On 07/08/2014 01:41 PM, Naoya Horiguchi wrote: >> > It would only set the first two bytes of a >> > 256k BMAP buffer since only two pages were encountered in the radix tree. > Hmm, this example shows me a problem, thanks. > > If the user knows the fd is for 1GB hugetlbfs file, it just prepares > the 2 bytes buffer, so no problem. > But if the user doesn't know whether the fd is from hugetlbfs file, > the user must prepare the large buffer, though only first few bytes > are used. And the more problematic is that the user could interpret > the data in buffer differently: > 1. only the first two 4kB-pages are loaded in the 2GB range, > 2. two 1GB-pages are loaded. > So for such callers, fincore() must notify the relevant page size > in some way on return. > Returning it via fincore_extra is my first thought but I'm not sure > if it's elegant enough. That does limit the interface to being used on a single page size per call, which doesn't sound too bad since we don't mix page sizes in a single file. But, you mentioned using this interface along with /proc/$pid/mem. How would this deal with a process which had two sizes of pages mapped? Another option would be to have userspace pass in its desired granularity. Such an interface could be used to find holes in a file fairly easily. But, introduces a whole new set of issues, like what BMAP means if only a part of the granule is in-core, and do you need a new option to differentiate BMAP_AND vs. BMAP_OR operations. I honestly think we need to take a step back and enumerate what you're trying to do here before going any further. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>