有关匿名页的反向映射的一个疑惑

flikelinux

在ULK中提到匿名共享页的线性地址是由：匿名线性区起始地址+page-->index，从而获得共享页的页表项。但我有个疑惑：一个page结构只有一个index字段，难道匿名共享页映射到不同的匿名内存区其index一定要相同吗？附上书上的一个图

firkraag

The mremap( ) system call may remap memory regions and move the pages into the User Mode address space by directly modifying the page table entries. In this particular case, object-based reverse mapping does not work, because the index field of the page descriptor cannot be used to determine the actual linear address of the page.

flikelinux

看这句话的意思，应该是要求所有的index必须一样，是吧回复 2# firkraag


   

firkraag

上面的English是书里的有的原话。匿名内存映射适用于具有亲属关系的进程之间.要是有亲属关系的话，我觉得这个index就应该相同了。

embeddedlwp

回复 3# flikelinux



对于匿名页的page->index表示的是page在vma中的偏移。共享匿名页的产生应该只有在fork，clone等时候。也就是说获得了parent process的VMAs。那么应该是一样的啊！


   

embeddedlwp

回复 2# firkraag



The mremap( ) system call may remap memory regions and move the pages into the User Mode address space by directly modifying the page table entries. In this particular case, object-based reverse mapping does not work, because the index field of the page descriptor cannot be used to determine the actual linear address of the page.


红色的部分怎么理解呢？与其他的方式比如mmap有什么区别？

firkraag

http://isec.pl/vulnerabilities/isec-0014-mremap-unmap.txt
Details:
========

The Linux kernel manages a list of user addressable valid memory locations on a
per process basis. Every process owns a single linked list of so called virtual
memory area descriptors (called from now on just VMAs). Every VMA describes the
start of a valid memory region, its length and moreover various memory flags
like page protection.

Every VMA in the list corresponds to a part of the process's page table. The
page table contains descriptors (in short page table entries PTEs) of physical
memory pages seen by the process. The VMA descriptor can be thus understood as a
high level description of a particular region of the process's page table
storing PTE properties like page R/W flag and so on.

The mremap() system call provides resizing (shrinking or growing) as well as
moving of existing virtual memory areas or any of its parts across process's
addressable space.

Moving a part of the virtual memory from inside a VMA area to a new location
requires creation of a new VMA descriptor as well as copying the underlying page
table entries described by the VMA from the old to the new location in the
process's page table.
......

firkraag

就是说该函数的功能之一是可以将一个vma的某部分移动到进程的其它地址空间中。

剑魂箫心

我也正有此疑惑