Messages in this thread |  | | | Date | Fri, 13 Apr 2007 11:14:20 +1000 | | From | Nick Piggin <> | | Subject | Re: [PATCH 0/13] maps: pagemap, kpagemap, and related cleanups |
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Andrew Morton wrote: > On Fri, 13 Apr 2007 10:15:24 +1000 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>>>>+ for (; i < 2 * chunk / KPMSIZE; i += 2, pfn++) { >>>>+ ppage = pfn_to_page(pfn); >>>>+ if (!ppage) { >>>>+ page[i] = 0; >>>>+ page[i + 1] = 0; >>>>+ } else { >>>>+ page[i] = ppage->flags; >>>>+ page[i + 1] = atomic_read(&ppage->_count); >>>>+ } >>>>+ } >>> >>> >>>Not a good idea to expose raw flags in this manner - it changes at the drop >>>of a hat. We'd need to also expose the kernel's PG_foo-to-bitnumber >>>mapping to make this viable. >> >>I don't think it is viable because that makes the flags part of the >>userspace ABI. > > > It *will* be viable. If the application wants to know if a page is dirty, > it looks up "PG_dirty" in /proc/pg_foo-to-bitnumber and uses PG_dirty's > numerical offset when inspecting fields in /proc/kpagemap. If correctly > designed, such a monitoring application will be able to report upon page > flags which we haven't even thought up yet.
Ooh, you wanted a _runtime_ mapping of flags, yeah then I guess that works. Still seems like a basically hit and miss affair to just use flags. What if you want to know the process mapping a page? With systemtap or something you could walk the rmap structures. What if you want to look at pages along the LRU list rather than per-pfn? What about connecting pages to inodes?
I thought this type of deep poking was the whole reason the probles thingies were merged. I'm saddened that they're no good for this. I thought it would be an ideal usage :(
>>I wonder what they are needed for. > > > Poking deeply into the kernel to provide information about kernel state. > > There are real-world needs for this, and the people who develop tools to > process this information will have decent kernel understanding and will > know that the file's contents may alter across kernel versions. It sure > beats poking around in /dev/kmem. > > I doubt if there's a sensible way in which we can prettify this interface > without losing information. But we should aim to make it as robust as > possible agaisnt future kenrel changes, of course. > > And we should satisfy ourselves that all the required information has been > made available. The fact that it will satisfy the Oracle requirement is > encouraging.
Yeah it is close, they need page_mapcount I think. But I was going to say that satisfying an Oracle requirement is a good reason _not_ to merge it ;) (I joke!)
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