From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Date: Mon, 30 May 2022 18:29:30 +0000 Subject: Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared memory types Message-Id: <58b163b1-22ec-7634-dcd3-5a0a54adec45@linux.ibm.com> List-Id: References: <20220527193936.30678-1-peterx@redhat.com> <33fd4731-9765-d78b-bdc3-f8243c98e81f@linux.ibm.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Xu Cc: Heiko Carstens , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Will Deacon , Matt Turner , linux-s390@vger.kernel.org, Andrew Morton , Brian Cain , Borislav Petkov , linux-alpha@vger.kernel.org, Alistair Popple , Jonas Bonn , linux-arm-kernel@lists.infradead.org, linux-ia64@vger.kernel.org, Michael Ellerman , Stefan Kristiansson , linux-snps-arc@lists.infradead.org, Vineet Gupta , Vasily Gorbik , Vlastimil Babka , Ivan Kokshaysky , Rich Felker , sparclinux@vger.kernel.org, Russell King , David Hildenbrand , Benjamin Herrenschmidt , Nicholas Piggin , "James E . J . Bottomley" , linux-xtensa@linux-xtensa.org, linux-sh@vger.kernel.org, Paul Walmsley , linux-m68k@lists.linux-m68k.org, linuxppc-dev@lists.ozlabs.org, Richard Henderson , Guo Ren , linux-parisc@vger.kernel.org, Andrea Arcangeli , Helge Deller , Al Viro , Albert Ou , linux-um@lists.infradead.org, "H . Peter Anvin" , Janosch Frank , Sven Schnelle , Anton Ivanov , openrisc@lists.librecores.org, Thomas Bogendoerfer , linux-hexagon@vger.kernel.org, Andy Lutomirski , Stafford Horne , linux-csky@vger.kernel.org, Thomas Gleixner , linux-mips@vger.kernel.org, Paul Mackerras , Alexander Gordeev , Dinh Nguyen , Palmer Dabbelt , "David S . Miller" , Johannes Weiner , Hugh Dickins , Ingo Molnar , Peter Zijlstra , linux-riscv@lists.infradead.org, Max Filippov , Catalin Marinas , Geert Uytterhoeven , Johannes Berg , Chris Zankel , Michal Simek , x86@kernel.org, Yoshinori Sato , Dave Hansen , Richard Weinberger , Ingo Molnar Am 30.05.22 um 18:00 schrieb Peter Xu: > On Mon, May 30, 2022 at 11:52:54AM -0400, Peter Xu wrote: >> On Mon, May 30, 2022 at 11:35:10AM +0200, Christian Borntraeger wrote: >>> >>> >>> Am 29.05.22 um 22:33 schrieb Heiko Carstens: >>> [...] >>>> >>>> Guess the patch below on top of your patch is what we want. >>>> Just for clarification: if gmap is not NULL then the process is a kvm >>>> process. So, depending on the workload, this optimization makes sense. >>>> >>>> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c >>>> index 4608cc962ecf..e1d40ca341b7 100644 >>>> --- a/arch/s390/mm/fault.c >>>> +++ b/arch/s390/mm/fault.c >>>> @@ -436,12 +436,11 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) >>>> /* The fault is fully completed (including releasing mmap lock) */ >>>> if (fault & VM_FAULT_COMPLETED) { >>>> - /* >>>> - * Gmap will need the mmap lock again, so retake it. TODO: >>>> - * only conditionally take the lock when CONFIG_PGSTE set. >>>> - */ >>>> - mmap_read_lock(mm); >>>> - goto out_gmap; >>>> + if (gmap) { >>>> + mmap_read_lock(mm); >>>> + goto out_gmap; >>>> + } >>>> + goto out; > > Hmm, right after I replied I found "goto out" could be problematic, since > all s390 callers of do_exception() will assume it an error condition (side > note: "goto out_gmap" contains one step to clear "fault" to 0). I'll > replace this with "return 0" instead if it looks good to both of you. > > I'll wait for a confirmation before reposting. Thanks, Yes, that sounds good and thank you for double checking.