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Sat, 11 Jan 2025 11:54:44 +0800 Subject: Re: [PATCH] mm: khugepaged: fix call hpage_collapse_scan_file() for anonymous vma To: Yang Shi , Matthew Wilcox References: <20250109070059.369257-1-liushixin2@huawei.com> <037d4442-4d2d-4aeb-8091-5efffc374d36@os.amperecomputing.com> <8e773a7c-e1dd-4e0e-8349-8aa52f39d85a@os.amperecomputing.com> <58595f1f-7959-4dc1-9ea2-470c44709d31@os.amperecomputing.com> CC: Andrew Morton , Chengming Zhou , Kefeng Wang , Nanyong Sun , Muchun Song , Qi Zheng , Johannes Weiner , , From: Liu Shixin Message-ID: Date: Sat, 11 Jan 2025 11:54:43 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:45.0) Gecko/20100101 Thunderbird/45.7.1 MIME-Version: 1.0 In-Reply-To: <58595f1f-7959-4dc1-9ea2-470c44709d31@os.amperecomputing.com> Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.174.179.24] X-ClientProxiedBy: dggems705-chm.china.huawei.com (10.3.19.182) To kwepemg200013.china.huawei.com (7.202.181.64) X-Rspamd-Queue-Id: 8968520003 X-Stat-Signature: 1pq4czi3id1tazukr6c9k6hsistkwbit X-Rspam-User: X-Rspamd-Server: rspam09 X-HE-Tag: 1736567689-497706 X-HE-Meta: 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 uIbYntFH aca5KxwlZjH9BhhMAWPMI+c2KqDzg2jqJ5R/l+K+KEORNPP2fMxQBLBMHXx5tO2zjfz1FpaYt5GMgSlNY1kB6UXxmABZhIglPk5YWl57BWvm8ayoT8Pfknuv2fVGkXY5RybsZI2f6d4ILtC5kh46AlzPCKYZR2BZxm/okNjbwOFNevK+6w6D7woHAntrsdZwegwYYf4bAk3HqTem7tn2cITGM9B6nIP9gt8iX 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: List-Subscribe: List-Unsubscribe: On 2025/1/11 3:40, Yang Shi wrote: > > > > On 1/10/25 11:01 AM, Matthew Wilcox wrote: >> On Fri, Jan 10, 2025 at 10:04:42AM -0800, Yang Shi wrote: >>> On 1/9/25 8:31 PM, Matthew Wilcox wrote: >>>> On Thu, Jan 09, 2025 at 09:00:24AM -0800, Yang Shi wrote: >>>>> Thanks for catching this. It sounds a little bit weird to have vm_file for >>>>> an anonymous VMA. I'm not sure why we should keep such special case. It >>>>> seems shared mapping is treated as shmem file mapping. So can we set vm_file >>>>> to NULL when mmap'ing /dev/zero for private mapping? Something like: >>>>> >>>>> diff --git a/drivers/char/mem.c b/drivers/char/mem.c >>>>> index 169eed162a7f..fc332efc5c11 100644 >>>>> --- a/drivers/char/mem.c >>>>> +++ b/drivers/char/mem.c >>>>> @@ -527,6 +527,7 @@ static int mmap_zero(struct file *file, struct >>>>> vm_area_struct *vma) >>>>> if (vma->vm_flags & VM_SHARED) >>>>> return shmem_zero_setup(vma); >>>>> vma_set_anonymous(vma); >>>>> + vma->vm_file = NULL; >>>>> return 0; >>>>> } >>>> I'm wary this might cause other bugs somewhere. rc6 is a bit late to be >>>> introducing such a subtle change. >>> Thanks for the extra caution. Applying the proposed fix in khugepaged code >>> is fine to me either. We can try to kill the special case later. >>> >>> Looking at the code further, I think we should do more to make private >>> /dev/zero mapping an anonymous mapping: >> I'm still nervous about this. We map device inodes in a lot of places. > > Yes, we do. But I don't think this change actually changes the semantic of /dev/zero. Shared /dev/zero mapping is still treated as shmem mapping, private /dev/zero mapping is treated as anonymous mapping, but the current implementation is actually half-baked. It has NULL vma->vm_ops which is used to tell kernel whether it is an anonymous vma or not, but it also has valid vma->vm_file and vma->vm_pgoff as in file offset. > > So this special case makes kernel has 3 types of VMA: > - anonymous VMA: vm_ops is NULL, vm_file is NULL, vm_pgoff is the linear address pgoff > - file VMA: vm_ops is *NOT* NULL, valid vm_file and vm_pgoff is index in file > - private /dev/zero mapping VMA > I have posted v2 to fix it in a safe way. Link: https://lore.kernel.org/all/20250111034511.2223353-1-liushixin2@huawei.com/ Maybe we can also revisit commit bfd40eaff5ab ("mm: fix vma_is_anonymous() false-positives") and fix it by another way? By the way, it seems we collpase the file even after cow for a private file mapping. Is that so? Thanks, > > . >