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 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 1/10/25 7:54 PM, Liu Shixin wrote: > > 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? What do you mean about revisiting this commit? Reset vm_file and recalculate vm_pgoff in vma_set_anonymous()? > > By the way, it seems we collpase the file even after cow for a private file mapping. Is that so? It seems so. > > Thanks, >> . >>