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b=aPYcJuS14avROKNJKvC5vVXubfl0lbQC93Dl8gbsR6nw1YWYh2GRbNGWKNRDA5FYD c2qpMAHwc9MpBCw13voMdBlmJpmhcrCbzC8cSqgsPJWGJhoGQou0RXoQEsRY+D/fIT ujpQjQg2UNq4OTHkNSX30wpJRmzuB4CJB7HxQyAU= Date: Wed, 17 Sep 2025 14:44:18 -0700 From: Andrew Morton To: Ankur Arora Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org, david@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, mingo@redhat.com, mjguzik@gmail.com, luto@kernel.org, peterz@infradead.org, acme@kernel.org, namhyung@kernel.org, tglx@linutronix.de, willy@infradead.org, raghavendra.kt@amd.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com Subject: Re: [PATCH v7 13/16] mm: memory: support clearing page ranges Message-Id: <20250917144418.25cb9117d64b32cb0c7908d9@linux-foundation.org> In-Reply-To: <20250917152418.4077386-14-ankur.a.arora@oracle.com> References: <20250917152418.4077386-1-ankur.a.arora@oracle.com> <20250917152418.4077386-14-ankur.a.arora@oracle.com> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.33; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit X-Stat-Signature: mm5stsqi46ofszqiox8iwzc6kfs6e9b3 X-Rspam-User: X-Rspamd-Queue-Id: D563E1C0009 X-Rspamd-Server: rspam10 X-HE-Tag: 1758145460-257676 X-HE-Meta: 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 H90mhNzM OWl/2YEa0708nXaNcnAtjZ1WHB2I8iFHkwRwBLrMMI9yp4fbM7HycrSUzIcQkTF3A4OGXvpCi/1lZ0fpdk1JSBIV+wr7RAFcFLROLfgZtOW6yvERkg4cRrFuoGzvsLr0yuGQIdU2/9mDC09HvlPMEngxesIjA3IL3lbpcDIR9R7iSRYNl6/ycN/0WW4x2BGVx+faNmn2aMyLw/+JzhvrrV1WGZ7y3CZuzB1Va3mpdlxNbSFA= 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: List-Subscribe: List-Unsubscribe: On Wed, 17 Sep 2025 08:24:15 -0700 Ankur Arora wrote: > Change folio_zero_user() to clear contiguous page ranges instead of > clearing using the current page-at-a-time approach. Exposing the largest > feasible length can be useful in enabling processors to optimize based > on extent. This patch is something which MM developers might care to take a closer look at. > However, clearing in large chunks can have two problems: > > - cache locality when clearing small folios (< MAX_ORDER_NR_PAGES) > (larger folios don't have any expectation of cache locality). > > - preemption latency when clearing large folios. > > Handle the first by splitting the clearing in three parts: the > faulting page and its immediate locality, its left and right > regions; with the local neighbourhood cleared last. Has this optimization been shown to be beneficial? If so, are you able to share some measurements? If not, maybe it should be removed? > ... > > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -7021,40 +7021,80 @@ static inline int process_huge_page( > return 0; > } > > -static void clear_gigantic_page(struct folio *folio, unsigned long addr_hint, > - unsigned int nr_pages) > +/* > + * Clear contiguous pages chunking them up when running under > + * non-preemptible models. > + */ > +static void clear_contig_highpages(struct page *page, unsigned long addr, > + unsigned int npages) Called "_highpages" because it wraps clear_user_highpages(). It really should be called clear_contig_user_highpages() ;) (Not serious) > { > - unsigned long addr = ALIGN_DOWN(addr_hint, folio_size(folio)); > - int i; > + unsigned int i, count, unit; > > - might_sleep(); > - for (i = 0; i < nr_pages; i++) { > + unit = preempt_model_preemptible() ? npages : PAGE_CONTIG_NR; Almost nothing uses preempt_model_preemptible() and I'm not usefully familiar with it. Will this check avoid all softlockup/rcu/etc detections in all situations (ie, configs)? > + for (i = 0; i < npages; ) { > + count = min(unit, npages - i); > + clear_user_highpages(nth_page(page, i), > + addr + i * PAGE_SIZE, count); > + i += count; > cond_resched(); > - clear_user_highpage(folio_page(folio, i), addr + i * PAGE_SIZE); > } > }