The runtime constant feature removes all the users of these variables, allowing the compiler to optimize them away. It's quite difficult to extract their values from the kernel text, and the memory saved by removing them is tiny, and it was never the point of this optimization. Since the dentry_hashtable is a core data structure, it's valuable for debugging tools to be able to read it easily. For instance, scripts built on drgn, like the dentrycache script[1], rely on it to be able to perform diagnostics on the contents of the dcache. Annotate it as used, so the compiler doesn't discard it. [1]: https://github.com/oracle-samples/drgn-tools/blob/3afc56146f54d09dfd1f6d3c1b7436eda7e638be/drgn_tools/dentry.py#L325-L355 Fixes: e3c92e81711d ("runtime constants: add x86 architecture support") Signed-off-by: Stephen Brennan <stephen.s.brennan@xxxxxxxxxx> --- Hi Christian, Al, Linus, I know this "fix" may seem a bit silly, but reading out the dentry hashtable contents from a core dump or live system has been a real life saver when debugging dentry cache bloat issues. Could we do something like this (even making it opt-in would be great) for v6.11? Thanks, Stephen fs/dcache.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-) diff --git a/fs/dcache.c b/fs/dcache.c index 3d8daaecb6d1a..69404b4d87546 100644 --- a/fs/dcache.c +++ b/fs/dcache.c @@ -96,11 +96,16 @@ EXPORT_SYMBOL(dotdot_name); * * This hash-function tries to avoid losing too many bits of hash * information, yet avoid using a prime hash-size or similar. + * + * Marking the variables "used" ensures that the compiler doesn't + * optimize them away completely on architectures with runtime + * constant infrastructure, this allows debuggers to see their + * values. But updating these values has no on those arches. */ -static unsigned int d_hash_shift __ro_after_init; +static unsigned int d_hash_shift __ro_after_init __used; -static struct hlist_bl_head *dentry_hashtable __ro_after_init; +static struct hlist_bl_head *dentry_hashtable __ro_after_init __used; static inline struct hlist_bl_head *d_hash(unsigned long hashlen) { -- 2.43.5
