userfaultfd现在支持文件支持的地图吗? [英] Does userfaultfd now support file backed map?

问题描述

我从userfaultfd的文档中看到了

I saw from the documentation of userfaultfd

https://manpages.debian.org/testing/manpages-dev/userfaultfd.2.en.html http://man7.org/linux/man-pages/man2/ioctl_userfaultfd.2.html

从内核4.11开始，userfaultfd将开始支持共享地图.但是，从某种程度上讲，该文档仍然看起来非常模棱两可，我仍然想知道这些功能是否包括支持文件支持的mmap(也可以是MAP_SHARED)?

that userfaultfd will start supporting shared map since kernel 4.11. However, the documentation still looks very ambiguous in the sense that I'm still wondering will these include supporting file-backed mmap (which can also be MAP_SHARED)?

推荐答案

要明确回答，由于该信息不在手册页中，因此我们可以查看源代码.

To answer definitively, since the information is not in the manual page(s), we can see the source.

尝试使用地址范围注册userfaultfd对象必须

Attempting to register a userfaultfd object with an address range must check whether that range is "compatible":

…
/* check not compatible vmas */
ret = -EINVAL;
if (!vma_can_userfault(cur))
    goto out_unlock;

和兼容性定义是:

static inline bool vma_can_userfault(struct vm_area_struct *vma)
{
    return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) || vma_is_shmem(vma);
}

因此，只有匿名映射( vma_is_anonymous )或以下位置的映射:

Thus, only anonymous mappings (vma_is_anonymous), or mappings in:

• tmpfs-支持，又名共享内存虚拟文件系统(tmpfs， shmget )映射
• 禁用CONFIG_SHMEM时，也可以兼容已重新映射(使用 generic_file_mmap )作为共享的文件支持范围

• tmpfs-backed, aka shared memory virtual filesystem (tmpfs, shmget) mappings are compatible
• when CONFIG_SHMEM is disabled, file-backed ranges which have been remapped (with generic_file_mmap) as shared are also compatible

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