Re: [RFC perf/core 05/11] uprobes: Add mapping for optimized uprobe trampolines
From: Andrii Nakryiko <hidden>
Date: 2024-11-21 00:10:32
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On Tue, Nov 19, 2024 at 7:14 AM Jiri Olsa [off-list ref] wrote:
On Mon, Nov 18, 2024 at 10:05:41PM -0800, Andrii Nakryiko wrote:quoted
On Sat, Nov 16, 2024 at 1:44 PM Jiri Olsa [off-list ref] wrote:quoted
On Thu, Nov 14, 2024 at 03:44:14PM -0800, Andrii Nakryiko wrote:quoted
On Tue, Nov 5, 2024 at 5:35 AM Jiri Olsa [off-list ref] wrote:quoted
Adding interface to add special mapping for user space page that will be used as place holder for uprobe trampoline in following changes. The get_tramp_area(vaddr) function either finds 'callable' page or create new one. The 'callable' means it's reachable by call instruction (from vaddr argument) and is decided by each arch via new arch_uprobe_is_callable function. The put_tramp_area function either drops refcount or destroys the special mapping and all the maps are clean up when the process goes down. Signed-off-by: Jiri Olsa <jolsa@kernel.org> --- include/linux/uprobes.h | 12 ++++ kernel/events/uprobes.c | 141 ++++++++++++++++++++++++++++++++++++++++ kernel/fork.c | 2 + 3 files changed, 155 insertions(+)diff --git a/include/linux/uprobes.h b/include/linux/uprobes.h index be306028ed59..222d8e82cee2 100644 --- a/include/linux/uprobes.h +++ b/include/linux/uprobes.h@@ -172,6 +172,15 @@ struct xol_area; struct uprobes_state { struct xol_area *xol_area; + struct hlist_head tramp_head; + struct mutex tramp_mutex; +}; + +struct tramp_area { + unsigned long vaddr; + struct page *page; + struct hlist_node node; + refcount_t ref;nit: any reason we are unnecessarily trying to save 4 bytes on refcount (and we don't actually, due to padding)hum, I'm not sure what you mean.. what's the alternative?atomic64_t ?hum, just because we have extra 4 bytes padding? we use refcount_t on other places so seems like better fit to me
My (minor) concern was that tramp_area is a very long-living object that can be shared across huge amounts of uprobes, and 2 billion uprobes is a lot, but still a number that one can, practically speaking, reach (with enough effort). And so skimping on refcount to save 4 bytes for something that we have one or two per *some* processes seemed (and still seems) like wrong savings to pursue.
jirka