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@xxxxxxxxxx/ 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, > > . >
