From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 81597431E6D for ; Wed, 1 Jul 2026 20:09:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782936552; cv=none; b=IdK+4K4WJKS20oWe6DrIi7iIBB17hPwt5xjAFnkyJ29Rx5TwzZOtPkkJ1pwhw6bcFbCsb0S+02BPcO5HzMZUZpNwWX4EC+T3hjJjVWFVGiNNZSYkqPIsCNuUOQlbPK4dmOBtkEdcGfB5+nU3GxRIFm1zd3Sl4HXl3ukLD4ZwCic= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782936552; c=relaxed/simple; bh=qzAfTbcd1aGjnu4hd04q81z+fieFRwbAitc5AR4KvSo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gESQMa8WaQt+PEhSZGiy2wrvL7M/pBb3rczrE3kbsW473KOmGfHRHG1TGsOEhdAbKx8eYr4DEWpkIzF4FGXTd5PfMdIhUtCpTFFtqOhQKABPdsf56Q+jyIo73K8e+GFwIlS5z1nsCPivjsGaQbr0HmiLRiCYIXM8ty6l7wLPIaA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=POD9t2s6; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="POD9t2s6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 71DB71F000E9; Wed, 1 Jul 2026 20:09:06 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782936551; bh=Yb6QajBml6PDRqEuDCEin9ZED3RkqNUP8x82FxgMrAI=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=POD9t2s65qU++mvFAjTjg4Q0W8maUZraS3umK+8oRj5Soj+OU+VQ0+AL6FQX8nzfF bOedh/d18VDJWBPVkpqMwCj6GesKeO1wwCxIE80qd6v7cEf7c43Mjw3DnhSD9aAdDa II9TOK4DRVeUNZUPEygCtnXSUa3J+QA1f8lgX05LZL8GzQ1HSWTQoSjCle3JeKgti7 7GSPnGZfmdfEAPUdHvLX723YqNd3ZCDeiAgMm5xPbtKUoZ93p9BRLdfyEEiVDvGAAV w0tNsGN0d5AUk+CMCcVPD1ZdZ7yeKzv/uwFQsoQEq46D1cQww8cqcTfgzULWJ6dxuz nNWXPlZN4Vraw== Message-ID: <22f9bdf3-576e-4146-89c7-64960207c5d2@kernel.org> Date: Wed, 1 Jul 2026 22:09:04 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 4/4] mm: try to free swapcache for non-LRU folios To: Barry Song Cc: Kairui Song , akpm@linux-foundation.org, linux-mm@kvack.org, baoquan.he@linux.dev, chrisl@kernel.org, jp.kobryn@linux.dev, kasong@tencent.com, liam@infradead.org, linux-kernel@vger.kernel.org, ljs@kernel.org, mhocko@suse.com, nphamcs@gmail.com, rppt@kernel.org, shakeel.butt@linux.dev, shikemeng@huaweicloud.com, surenb@google.com, usama.arif@linux.dev, vbabka@kernel.org, youngjun.park@lge.com References: <20260623231635.43086-1-baohua@kernel.org> <20260623231635.43086-5-baohua@kernel.org> <1e7a712b-33f4-44c2-85ed-6333ddca421c@kernel.org> <430e75bc-3993-40da-9996-71583e44c2d5@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit On 7/1/26 11:50, Barry Song wrote: > On Wed, Jul 1, 2026 at 4:18 PM David Hildenbrand (Arm) wrote: > [...] >>> >>> The newly allocated folio will likely still be sitting in this >>> CPU's local LRU cache. If we drop the drain in patch 1, it seems >>> there is little point in calling do_wp_page() from >>> do_swap_page(), since we won't be able to reuse the folio while >>> it remains in the local LRU cache. >> >> Well, we call do_wp_page() here to avoid another page fault where we would end >> up in do_wp_page() and actually break COW. > > Yep. We still need do_wp_page() to handle CoW, but we could > actually reuse the folio if we don't drop the LRU drain. > >> >>> >>> So I'm wondering whether we should keep the drain in >>> patch 1 by checking for a refcount of 3, or instead do >>> something like the following in patch 4: >>> >>> >>> if ((vmf->flags & FAULT_FLAG_WRITE) && !pte_write(pte)) { >>> lru_add_drain(); >>> ret |= do_wp_page(vmf); >>> if (ret & VM_FAULT_ERROR) >>> ret &= VM_FAULT_ERROR; >>> } >>> >>> Otherwise, the combination of patches 1 and 4 seems somewhat >>> inconsistent. It feels like a strange mix of sweet, spicy, >>> hot, and cold—kind of contradictory :-) >> >> Yes, this needs a proper thought. >> >> What you describe is relevant for >> >> e.g., >> >> swapout >> fork -> exclusive bit cleared >> child exit -> parent is single user >> swapin -> added to swapcache and LRU cache >> >> >> This is not very odd, so we should keep that working (I think I have >> micro-benchmarks for that). > > Maybe the simplest approach is to keep the drain in patch 1 and > just make the code improvements you suggested in [1]? Yeah, probably best for that scenario. -- Cheers, David