Re: [PATCH v3 1/3] mm: introduce fincore()

On 07/08/2014 01:41 PM, Naoya Horiguchi wrote:

quoted

quoted

 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.

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