This is an assignment problem which asks for partial implementation of process checkpointing:
The test program allocates an array, does a system call and passes the start and end address of array to the call. In the system call function I have to save the contents in the give range to a file.
From my understanding, I could simply use copy_from_usr function to save the contents from the give range. However since the assignment is based on topic "Process address space", I probably need to walk through page tables. Say I manage to get the struct pages that correspond to given range. How do I get the data corresponding to the pages?
Can I just use page_to_virt function and access data directly?
Since the array is contiguous in virtual space, I guess I will just need to translate the starting address to page and then back to virtual address and then just copy the range size of data to file. Is that right?
I think copy_from_user() is ok, nothing else needed. When executing the system call, although it trap to kernel space, the context is still the process context which doing the system call. The kernel still use the process's page table. So just to use copy_from_user(), and nothing else needed.
Okey, if you want to do this experiment, I think you can use the void __user *vaddr to traverse the mm->pgd(page table), using pgd_offset/pud_offset/pmd_offset/pte_offset to get the page physical address(page size alignment). Then in kernel space, using ioremap() to create a kernel space mapping, then using the kernel virtual address(page size) + offset(inside the page), you get the start virtual address of the array. Now in kernel, you can using the virtual address to access the array.
Related
So I saw the virt_to_phy function in asm/io.h but this only works for kmalloc allocated memory, is there an equivalent for memory allocated in the data segment? For example, global variables (which includes a page table walk right?).
Such a function would make no sense. In fact, it only makes sense in a kernel that does not page, as in Linux.
In the user space, the mapping changes. By the time you got a return value from such a function, the mapping could already be changed.
I am trying to get an idea on how does memory mapping take place using the system call mmap.
So far I know mmap takes arguments from the user and returns a logical address of where the file is stored. When the user tries to access it takes this address to the map table converts it to a a physical address and carries the operation as requested.
However I found articles as code example and Theoretical explanation
What it mentions is the memory mapping is carried out as:
A. Using system call mmap ()
B. file operations using (struct file *filp, struct vm_area_struct *vma)
What I am trying to figure out is:
How the arguments passed in the mmap system call are used in the struct vm_area_struct *vma) More generally how are these 2 related.
for instance: the struct vm_area_struct has arguments such as starting address, ending address permissions,etc. How are the values sent by the user used to fill values of these variables.
I am trying to write a driver so, Does the kernal fill the values for variables in the structure for us and I simply use it to call and pass values to remap_pfn_range
And a more fundamental question, why is a different file systems operation needed. The fact that mmap returns the virtual address means that it has already achieved a mapping doesnt it ?
Finally I am not that clear about how the entire process would work in user as well as kernal space. Any documentation explaining the process in details would be helpful.
For some reason, I need to translate the virtual address of the code section to physical address. I did following experiment:
I get the virtual address from the start_code and end_code in mm_struct of process A, which are the initial address and final address of the executable code.
I get the CR3 of process A.
I translate the virtual address to physical address page by page. For example, there are 10 pages for code section in process A. I will translate 10 virtual address of each beginning of the page.
I found out some pages will get Page Table Entry(PTE) == 0.Some pages could successfully translate to a physical address.
I tried Firefox and Minicom as my Process, and both of them will get into situation.
I guess my question is: could anyone explain to me why PTE == 0? Does it mean these pages have been swap out to disk? If this is the case, how can I find these pages?
Thanks for any input!!
It looks as if you are trying to perform page table introspection without using the kernel APIs for it. Note that the address space is arranged in a red-black tree of vm_area_struct structs and you should probably use the APIs that traverse them. The mappings might change at any time so using the proper locking for these data structures is necessary.
For example, see the get_user_pages() function. It can be used to swap-in and temporarily pin the pages into memory. Using this function for page table introspection is usually asked for because once have the physical address in hand then the kernel can swap out the page at any time.
I have some code (which I cannot change) that I need to get working in a native Win32 environment. This code calls mmap() and munmap(), so I have created those functions using CreateFileMapping(), MapViewOfFile(), etc., to accomplish the same thing. Initially this works fine, and the code is able to access files as expected. Unfortunately the code goes on to munmap() selected parts of the file that it no longer needs.
x = mmap(0, size, PROT_READ, MAP_SHARED, fd, 0);
...
munmap(x, hdr_size);
munmap(x + foo, bar);
...
Unfortunately, when you pass a pointer into the middle of the mapped range to UnmapViewOfFile() it destroys the entire mapping. Even worse, I can't see how I would be able to detect that this is a partial un-map request and just ignore it.
I have tried calling VirtualFree() on the range but, unsurprisingly, this produces ERROR_INVALID_PARAMETER.
I'm beginning to think that I will have to use static/global variables to track all the open memory mappings so that I can detect and ignore partial unmappings, but I hope you have a better idea...
edit:
Since I wasn't explicit enough above: the docs for UnMapViewOfFile do not accurately reflect the behavior of that function.
Un-mapping the whole view and remapping pieces is not a good solution because you can only suggest a base address for a new mapping, you can't really control it. The semantics of munmap() don't allow for a change to the base address of the still-mapped portion.
What I really need is a way to find the base address and size of a already-mapped memory area.
edit2: Now that I restate the problem that way, it looks like the VirtualQuery() function will suffice.
It is quite explicit in the MSDN Library docs for UnmapViewOfFile:
lpBaseAddress A pointer to the
base address of the mapped view of a
file that is to be unmapped. This
value must be identical to the value
returned by a previous call to the
MapViewOfFile or MapViewOfFileEx
function.
You changing the mapping by unmapping the old one and creating a new one. Unmapping bits and pieces isn't well supported, nor would it have any useful side-effects from a memory management point of view. You don't want to risk getting the address space fragmented.
You'll have to do this differently.
You could keep track each mapping and how many pages of it are still allocated by the client and only free the mapping when that counter reaches zero. The middle sections would still be mapped, but it wouldn't matter since the client wouldn't be accessing that memory anyway.
Create a global dictionary of memory mappings through this interface. When a mapping request comes through, record the address, size and number of pages that are in the range. When a unmap request is made, find out which mapping owns that address and decrease the page count by the number of pages that are being freed. When that count reaches zero, really unmap the view.
I'd like pass a pointer from a user space memory into a function in my kernel module. I don't want to use copy_from_user. I've read that I should use get_user_pages function.
For example one page.
struct page **pages;
pages = kmalloc(1 * sizeof(*pages), GFP_KERNEL);
down_read(¤t->mm->mmap_sem);
get_user_pages(current,current->mm,uaddr, 1, 1, 0,pages,NULL);
up_read(¤t->mm->mmap_sem);
uaddr is an address in User Space.
After doing this, am I allowed to cast and pass uaddr into my kernel module function? Or maybe I have to use these struct pages in some way?
Why do I have to use down/up read?
After everything do I have to use SetPageDirty() and page_cache_release() functions ?
No, you cannot directly access the userspace pages via uaddr. The struct pages are filled out to allow the kernel to access the physical pages that correspond to the userspace pages. Note also that they are most unlikely to be contiguous, so one must be careful to use the correct page index into the array from the start of the uaddr.
You are changing the page mapping structures for this process, so need to protect them while the page mapping in the kernel is set up.
When you are done with the mappings that were setup by get_user_pages(), you must 'release' them by the referenced functions.
This is not what get_user_pages is for (and no - you can't then just cast and pass uaddr into your kernel module function).
If you don't want to call copy_from_user in the calling function, then just pass a void __user * to your module function and have it do the copy_from_user.
You can only use the user pages for page type activity, for example setting up Scatter/Gather DMA into userspace memory. You cannot use it to directly access user space from kernel mode code. Hence the copy_to/from functions that are there for that reason. Unless your moving large amounts of data why not use these functions?
Once you get the valid user space address, use get_user_pages to get the struct page pointer. once struct page pointer received , to access it in the kernel mode you have to map it to kernel virtual address by using kmap. hope that helps