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@xxxxxxxx> 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. ;)
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