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Tue, 31 Aug 2021 10:20:19 +0000 (UTC) Received: from dovecot-director2.suse.de ([192.168.254.65]) by imap1.suse-dmz.suse.de with ESMTPSA id E/+2D2MCLmE8egAAGKfGzw (envelope-from ); Tue, 31 Aug 2021 10:20:19 +0000 Message-ID: <17a36dfc-cbd1-56c5-46a9-9f4043bb56e6@suse.cz> Date: Tue, 31 Aug 2021 12:20:18 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.0.3 Subject: Re: [PATCH v1 2/2] mm: remove redundant smp_wmb() Content-Language: en-US To: Qi Zheng , akpm@linux-foundation.org, tglx@linutronix.de, hannes@cmpxchg.org, mhocko@kernel.org, vdavydov.dev@gmail.com, kirill.shutemov@linux.intel.com, mika.penttila@nextfour.com, david@redhat.com Cc: linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, songmuchun@bytedance.com References: <20210828042306.42886-1-zhengqi.arch@bytedance.com> <20210828042306.42886-3-zhengqi.arch@bytedance.com> From: Vlastimil Babka In-Reply-To: <20210828042306.42886-3-zhengqi.arch@bytedance.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Authentication-Results: imf11.hostedemail.com; dkim=pass header.d=suse.cz header.s=susede2_rsa header.b=bKJb7owM; dkim=pass header.d=suse.cz header.s=susede2_ed25519 header.b=AzUwivJ+; spf=pass (imf11.hostedemail.com: domain of vbabka@suse.cz designates 195.135.220.28 as permitted sender) smtp.mailfrom=vbabka@suse.cz; dmarc=none X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: A68D5F0000BA X-Stat-Signature: ectr39sntxja5xsa4y79gmfeb4zhfnq7 X-HE-Tag: 1630405220-261852 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 8/28/21 06:23, Qi Zheng wrote: > The smp_wmb() which is in the __pte_alloc() is used to > ensure all ptes setup is visible before the pte is made > visible to other CPUs by being put into page tables. We > only need this when the pte is actually populated, so > move it to pte_install(). __pte_alloc_kernel(), It's named pmd_install()? > __p4d_alloc(), __pud_alloc() and __pmd_alloc() are similar > to this case. > > We can also defer smp_wmb() to the place where the pmd entry > is really populated by preallocated pte. There are two kinds > of user of preallocated pte, one is filemap & finish_fault(), > another is THP. The former does not need another smp_wmb() > because the smp_wmb() has been done by pte_install(). Same here. > Fortunately, the latter also does not need another smp_wmb() > because there is already a smp_wmb() before populating the > new pte when the THP uses a preallocated pte to split a huge > pmd. > > Signed-off-by: Qi Zheng > Reviewed-by: Muchun Song > --- > mm/memory.c | 47 ++++++++++++++++++++--------------------------- > mm/sparse-vmemmap.c | 2 +- > 2 files changed, 21 insertions(+), 28 deletions(-) > > diff --git a/mm/memory.c b/mm/memory.c > index ef7b1762e996..9c7534187454 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -439,6 +439,20 @@ void pmd_install(struct mm_struct *mm, pmd_t *pmd, pgtable_t *pte) > > if (likely(pmd_none(*pmd))) { /* Has another populated it ? */ > mm_inc_nr_ptes(mm); > + /* > + * Ensure all pte setup (eg. pte page lock and page clearing) are > + * visible before the pte is made visible to other CPUs by being > + * put into page tables. > + * > + * The other side of the story is the pointer chasing in the page > + * table walking code (when walking the page table without locking; > + * ie. most of the time). Fortunately, these data accesses consist > + * of a chain of data-dependent loads, meaning most CPUs (alpha > + * being the notable exception) will already guarantee loads are > + * seen in-order. See the alpha page table accessors for the > + * smp_rmb() barriers in page table walking code. > + */ > + smp_wmb(); /* Could be smp_wmb__xxx(before|after)_spin_lock */ So, could it? :)