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Fri, 31 May 2024 09:36:23 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id rHqcDReaWWY2HgAAD6G6ig (envelope-from ); Fri, 31 May 2024 09:36:23 +0000 Message-ID: <12c1dbf0-d458-49a3-b4b3-b0e8a8bd2cd8@suse.cz> Date: Fri, 31 May 2024 11:36:22 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] mm: don't call should_failslab() for !CONFIG_FAILSLAB Content-Language: en-US To: Mateusz Guzik Cc: Jens Axboe , LKML , Linux Memory Management List , Andrew Morton , Alexei Starovoitov , Daniel Borkmann , Andrii Nakryiko , Martin KaFai Lau , Song Liu , Jesper Dangaard Brouer , Christoph Lameter , David Rientjes , Pekka Enberg , Joonsoo Kim , Yonghong Song , John Fastabend , KP Singh , Howard McLauchlan , bpf@vger.kernel.org, torvalds@linux-foundation.org References: <2dfc6273-6cdd-f4f5-bed9-400873ac9152@suse.cz> <3j5d3p22ssv7xoaghzraa7crcfih3h2qqjlhmjppbp6f42pg2t@kg7qoicog5ye> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Action: no action X-Rspamd-Queue-Id: 149E0120018 X-Stat-Signature: t4oo1bmtj1ph1pfhqxf8c8u6qg5u8ewx X-Rspam-User: X-Rspamd-Server: rspam04 X-HE-Tag: 1717148185-803508 X-HE-Meta: U2FsdGVkX1+QfbKTRSU3U2WXft3IxAnS7mngGE8H3iyFazLuvK/Q0zR6PYTzKWUlCuUX+eLrOnf7Zdir52MLigHTMDCiUmaTVG3n3G/qe9+Ast45WHnRX9eQHFBFmZXMKyipGr3++0KCemPGbu8iQX2dhGuSNJViVY0UmFKxNUl0DW1OmxtB6nllJSxabXhJtFMHxxwNPX06fd8swXdQmFAPRu54LmIsSsZFlo1S20EvKN/k/RT05tBQJU8inZOy3ntcFcM+n2bYLbHBjcDJmctL+mFu9YxOgupCNTEVEnjLeZ6V+0EFSnIZlorGBIN0Gfc9RmBPhQ/Jkj7T2Eq6fjUdvVQyXwLSCWdhI5X1X7vmdRujVwiQi9l7FrvXAELChTJXeaiyzWWI+ykFOzVYfLL8A/6lP2mjqPiNV1e26RYymZm1OiCUbjTSEpmhugdoN3FX8PporYdvuQ1kCn+u6718vM3VLr9RCu+BjeiTn75jhhq+EN0OAaNcG1rvmHQJVKisn33WPTJepX68jtbJX+5WiI2KWamgHWycqqIAOmyC+wBDR0J3uMlGVBpwl7QzQQstngsbzc8+8BAJsfYL6UKWD0ips04VDmCAnqihWPImh124Iu7GsvREkYaho9SZqiblKWf2J6ZFAwK5L/j2dNy4Ol3+vS0e75vFbpZuns1+gs1DlCtJQglj5C/AjKlWnqqdQWgWkfNm+av2Lw53uhPmwElGAYSCk3dQb3NCm1+EZSuIqKKIWJWoO5zeVMlAvFJxlH0C91bushc735xDCdogAxIp8OwpqY4DAQZmt/ZfxnI4ZgqUSb8KxYt7E8/RzMgscX0zXepbHsxqoSxwo8/K/hJApQ3Jv1UHdsFSPHGCBGb2JUQxhbh1S6LeASfsz8jJOHmKzTvqUPPCXvIJ6Hv0taePBKMONR6OlXC1XHmPl8qx+nJ2czcG8c4e7apIh/dvxoYNwJwXPf/2+zg dAAUpx8h 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/27/24 11:34 AM, Mateusz Guzik wrote: > +cc Linus > > On Thu, Oct 07, 2021 at 05:32:52PM +0200, Vlastimil Babka wrote: >> On 10/5/21 17:31, Jens Axboe wrote: >> > Allocations can be a very hot path, and this out-of-line function >> > call is noticeable. >> > >> > Signed-off-by: Jens Axboe >> >> It used to be inline b4 (hi, Konstantin!) and then was converted to be like >> this intentionally :/ >> >> See 4f6923fbb352 ("mm: make should_failslab always available for fault >> injection") >> >> And now also kernel/bpf/verifier.c contains: >> BTF_ID(func, should_failslab) >> >> I think either your or Andrew's version will break this BTF_ID thing, at the >> very least. >> >> But I do strongly agree that putting unconditionally a non-inline call into >> slab allocator fastpath sucks. Can we make it so that bpf can only do these >> overrides when CONFIG_FAILSLAB is enabled? >> I don't know, perhaps putting this BTF_ID() in #ifdef as well, or providing >> a dummy that is always available (so that nothing breaks), but doesn't >> actually affect slab_pre_alloc_hook() unless CONFIG_FAILSLAB has been enabled? >> > > I just ran into it while looking at kmalloc + kfree pair. > > A toy test which calls this in a loop like so: > static long noinline custom_bench(void) > { > void *buf; > > while (!signal_pending(current)) { > buf = kmalloc(16, GFP_KERNEL); > kfree(buf); > cond_resched(); > } > > return -EINTR; > } > > ... shows this with perf top: > 57.88% [kernel] [k] kfree > 31.38% [kernel] [k] kmalloc_trace_noprof > 3.20% [kernel] [k] should_failslab.constprop.0 > > A side note is that I verified majority of the time in kfree and > kmalloc_trace_noprof is cmpxchg16b, which is both good and bad news. > > As for should_failslab, it compiles to an empty func on production > kernels and is present even when there are no supported means of > instrumenting it. As in everyone pays for its existence, even if there > is no way to use it. > > Also note there are 3 unrelated mechanisms to alter the return code, > which imo is 2 too many. But more importantly they are not even > coordinated. > > A hard requirement for a long term solution is to not alter the fast > path beyond nops for hot patching. > > So far I think implementing this in a clean manner would require > agreeing on some namespace for bpf ("failprobes"?) and coordinating > hotpatching between different mechanisms. Maybe there is a better, I > don't know. I've attempted something (not complete yet) here: https://lore.kernel.org/all/20240531-fault-injection-statickeys-v1-0-a513fd0a9614@suse.cz/ > Here is the crux of my e-mail though: > 1. turning should_failslab into a mandatory func call is an ok local > hack for the test farm, not a viable approach for production > 2. as such it is up to the original submitter (or whoever else > who wants to pick up the slack) to implement something which > hotpatches the callsite as opposed to inducing a function call for > everyone > > In the meantime the routine should disappear unless explicitly included > in kernel config. The patch submitted here would be one way to do it.