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Date: Tue, 20 Dec 2016 18:50:05 +0100 From: Oleg Nesterov <oleg@...hat.com> To: Marcin Nowakowski <marcin.nowakowski@...tec.com> Cc: tip-bot for Marcin Nowakowski <tipbot@...or.com>, linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org, peterz@...radead.org, alexander.shishkin@...ux.intel.com, acme@...nel.org, acme@...hat.com, mingo@...nel.org, victor.kamensky@...aro.org, jolsa@...hat.com, hpa@...or.com, torvalds@...ux-foundation.org, tglx@...utronix.de Subject: Re: [tip:perf/urgent] uprobes: Fix uprobes on MIPS, allow for a cache flush after ixol breakpoint creation On 12/20, Marcin Nowakowski wrote: > > Hi Oleg, > > On 20.12.2016 14:08, Oleg Nesterov wrote: >> On 12/19, tip-bot for Marcin Nowakowski wrote: >>> >>> uprobes: Fix uprobes on MIPS, allow for a cache flush after ixol breakpoint creation >>> >>> Commit: >>> >>> 72e6ae285a1d ('ARM: 8043/1: uprobes need icache flush after xol write' >>> >>> ... has introduced an arch-specific method to ensure all caches are >>> flushed appropriately after an instruction is written to an XOL page. >> >> when this page is already mmaped, >> >>> However, when the XOL area is created and the out-of-line breakpoint >>> instruction is copied, caches are not flushed at all and stale data may >>> be found in icache. >> >> but in this case the page is not mmaped yet, the probed application will >> take a page fault if it tries to execute this insn, > > In case of MIPS (and AFAICT ARM as well, and these are the only > architectures that implement arch_uprobe_copy_ixol), the cache flushing > is done through the kernel addresses of that page, so the fact that it > is not mapped yet is not an issue. OK, thanks, > Do I understand correctly that your statement implies that after the > page fault and mmapping the xol page, the page is guaranteed to be > updated in the cache? As definitely that is not something that is > happening at the moment. Well, I do not know. Let me repeat I don't understand this flush_.*cache magic. But. do_read_fault() does __do_fault(..., &fault_page, ...); alloc_set_pte(fault_page); and alloc_set_pte() does flush_icache_page(vma, page)... Hmm, which is nop on MIPS. >> OK, I know nothing about MIPS, but could you help me understand this change? >> >> See above. If we really need flush_icache_range() here then perhaps we should >> modify install_special_mapping() and/or __do_fault/special_mapping_fault paths >> instead? > > Are you suggesting that those should be updated to force a cache update? Again, I do not know. But perhaps it makes more sense to actually implement flush_icache_page() ? Otherwise another user of install_special_mapping() can hit the same problem? Oleg.
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