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The user does not know how many page allocations I should probably clarify my previous reply, so here is the more detailed version: >> are there. I think I can simplify this by trying to use all unused >> page flag bits for addressing the tags. Then, after compilation we can Yes. >> follow the rules I mentioned before: >> - If the available bits are not enough to address all kernel page >> allocations, we issue an error. The user should disable >> CONFIG_PGALLOC_TAG_USE_PAGEFLAGS. The configuration should disable itself, in this case. But if that is too big of a change for now, I suppose we could fall back to an error message to the effect of, "please disable CONFIG_PGALLOC_TAG_USE_PAGEFLAGS because the kernel build system is still too primitive to do that for you". :) >> - If there are enough unused bits but we have to push last_cpupid out >> of page flags, we issue a warning and continue. The user can disable >> CONFIG_PGALLOC_TAG_USE_PAGEFLAGS if last_cpupid has to stay in page >> flags. Let's try to decide now, what that tradeoff should be. Just pick one based on what some of us perceive to be the expected usefulness and frequency of use between last_cpuid and these tag refs. If someone really needs to change the tradeoff for that one bit, then that someone is also likely able to hack up a change for it. thanks, -- John Hubbard >> - If we run out of addressing space during module loading, we disable >> allocation tagging and continue. The user should disable >> CONFIG_PGALLOC_TAG_USE_PAGEFLAGS. > > If the computer already knows what to do, it should do it, rather than > prompting the user to disable a deeply mystifying config parameter. > >> >> This leaves one outstanding case: >> - If we run out of addressing space during module loading but we would >> not run out of space if we pushed last_cpupid out of page flags during >> compilation. >> In this case I would want the user to have an option to request a >> larger addressing space for page allocation tags at compile time. >> Maybe I can keep CONFIG_PGALLOC_TAG_REF_BITS for such explicit >> requests for a larger space? This would limit the use of >> CONFIG_PGALLOC_TAG_REF_BITS to this case only. In all other cases the >> number of bits would be set automatically. WDYT? > > Manually dealing with something like this is just not going to work. > > The more I read this story, the clearer it becomes that this should be > entirely done by the build system: set it, or don't set it, automatically. > > And if you can make it not even a kconfig item at all, that's probably even > better. > > And if there is no way to set it automatically, then that probably means > that the feature is still too raw to unleash upon the world. > > thanks,