296-7670-7b60fdcc092a@oracle.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 1/4/21 11:55 AM, Mike Kravetz wrote: > I believe the only case where your patch produced incorrect results is > when the range was within a vma that was smaller than PUD_SIZE. Do you > agree? > Not exactly. We need to consider for vma which span at least one PUD_SIZE after align its vm_start and vm_end. > If that is the case, then how about just adding the following to your patch? > I think this is simpler and faster than the 'range_in_vma' checking I proposed. > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 49990c0a02a3..716d1e58a7ae 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -5261,7 +5261,9 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma, > { > unsigned long a_start, a_end; > > - if (!(vma->vm_flags & VM_MAYSHARE)) > + /* Quick check for vma capable of pmd sharing */ > + if (!(vma->vm_flags & VM_MAYSHARE) || > + (vma->vm_start - vma->vm_end) < PUD_SIZE) > return; > > /* Extend the range to be PUD aligned for a worst case scenario */ Your suggestion is good, we are able to simplify the code a bit, with less comparison, and maybe easier to follow. I will send a new patch to review.