From mboxrd@z Thu Jan 1 00:00:00 1970 From: Heiko Carstens Date: Sun, 29 May 2022 20:33:23 +0000 Subject: Re: [PATCH v4] mm: Avoid unnecessary page fault retires on shared memory types Message-Id: List-Id: References: <20220527193936.30678-1-peterx@redhat.com> In-Reply-To: <20220527193936.30678-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Peter Xu Cc: 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 , Christian Borntraeger , Johannes Berg , Chris Zankel , Michal Simek , x86@kernel.org, Yoshinori Sato , Dave Hansen , Richard Weinberger , Ingo Molnar On Fri, May 27, 2022 at 03:39:36PM -0400, Peter Xu wrote: > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c > index e173b6187ad5..4608cc962ecf 100644 > --- a/arch/s390/mm/fault.c > +++ b/arch/s390/mm/fault.c > @@ -433,6 +433,17 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > goto out_up; > goto out; > } > + > + /* 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 (unlikely(fault & VM_FAULT_ERROR)) > goto out_up; > 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; } if (unlikely(fault & VM_FAULT_ERROR))