Am 12.03.2013 14:08, schrieb Prabhu nath:

On Sun, Mar 10, 2013 at 11:30 PM, Christoph Seitz <c.seitz@tu-bs.de <mailto:c.seitz@tu-bs.de>> wrote:

Hi all,

I have some problems allocation Memory the right way and use it in my kernel module.

I use a char device for reading and writing from/to a pcie dma card. Especially the read function makes me some headache. The user allocates some memory with posix_memalign and call the read function on the device, so that the devices knows where to write to. My driver now uses get_user_pages() to pin the user pages. The memory has never been written or read by the user, so it's not yet in the RAM, right? And get_user_pages returns a valid number of pages, but for every page the same struct. (respectively the same pointer). Is there any way to ensure that the user pages are in the ram and get_user_pages returns a valid page array?

If you know the RAM physical address range you can figure out by doing the following *page_to_pfn(page_ptr) << 12*; where page_ptr is a struct page * returned by get_user_pages(). *page_to_pfn()* will return the pfn of the corresponding page frame and left shifting by 12 bits will give you page frame base address.

Maybe my description was a little bit misleading. I have the problem, that if I don't write to that allocated memory in the user application, I won't get any valid pages from get_user_pages. The reason seems to be, that thees pages never got faulted and so the user memory never gets a page frame. Is there any chance for a kernel module to force a page fault or to assign the user memory a page frame? I found out, if I use the force flag with get_user_pages, the pages get faulted, but there has to be a nicer way than using the force flag. Regards, Chris

On Tue, 12 Mar 2013 18:38:05 +0530, Prabhu nath said:

On Sun, Mar 10, 2013 at 11:30 PM, Christoph Seitz <c.seitz@tu-bs.de> wrote:

...

I use a char device for reading and writing from/to a pcie dma card. Especially the read function makes me some headache. The user allocates some memory with posix_memalign and call the read function on the device, so that the devices knows where to write to. My driver now uses get_user_pages() to pin the user pages. The memory has never been written or read by the user, so it's not yet in the RAM, right? And get_user_pages returns a valid number of pages, but for every page the same struct. (respectively the same pointer). Is there any way to ensure that the user pages are in the ram and get_user_pages returns a valid page array?

If you know the RAM physical address range you can figure out by doing the following *page_to_pfn(page_ptr) << 12*; where page_ptr is a struct page * returned by get_user_pages(). * page_to_pfn()* will return the pfn of the corresponding page frame and left shifting by 12 bits will give you page frame base address.

Unfortunately, that doesn't actually tell you what Christoph was worried about - is the page *currently* in RAM? For that, you need to check some bits in the pfn once you find it. Also, note the following: It's not always 12, because not everything uses a 4K page - consider hugepage support, or Power and Itanium where the pages are bigger and often several different sizes are supported. There's an API for the current page size. Use it. :) Also, there's an API for pinning pages so they *stay* in RAM so you can target them for I/O. Use that. ;)