On Thu, Oct 06, 2011 at 08:22:17PM +0400, Pavel Emelyanov wrote: > Hi. > > > While doing the checkpoint-restore in the userspace we need to determine > whether various kernel objects (like mm_struct-s of file_struct-s) are shared > between tasks and restore this state. > The 2nd step can for now be solved by using respective CLONE_XXX flags and > the unshare syscall, while there's currently no ways for solving the 1st one. > > One of the ways for checking whether two tasks share e.g. an mm_struct is to > provide some mm_struct ID of a task to its proc file. The best from the > performance point of view ID is the object address in the kernel, but showing > them to the userspace is not good for performance reasons. Thus the ID should > not be calculated based on the object address. > > The proposal is to have the ID for slab objects as the mixture of two things - > the number of an object on the slub and the ID of a slab, which is calculated > simply by getting a monotonic 64 bit number at the slab allocation time which > gives us 200+ years of stable work (see comment in the patch #2) :) This just strikes me as the wrong approach. Userspace should not need to know the structures the kernel is using to implement the sharing that's possible with the clone flags. The userspace interface should be framed such that the kernel is not exporting the relationship between these structures so much as the relationship between the tasks which those structures reflect. Perhaps you could write the interface so that it shows the clone flags one would use to re-create the child task from the parent instead of trying to output these ids. Also, putting this in slab seems like a poor choice -- isn't instrumenting the allocator rather invasive? Especialy when we're talking about a handful of structs in comparison to everything else the allocators handle? Cheers, -Matt Helsley -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>