From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 68721369204; Fri, 24 Apr 2026 19:15:21 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777058121; cv=none; b=iSENtm444a7JdmCh0BdtAXSpGNC+4E5ZB3kyxfcYZume7Qdmc6OjDyIeo3yFz3/zu3G/HJUluiK130V/V80EVFGgvjhs+fl11gRQxZeXUDkM2sJOLZVoyU2zBFehCzwJ83+WRfGZPAVnCpf33NxN0TyCJOpmHNY0yTzRVUtUWjQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777058121; c=relaxed/simple; bh=elAiMYYB8thnLDPWGWuzFWDG9U2HmB7k3ICkW3UnRss=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=qArS3lAs0qYv4UcYEFPzmTAW1ZwYuOs5YNAfP/vB8KcWTonP/jVpvTPX9Kqoir6Z4QvO/VJSlhuiV57To1sTB2RpM7td1r4Pa7Bd/ClyzazDCNVDizjcx4ajbTkZWieum8mvAgUO/ytWmrLndN7cU6zmYS+PtqN4wJQ7Abl0fW8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ZqMFc8Um; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ZqMFc8Um" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3CC6DC19425; Fri, 24 Apr 2026 19:15:20 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1777058120; bh=elAiMYYB8thnLDPWGWuzFWDG9U2HmB7k3ICkW3UnRss=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=ZqMFc8Ummu0HNQYO0qnqYhFMafW5AXr1X1GDRJ7ovxKkZyWFWtJKmS2mWc4jHDW1D YLptaVzHms+E8S11zRFdecHEn7H4PsyF+GZxHXAuTuBCxSL8ALxMUlrQUu0QUPLFIJ jF3AQsgzAKV9C6siKoTwRRxC0xQKhVVbkyYvp61FIZ5L2sOLM9vXGxvV8i0M9jLU+H G6T1uQjLRLF8ciQ+VDJgJtttVdDtWSV2OmIo0IAtFVvBu3WxAh5Sc0w60jiqnE8OGb sYgZ1Aca5NVmjdLZGq2HdClqASNSDiJM40TcOLWWW/594ed2rJ54G70Ga59yuNZvlw Pkhr4HzGB/jnA== Date: Fri, 24 Apr 2026 12:15:18 -0700 From: Minchan Kim To: "David Hildenbrand (Arm)" Cc: Michal Hocko , akpm@linux-foundation.org, hca@linux.ibm.com, linux-s390@vger.kernel.org, brauner@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, surenb@google.com, timmurray@google.com Subject: Re: [PATCH v1 2/3] mm: process_mrelease: skip LRU movement for exclusive file folios Message-ID: References: <20260421230239.172582-1-minchan@kernel.org> <20260421230239.172582-3-minchan@kernel.org> <7c7da8ae-cd39-4edf-b94f-c79ab85df456@kernel.org> Precedence: bulk X-Mailing-List: linux-s390@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <7c7da8ae-cd39-4edf-b94f-c79ab85df456@kernel.org> On Fri, Apr 24, 2026 at 09:57:16AM +0200, David Hildenbrand (Arm) wrote: > On 4/24/26 09:51, Michal Hocko wrote: > > On Tue 21-04-26 16:02:38, Minchan Kim wrote: > >> For the process_mrelease reclaim, skip LRU handling for exclusive > >> file-backed folios since they will be freed soon so pointless > >> to move around in the LRU. > >> > >> This avoids costly LRU movement which accounts for a significant portion > >> of the time during unmap_page_range. > >> > >> - 91.31% 0.00% mmap_exit_test [kernel.kallsyms] [.] exit_mm > >> exit_mm > >> __mmput > >> exit_mmap > >> unmap_vmas > >> - unmap_page_range > >> - 55.75% folio_mark_accessed > >> + 48.79% __folio_batch_add_and_move > >> 4.23% workingset_activation > >> + 12.94% folio_remove_rmap_ptes > >> + 9.86% page_table_check_clear > >> + 3.34% tlb_flush_mmu > >> 1.06% __page_table_check_pte_clear > >> > >> Signed-off-by: Minchan Kim > > > > As pointed out in the previous version of the patch. I really dislike > > this to be mrelease or OOM specific. Behavior. You do not explain why > > this needs to be this way, except for the performance reasons. My main > > question is still unanswered (and NAK before this is sorted out). Why > > this cannot be applied in general for _any_ exiting task. As you argue > > the memory will just likely go away so why to bother? > > I think there was a lengthy discussion involving Johannes from a previous series. > > That should be linked here indeed. How about this? mm: process_mrelease: skip LRU movement for exclusive file folios During process_mrelease() or OOM reaping, unmapping file-backed folios spends a significant portion of CPU time in folio_mark_accessed() to maintain accurate LRU state (~55% of unmap time as shown in the profile below). This patch skips LRU handling for exclusive file-backed folios during such emergency memory reclaim. One might ask why this optimization shouldn't be applied to any exiting task in general. The reason is that for a normal, orderly exit or just pure kill, it is worth paying the CPU cost to preserve the active state of clean file folios in case they are reused soon. Preserving cache hits is beneficial for overall system performance. However, process_mrelease() and OOM reaping are emergency operations triggered under extreme memory pressure. In these scenarios, the highest priority is to recover memory as quickly as possible to avoid further kills or system jank. Spending half of the unmap time on LRU maintenance for pages belonging to a victim process is a bad trade-off. If speeding up the victim's reclaim by avoiding LRU movement and evicting cache negatively affects the workflow (due to immediate restart), it implies a sub-optimal kill target selection by the userspace policy (e.g., LMKD), rather than a problem in this expedited APIs. Therefore, we choose to prioritize immediate CPU savings and faster memory recovery over potential future cache hits for the specific victim's files. Profile showing the overhead of folio_mark_accessed during unmap: - 91.31% 0.00% mmap_exit_test [kernel.kallsyms] [.] exit_mm exit_mm __mmput exit_mmap unmap_vmas - unmap_page_range - 55.75% folio_mark_accessed + 48.79% __folio_batch_add_and_move 4.23% workingset_activation + 12.94% folio_remove_rmap_ptes + 9.86% page_table_check_clear + 3.34% tlb_flush_mmu 1.06% __page_table_check_pte_clear Signed-off-by: Minchan Kim