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[173.79.60.52]) by smtp.gmail.com with ESMTPSA id af79cd13be357-93982db6a95sm207374585a.9.2026.09.04.11.20.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 04 Sep 2026 11:20:19 -0700 (PDT) From: Gregory Price To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, kprateek.nayak@amd.com, ziy@nvidia.com, matthew.brost@intel.com, joshua.hahnjy@gmail.com, rakie.kim@sk.com, byungchul@sk.com, gourry@gourry.net, ying.huang@linux.alibaba.com, apopple@nvidia.com, raghavendra.kt@amd.com, shy828301@gmail.com, baolin.wang@linux.alibaba.com, osalvador@suse.de Subject: [PATCH 2/2] sched/numa: scan read-only file mappings in tiering mode Date: Fri, 4 Sep 2026 14:20:06 -0400 Message-ID: <20260904182006.1562449-3-gourry@gourry.net> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260904182006.1562449-1-gourry@gourry.net> References: <20260904182006.1562449-1-gourry@gourry.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit task_numa_work() refuses to scan any read-only file-backed mapping: if (vma->vm_file && (vma->vm_flags & (VM_READ|VM_WRITE)) == (VM_READ)) { trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_SHARED_RO); continue; } commit 4591ce4f2d22 ("sched/numa: Do not trap hinting faults for shared libraries") added this because such pages are expected to be cache replicated and bounce between accessor nodes, so: If we are never going to migrate the page, it is overhead for no gain Memory tiering broke that premise. A slow tier folio is promoted on hint fault based on hotness, so these can migrate now - they just never get the chance, because the VMA is never scanned. Observed on a 768G DRAM + 256G CXL host running a ~430G database service: 91% of its main binary (169M of 185M) accumulated on CXL. After this patch the binary's tier residency tracked runtime load. Scan these VMAs with the slow_only restriction. Only non-toptier folios are made hint-faultable, so the scan never marks a toptier folio in a read-only file mapping in any mode. A page already on the fast tier costs nothing, and a page on the slow tier costs one hint fault, which is what buys the promotion. That fault is not free of placement effect: a successful promotion reports its toptier destination node to task_numa_fault(), so it lands in numa_faults[] like any other promotion. Only the set of VMAs that can produce one changes. Fixes: c574bbe91703 ("NUMA balancing: optimize page placement for memory tiering system") Signed-off-by: Gregory Price (Meta) Assisted-by: Claude:claude-opus-5 --- kernel/sched/fair.c | 18 ++++++++++++++++-- 1 file changed, 16 insertions(+), 2 deletions(-) diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 990af0e45d69..5c5c80b17649 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4075,6 +4075,11 @@ static void reset_ptenuma_scan(struct task_struct *p) p->mm->numa_scan_offset = 0; } +static bool vma_is_ro_file(struct vm_area_struct *vma) +{ + return vma->vm_file && (vma->vm_flags & (VM_READ | VM_WRITE)) == VM_READ; +} + static bool vma_is_accessed(struct mm_struct *mm, struct vm_area_struct *vma) { unsigned long pids; @@ -4211,6 +4216,8 @@ static void task_numa_work(struct callback_head *work) } for (; vma; vma = vma_next(&vmi)) { + bool ro_file; + if (!vma_migratable(vma) || !vma_policy_mof(vma) || is_vm_hugetlb_page(vma) || (vma->vm_flags & VM_MIXEDMAP)) { trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_UNSUITABLE); @@ -4222,9 +4229,16 @@ static void task_numa_work(struct callback_head *work) * migrated as it is expected they are cache replicated. Avoid * hinting faults in read-only file-backed mappings or the vDSO * as migrating the pages will be of marginal benefit. + * + * Tiering still wants them. A read-only mapping strands just + * as much on the slow tier as any other, so scan it for slow + * tier folios alone. Nothing on the top tier is marked here, + * in any mode. */ + ro_file = vma_is_ro_file(vma); if (!vma->vm_mm || - (vma->vm_file && (vma->vm_flags & (VM_READ|VM_WRITE)) == (VM_READ))) { + (ro_file && !(sysctl_numa_balancing_mode & + NUMA_BALANCING_MEMORY_TIERING))) { trace_sched_skip_vma_numa(mm, vma, NUMAB_SKIP_SHARED_RO); continue; } @@ -4302,7 +4316,7 @@ static void task_numa_work(struct callback_head *work) * VMA can cause others to become "permanently unaccessed" if a scan * cycle is consumed entirely by the large VMA. */ - vma->numab_state->slow_only = false; + vma->numab_state->slow_only = ro_file; if (!vma_pids_forced && !vma_is_accessed(mm, vma)) { if (!(sysctl_numa_balancing_mode & NUMA_BALANCING_MEMORY_TIERING)) { -- 2.55.0