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Wed, 1 Jul 2026 20:09:14 +0000 (UTC) X-FDA: 84941297028.22.A22D659 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf01.hostedemail.com (Postfix) with ESMTP id 6DFB040009 for ; Wed, 1 Jul 2026 20:09:12 +0000 (UTC) Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=POD9t2s6; spf=pass (imf01.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1782936552; b=sAdQJGWByN30M2UuAuepcHEA+NuLIzUnLLJNy2I0wFIM8UiZkeAEvOAHUpplakq9Is6LJx NTtUhdi1Z0MTh4v9yuq4gtZv2QjiIstTA3gK4/OdV5F1iEZJ2M5Sfc5Ja7EC0f5ulmhArA llHdcgEd0LaIxaRuVMLL7B5OoST2Ddg= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1782936552; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; 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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 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 X-Stat-Signature: 9u9xjo9d47wdhxu817jw4awwog5r4tu3 X-Rspam-User: X-Rspamd-Queue-Id: 6DFB040009 X-Rspamd-Server: rspam02 X-HE-Tag: 1782936552-802999 X-HE-Meta: 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 /xhRlCkD 4VOduCbL/++Jc4KgbEejiywGkqrpkI+Sn88o77OLdHpopPJKNCMyHp8eKjYLZ0sqAFPnaQRhd/KbP9hqvkuI6uFrRiKUSUNdwfeNLNmdZq5hqvJ5rJn7Vu4IEZXotSx2lzj7V++PCrc/8iD/Wlqqv8muZWPfxAT4AWfmIMJP8ovtVevkHOftIMDeaZ1OwCsqxxaWix/kT5CLSb1/+UdZL8fuptI2Oc2k2NgBhHF0WjeHVs0KIOHCg4uqse3I0eUTQjf5TkZCjUC1aGye8MjTXmf0SyDNUgwGLttrVjxIsgpgCEwq0ANQd7aNWHQ== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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