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Peter Anvin" , Andrea Arcangeli , openrisc@lists.librecores.org, linux-s390@vger.kernel.org, Ingo Molnar , linux-m68k@lists.linux-m68k.org, Palmer Dabbelt , Chris Zankel , Peter Zijlstra , Alistair Popple , linux-csky@vger.kernel.org, linux-hexagon@vger.kernel.org, Vlastimil Babka , Thomas Gleixner , sparclinux@vger.kernel.org, Christian Borntraeger , Stafford Horne , Michael Ellerman , x86@kernel.org, Thomas Bogendoerfer , Paul Mackerras , linux-arm-kernel@lists.infradead.org, Sven Schnelle , Benjamin Herrenschmidt , linux-xtensa@linux-xtensa.org, Nicholas Piggin , linux-sh@vger.kernel.org, Vasily Gorbik , Borislav Petkov , linux-mips@vger.kernel.org, Max Filippov , Helge Deller , Vineet Gupta , Al Viro , Paul Walmsley , Johannes Weiner , Anton Ivanov , Catalin Marinas , linux-um@lists.infradead.org, linux-alpha@vger.kernel.org, Johannes Berg , linux-ia64@vger.kernel.org, Geert Uytterhoeven , Dinh Nguyen , Guo Ren , linux-snps-arc@lists.infradead.org, Hugh Dickins , Rich Felker , Andy Lutomirski , Richard Weinberger , linuxppc-dev@lists.ozlabs.org, Brian Cain , Yoshinori Sato , Andrew Morton , Stefan Kristiansson , linux-parisc@vger.kernel.org, "David S . Miller" , Janosch Frank Subject: Re: [PATCH v3] mm: Avoid unnecessary page fault retires on shared memory types Message-ID: References: <20220524234531.1949-1-peterx@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20220524234531.1949-1-peterx@redhat.com> X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: B856HEmp0-oPmnQmmWvfQf-2fQL_698C X-Proofpoint-GUID: YnY8uuN2fOATVDxq6iTcIl1znLlGk8-q X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.874,Hydra:6.0.486,FMLib:17.11.64.514 definitions=2022-05-27_03,2022-05-27_01,2022-02-23_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 suspectscore=0 bulkscore=0 phishscore=0 priorityscore=1501 malwarescore=0 lowpriorityscore=0 mlxscore=0 clxscore=1011 mlxlogscore=999 spamscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2204290000 definitions=main-2205270057 X-Rspamd-Queue-Id: BB8FF80018 X-Rspam-User: Authentication-Results: imf30.hostedemail.com; dkim=pass header.d=ibm.com header.s=pp1 header.b=CdGIqDdz; spf=pass (imf30.hostedemail.com: domain of hca@linux.ibm.com designates 148.163.156.1 as permitted sender) smtp.mailfrom=hca@linux.ibm.com; dmarc=pass (policy=none) header.from=ibm.com X-Stat-Signature: dmo1swy833zh44wo5mw7s9mxrx3f7tn8 X-Rspamd-Server: rspam05 X-HE-Tag: 1653654210-112491 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, May 24, 2022 at 07:45:31PM -0400, Peter Xu wrote: > I observed that for each of the shared file-backed page faults, we're very > likely to retry one more time for the 1st write fault upon no page. It's > because we'll need to release the mmap lock for dirty rate limit purpose > with balance_dirty_pages_ratelimited() (in fault_dirty_shared_page()). > > Then after that throttling we return VM_FAULT_RETRY. > > We did that probably because VM_FAULT_RETRY is the only way we can return > to the fault handler at that time telling it we've released the mmap lock. > > However that's not ideal because it's very likely the fault does not need > to be retried at all since the pgtable was well installed before the > throttling, so the next continuous fault (including taking mmap read lock, > walk the pgtable, etc.) could be in most cases unnecessary. > > It's not only slowing down page faults for shared file-backed, but also add > more mmap lock contention which is in most cases not needed at all. > > To observe this, one could try to write to some shmem page and look at > "pgfault" value in /proc/vmstat, then we should expect 2 counts for each > shmem write simply because we retried, and vm event "pgfault" will capture > that. > > To make it more efficient, add a new VM_FAULT_COMPLETED return code just to > show that we've completed the whole fault and released the lock. It's also > a hint that we should very possibly not need another fault immediately on > this page because we've just completed it. > > This patch provides a ~12% perf boost on my aarch64 test VM with a simple > program sequentially dirtying 400MB shmem file being mmap()ed and these are > the time it needs: > > Before: 650.980 ms (+-1.94%) > After: 569.396 ms (+-1.38%) > > I believe it could help more than that. > > We need some special care on GUP and the s390 pgfault handler (for gmap > code before returning from pgfault), the rest changes in the page fault > handlers should be relatively straightforward. > > Another thing to mention is that mm_account_fault() does take this new > fault as a generic fault to be accounted, unlike VM_FAULT_RETRY. > > I explicitly didn't touch hmm_vma_fault() and break_ksm() because they do > not handle VM_FAULT_RETRY even with existing code, so I'm literally keeping > them as-is. > > Signed-off-by: Peter Xu ... > diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c > index e173b6187ad5..9503a7cfaf03 100644 > --- a/arch/s390/mm/fault.c > +++ b/arch/s390/mm/fault.c > @@ -339,6 +339,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > unsigned long address; > unsigned int flags; > vm_fault_t fault; > + bool need_unlock = true; > bool is_write; > > tsk = current; > @@ -433,6 +434,13 @@ 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) { > + need_unlock = false; > + goto out_gmap; > + } > + > if (unlikely(fault & VM_FAULT_ERROR)) > goto out_up; > > @@ -452,6 +460,7 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > mmap_read_lock(mm); > goto retry; > } > +out_gmap: > if (IS_ENABLED(CONFIG_PGSTE) && gmap) { > address = __gmap_link(gmap, current->thread.gmap_addr, > address); > @@ -466,7 +475,8 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access) > } > fault = 0; > out_up: > - mmap_read_unlock(mm); > + if (need_unlock) > + mmap_read_unlock(mm); > out: This seems to be incorrect. __gmap_link() requires the mmap_lock to be held. Christian, Janosch, or David, could you please check?