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Fri, 29 May 2026 10:01:21 +0000 Date: Fri, 29 May 2026 11:01:19 +0100 From: Pedro Falcato To: Usama Arif Cc: Andrew Morton , david@kernel.org, willy@infradead.org, ryan.roberts@arm.com, linux-mm@kvack.org, r@hev.cc, jack@suse.cz, Andrew Donnellan , apopple@nvidia.com, baohua@kernel.org, baolin.wang@linux.alibaba.com, brauner@kernel.org, catalin.marinas@arm.com, dev.jain@arm.com, kees@kernel.org, kevin.brodsky@arm.com, lance.yang@linux.dev, "Liam R. Howlett" , linux-arm-kernel@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, ljs@kernel.org, mhocko@suse.com, npache@redhat.com, pasha.tatashin@soleen.com, rmclure@linux.ibm.com, rppt@kernel.org, surenb@google.com, vbabka@kernel.org, Al Viro , wilts.infradead.org@pedro-suse.lan, ziy@nvidia.com, hannes@cmpxchg.org, kas@kernel.org, shakeel.butt@linux.dev, kernel-team@meta.com Subject: Re: [PATCH v6 2/2] mm: use mapping_max_folio_order() for force_thp_readahead order Message-ID: References: <20260528165635.2068012-1-usama.arif@linux.dev> <20260528165635.2068012-3-usama.arif@linux.dev> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260528165635.2068012-3-usama.arif@linux.dev> X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Queue-Id: DDF066AADA X-Rspamd-Action: no action X-Spamd-Result: default: False [-3.01 / 50.00]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; FUZZY_RATELIMITED(0.00)[rspamd.com]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; RCPT_COUNT_TWELVE(0.00)[37]; TAGGED_RCPT(0.00)[kernel]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_EQ_ENVFROM(0.00)[]; FROM_HAS_DN(0.00)[]; R_RATELIMIT(0.00)[to_ip_from(RL764437jfm1qe6abtk9nwyx8m)]; SPAMHAUS_XBL(0.00)[2a07:de40:b281:104:10:150:64:97:from]; TO_MATCH_ENVRCPT_ALL(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[linux.dev:email,pedro-suse.lan:mid,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo,suse.de:dkim]; RCVD_COUNT_TWO(0.00)[2]; DKIM_TRACE(0.00)[suse.de:+] X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260529_030130_093479_9C641011 X-CRM114-Status: GOOD ( 31.21 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, May 28, 2026 at 09:55:20AM -0700, Usama Arif wrote: > The force_thp_readahead path in do_sync_mmap_readahead() is gated on > HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER and always requests > HPAGE_PMD_ORDER / HPAGE_PMD_NR. On configurations where HPAGE_PMD_ORDER > exceeds MAX_PAGECACHE_ORDER, notably arm64 with a 64K base page size, > VM_HUGEPAGE mappings cannot use this path and fall back to the non-forced > mmap readahead path even when the mapping supports useful large folios. > > Keep the existing PMD-sized behavior when HPAGE_PMD_ORDER fits in the > page cache. When it does not, enable forced readahead for mappings that > support large folios and request an order capped by both > mapping_max_folio_order(mapping) and 2MB. > > 2MB is chosen as the cap because it matches the PMD size on x86_64 > and on arm64 with 4K or 16K base pages, so the size/memory-pressure 16K base page size's PMDs should be 32M, no? Am I misunderstanding what you mean here? > tradeoff for folios of that size is already well understood. On arm64 > with a 64K base page size, 2MB is also the contiguous-PTE (contpte) > block size, so the resulting folios coalesce into a single TLB entry > and reduce TLB pressure on the readahead path. > > The final allocation order may still be clamped by page_cache_ra_order() > to the mapping and request geometry, but this gives VM_HUGEPAGE mappings > on such configurations a large-folio readahead request instead of > dropping back to base-page readahead. > > Signed-off-by: Usama Arif > --- > mm/filemap.c | 27 +++++++++++++++++++-------- > 1 file changed, 19 insertions(+), 8 deletions(-) > > diff --git a/mm/filemap.c b/mm/filemap.c > index a16b33e0fc71..bfb891d9da1f 100644 > --- a/mm/filemap.c > +++ b/mm/filemap.c > @@ -3312,14 +3312,23 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) > struct file *fpin = NULL; > vm_flags_t vm_flags = vmf->vma->vm_flags; > bool force_thp_readahead = false; > + unsigned int thp_order = 0; > unsigned short mmap_miss; > > ractl._max_index = vmf->vma->vm_pgoff + vma_pages(vmf->vma) - 1; > > /* Use the readahead code, even if readahead is disabled */ > - if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && > - (vm_flags & VM_HUGEPAGE) && HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) > - force_thp_readahead = true; > + if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) && (vm_flags & VM_HUGEPAGE)) { > + if (HPAGE_PMD_ORDER <= MAX_PAGECACHE_ORDER) { > + force_thp_readahead = true; > + thp_order = HPAGE_PMD_ORDER; > + } else if (mapping_large_folio_support(mapping)) { > + force_thp_readahead = true; > + thp_order = min_t(unsigned int, > + mapping_max_folio_order(mapping), > + get_order(SZ_2M)); This looks somewhat arbitrary to me (as does the old logic). It seems more natural (correct?) to do this: if (THP_ENABLED && (vm_flags & VM_HUGEPAGE)) { if (mapping_large_folio_support(mapping)) { force_thp_readahead = true; /* * Cap THP readahead to either the max folio order the * mapping supports, or the max order the page cache * supports (useless to try more), or the hugepage PMD * order (CPU can't benefit from larger). */ thp_order = min3(mapping_max_folio_order(mapping), MAX_PAGECACHE_ORDER, HPAGE_PMD_ORDER); } } > + } > + } > > if (!force_thp_readahead) { > /* > @@ -3354,17 +3363,19 @@ static struct file *do_sync_mmap_readahead(struct vm_fault *vmf) > } > > if (force_thp_readahead) { > + unsigned long folio_nr_pages = 1UL << thp_order; > + > fpin = maybe_unlock_mmap_for_io(vmf, fpin); > - ractl._index &= ~((unsigned long)HPAGE_PMD_NR - 1); > - ra->size = HPAGE_PMD_NR; > + ractl._index &= ~(folio_nr_pages - 1); > + ra->size = folio_nr_pages; > /* > - * Fetch two PMD folios, so we get the chance to actually > + * Fetch two folios so we get the chance to actually ^^ large folios? > * readahead, unless we've been told not to. > */ > if (!(vm_flags & VM_RAND_READ)) > ra->size *= 2; > - ra->async_size = HPAGE_PMD_NR; > - ra->order = HPAGE_PMD_ORDER; > + ra->async_size = folio_nr_pages; > + ra->order = thp_order; This part LGTM. -- Pedro