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Shutemov" , "Dr . David Alan Gilbert" Subject: Re: [PATCH v4 05/10] mm: Return faster for non-fatal signals in user mode faults Message-ID: <20190925034627.GJ28074@xz-x1> References: <20190923042523.10027-1-peterx@redhat.com> <20190923042523.10027-6-peterx@redhat.com> <20190924024721.GD28074@xz-x1> <20190924025447.GE1855@bombadil.infradead.org> <20190924031908.GF28074@xz-x1> <20190924154518.GG1855@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20190924154518.GG1855@bombadil.infradead.org> User-Agent: Mutt/1.11.4 (2019-03-13) 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: On Tue, Sep 24, 2019 at 08:45:18AM -0700, Matthew Wilcox wrote: > On Tue, Sep 24, 2019 at 11:19:08AM +0800, Peter Xu wrote: > > On Mon, Sep 23, 2019 at 07:54:47PM -0700, Matthew Wilcox wrote: > > > On Tue, Sep 24, 2019 at 10:47:21AM +0800, Peter Xu wrote: > > > > On Mon, Sep 23, 2019 at 11:03:49AM -0700, Linus Torvalds wrote: > > > > > On Sun, Sep 22, 2019 at 9:26 PM Peter Xu wrote: > > > > > > > > > > > > This patch is a preparation of removing that special path by allowing > > > > > > the page fault to return even faster if we were interrupted by a > > > > > > non-fatal signal during a user-mode page fault handling routine. > > > > > > > > > > So I really wish saome other vm person would also review these things, > > > > > but looking over this series once more, this is the patch I probably > > > > > like the least. > > > > > > > > > > And the reason I like it the least is that I have a hard time > > > > > explaining to myself what the code does and why, and why it's so full > > > > > of this pattern: > > > > > > > > > > > - if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) > > > > > > + if ((fault & VM_FAULT_RETRY) && > > > > > > + fault_should_check_signal(user_mode(regs))) > > > > > > return; > > > > > > > > > > which isn't all that pretty. > > > > > > > > > > Why isn't this just > > > > > > > > > > static bool fault_signal_pending(unsigned int fault_flags, struct > > > > > pt_regs *regs) > > > > > { > > > > > return (fault_flags & VM_FAULT_RETRY) && > > > > > (fatal_signal_pending(current) || > > > > > (user_mode(regs) && signal_pending(current))); > > > > > } > > > > > > > > > > and then most of the users would be something like > > > > > > > > > > if (fault_signal_pending(fault, regs)) > > > > > return; > > > > > > > > > > and the exceptions could do their own thing. > > > > > > > > > > Now the code is prettier and more understandable, I feel. > > > > > > > > > > And if something doesn't follow this pattern, maybe it either _should_ > > > > > follow that pattern or it should just not use the helper but explain > > > > > why it has an unusual pattern. > > > > > > > +++ b/arch/alpha/mm/fault.c > > > > @@ -150,7 +150,7 @@ do_page_fault(unsigned long address, unsigned long mmcsr, > > > > the fault. */ > > > > fault = handle_mm_fault(vma, address, flags); > > > > > > > > - if ((fault & VM_FAULT_RETRY) && fatal_signal_pending(current)) > > > > + if (fault_signal_pending(fault, regs)) > > > > return; > > > > > > > > if (unlikely(fault & VM_FAULT_ERROR)) { > > > > > > > +++ b/arch/arm/mm/fault.c > > > > @@ -301,6 +301,11 @@ do_page_fault(unsigned long addr, unsigned int fsr, struct pt_regs *regs) > > > > return 0; > > > > } > > > > > > > > + /* Fast path to handle user mode signals */ > > > > + if ((fault & VM_FAULT_RETRY) && user_mode(regs) && > > > > + signal_pending(current)) > > > > + return 0; > > > > > > But _why_ are they different? This is a good opportunity to make more > > > code the same between architectures. > > > > (Thanks for joining the discussion) > > > > I'd like to do these - my only worry is that I can't really test them > > well simply because I don't have all the hardwares. For now the > > changes are mostly straightforward so I'm relatively confident (not to > > mention the code needs proper reviews too, and of course I would > > appreciate much if anyone wants to smoke test it). If I change it in > > a drastic way, I won't be that confident without some tests at least > > on multiple archs (not to mention that even smoke testing across major > > archs will be a huge amount of work...). So IMHO those might be more > > suitable as follow-up for per-arch developers if we can at least reach > > a consensus on the whole idea of this patchset. > > I think the way to do this is to introduce fault_signal_pending(), > converting the architectures to it that match that pattern. Then one > patch per architecture to convert the ones which use a different pattern > to the same pattern. Fair enough. I can start with a fault_signal_pending() only keeps the sigkill handling just like before, then convert all the archs, with the last patch to only touch fault_signal_pending() for non-fatal signals. > > Oh, and while you're looking at the callers of handle_mm_fault(), a > lot of them don't check conditions in the right order. x86, at least, > handles FAULT_RETRY before handling FAULT_ERROR, which is clearly wrong. > > Kirill and I recently discussed it here: > https://lore.kernel.org/linux-mm/20190911152338.gqqgxrmqycodfocb@box/T/ Hmm sure. These sound very reasonable. I must admit that I am not brave enough to continue grow my patchset on my own. The condition I'm facing right now is that I can't really find enough reviewers for this series (Linus helped me quite a lot, I really, really, appreciated that), while it's still growing. I hope the started discussion means that you'll be at least another potential reviewer (oh, should I count Kirill in as well? :) at least to the coming patches for the things mentioned above. Thanks, -- Peter Xu