register MADV_HUGEPAGE

From: Andrea Arcangeli <[email protected]>

In order to allow early shipping transparent hugepage feature enabled only inside MADV_HUGEPAGE and not globally to diminish the risk of unexpected performance regressions on non-hypervisor related usages I'd need this little define registered. This is also to avoid things like this:

#define MADV_DOFORK 11 /* do inherit across fork */
#define MADV_HWPOISON 100 /* poison a page for testing */

Picking random number not so nice... and with my proof of concept patch posted working and boosting performance equally to hugetlbfs but without its limitations, I think it is a given that eventually KVM will run inside MADV_HUGEPAGE madvise regions only, so I see a negligeable risk to pollute the kernel in including this as first bit of the full patchset.

---

The rest of the transparent hugepage patchset is very ready for inclusion too with all cleanups requested and after full handling of all review on linux-mm so far, with the only exception of khugepaged that is about to be finished, expect a new submit on linux-mm in a few days. Then we'll have to plug Mel memory compaction on top of it.

(in addition to the below, /sys/kernel/mm/transparent_hugepage also has been moved to some other distro specific location to avoid any possible risk of clashes or confusion with future mainline behavior)

Thanks,
Andrea

@@ -45,6 +45,8 @@
 #define MADV_MERGEABLE   12		/* KSM may merge identical pages */
 #define MADV_UNMERGEABLE 13		/* KSM may not merge identical pages */
 
+#define MADV_HUGEPAGE	14		/* Worth backing with hugepages */
+
 /* compatibility flags */
 #define MAP_FILE	0
 

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NACK, usespace ABIs only go in when features are added. There's a more than large enough chance that it will change for one reason or another.

And while transparent hugepages are a good feature to add, it's still in a far to early stage to go on with it.

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