(2014/11/27 19:52), Petr Mladek wrote: > On Thu 2014-11-27 19:06:37, Masami Hiramatsu wrote: >> (2014/11/27 0:27), Josh Poimboeuf wrote: >>> On Wed, Nov 26, 2014 at 10:18:24AM +0100, Jiri Kosina wrote: >>>> On Wed, 26 Nov 2014, Masami Hiramatsu wrote: >>>> >>>>>> Note to Steve: >>>>>> Masami's IPMODIFY patch is heading for -next via your tree. Once it arrives, >>>>>> I'll rebase and make the change to set IPMODIFY. Do not pull this for -next >>>>>> yet. This version (v4) is for review and gathering acks. >>>>> >>>>> BTW, as we discussed IPMODIFY is an exclusive flag. So if we allocate >>>>> ftrace_ops for each function in each patch, it could be conflict each >>>>> other. >>>> >>>> Yup, this corresponds to what Petr brought up yesterday. There are cases >>>> where all solutions (kpatch, kgraft, klp) would allocate multiple >>>> ftrace_ops for a single function entry (think of patching one function >>>> multiple times in a row). >>>> >>>> So it's not as easy as just setting the flag. >>>> >>>>> Maybe we need to have another ops hashtable to find such conflict and >>>>> new handler to handle it. >>>> >>>> If I understand your proposal correctly, that would sound like a hackish >>>> workaround, trying to basically trick the IPMODIFY flag semantics you just >>>> implemented :) >>> >>> I think Masami may be proposing something similar to what we do in >>> kpatch today. We have a single ftrace_ops and handler which is used for >>> all functions. The handler accesses a global hash of kpatch_func >>> structs which is indexed by the original function's IP address. >> >> Hmm, I think both is OK. kpatch method is less memory consuming and >> will have a bigger overhead. However, as Steven talked at Plumbers Conf., >> he will introduce a direct code modifying interface for ftrace. After >> that is introduced, we don't need to care about performance degradation >> by patching :) > > Yup, I would prefer to have ftrace_ops per (original) function entry. I mean > that new patches will reuse the existing ftrace_ops for already > patched functions. They will just create new ftrace_ops from the > not-yet-patched symbols. However, too many ftrace_ops will get bigger overhead if conflicts happened on any entry, since on such entry ftrace walks through all registered ftrace_ops and checks its filter. It's a downside. Perhaps, ftrace needs to have 2 different ftrace_ops lists, one is for managing, and one is for walk through. And the latter list drops the ftrace_ops which has trampoline and whose all filtered ip is exclusively used (iow, such ftrace_ops never be hit in the walk through). > Using a single ftrace_ops everywhere would kill the win from > Steven's direct ftrace optimization. It depends on the implementation and interface. E.g. an explicit path-change optimization interface for each address with ftrace_ops, like as ftrace_change_path_ip(ftrace_ops *ops, unsigned long old_addr, unsigned long new_addr); This checks ftrace_ops is already registered and has given old_addr in filter, ensures no other ftrace_ops are registered on given address, and then optimizes the fentry call to jump to the new_addr. Anyway, at this point it is not a major discussion point, it's a kind of minor implementation issue for performance and memory consuming. We can switch it without changing API. Thank you, >>> It actually works out pretty well because it nicely encapsulates the >>> knowledge about which functions are patched in a single place. And it >>> makes it easy to track function versions (for incremental patching and >>> rollback). >>> >>>> What I'd propose instead is to make sure that we always have >>>> just a ftrace_ops per function entry, and only update the pointers there >>>> as necessary. Fortunately we can do the switch atomically, by making use >>>> of ->private. >>> >>> But how would you update multiple functions atomically, to enforce >>> per-thread consistency? >> >> At this point, both can do it atomically. We just need an atomic flag >> for applying patches. > > By other words, we would need something like the "kgr_immutable" flag from > kGraft. It will make sure that everybody stays with the current code until > all function entries are updated. > > Best Regards, > Petr -- Masami HIRAMATSU Software Platform Research Dept. Linux Technology Research Center Hitachi, Ltd., Yokohama Research Laboratory E-mail: masami.hiramatsu.pt@xxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
