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bh=nDL3ilZVm2uMCF1pO9rBAggewE0THWnlKKczkxpeYj8=; b=tHVgrhFw4io4z5rNb8lIjFGGYNdthpCsih4+UUbbOaJLgYTLB4CVslVsdolOE4upE2rTVZ 387lONTRzkl8ppSXgbxcT5FngDU34PFzQtv3oWIllsbCbtuoq2I0wi03d8vcWFAZbk4UTe gOpTXf1c6JnT2uAAx4pr4nvbkkhUDPc= ARC-Authentication-Results: i=1; imf14.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=LKoNVL4g; spf=pass (imf14.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id E862B4401A; Wed, 1 Jul 2026 08:18:25 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A5CFF1F000E9; Wed, 1 Jul 2026 08:18:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782893905; bh=nDL3ilZVm2uMCF1pO9rBAggewE0THWnlKKczkxpeYj8=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=LKoNVL4gjYpGxBtnO4v3Q2F8NC1jF1fN/yySyi2CjqRa5Ts5CGAgrUVgiXYAxUoWT 6HOzYQRI7yPU53Gzp3Jz+LBMhzfoEzAQW2AOXgkI0+UqECGwrmlHRyQUV5pG19unAT i5ioIfQh3KKBswp9pR5E+Zu2ybe19hhTuxAZ6kYVk4xnKOXq6iB7ade+OKJEr+2oG9 AFHqpQ0iMG5WyiyjBN6MZ3d+BrBnbzY8yZ4QYY/RTKHXBtLs6IMZrODqT1zmx9p8ID gcXaWVYXsw36zUNnSYzUVBQwHLs8PdZnXGdprytdDmGDeMRxmzVZ0S8cUX77iaaYpU dV6G33jjw6ZJQ== Message-ID: <430e75bc-3993-40da-9996-71583e44c2d5@kernel.org> Date: Wed, 1 Jul 2026 10:18:19 +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> 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-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: CA4C910000D X-Rspam-User: X-Stat-Signature: wd4he19h7xyh5uj341ni1agfz745ptrs X-HE-Tag: 1782893906-138439 X-HE-Meta: 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 0d/HJiMT brcCe7MKPJUOq6VJs4SOBv2kTrAYSOKwVJf1Erc6O5GG4YgwidBMxVzQ7hii5qhEeZBng56lRMkdVITbQ7A13lEbh9CYhWZJM2+qvAyYdWYWcQ0kZMnraMVFdr0EtiUQxmEg+3F8IussHx5/6AgJOpnu5CF5Z6oVcJEbKoz6W0ZJxC/pMSlLDwBK3HGzGyk6CVyFQfHZKJk6UJYUpwaGtrOe29vZtF0zrnzpnwKdVvwxOcyKfpCNAllAbs4ovBCyWXRPUuhR5Sc7PG1x7NXnYoymofzv6QqsWUcBZ0kVVX+zAXEQjf5KTFtHSPOHZZ/xoz4VRQiuZe1vY2KMAY7qu3upr3pK25hlb1xT0 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/1/26 00:36, Barry Song wrote: > On Tue, Jun 30, 2026 at 1:48 PM David Hildenbrand (Arm) > wrote: > [...] >> >> - if (vmf->flags & FAULT_FLAG_WRITE) { >> + if ((vmf->flags & FAULT_FLAG_WRITE) && !pte_write(pte)) { >> ret |= do_wp_page(vmf); >> if (ret & VM_FAULT_ERROR) >> ret &= VM_FAULT_ERROR; >> > > Thanks! I'm working on a v3, but one thing still seems a bit odd to me. > > In patch 1, we're removing the local LRU drain from > do_wp_page(). But in patch 4, do_swap_page() calls > do_wp_page() for folios without the "exclusive" hint, > since folios with the hint have already been reused in > do_swap_page(). > > This can happen when we allocate a new folio whose swap > entry lacks the exclusive flag, even though the folio is > still reusable. For example, in the scenario I mentioned > in patch 1, the swap count is effectively 1, but there is > no exclusive flag: > > #include > > int main(int argc, char *argv[]) > { > int i; > #define SIZE 30*1024*1024 > while(1) { > volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE, > MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); > volatile int q; > > for (int i = 0; i < SIZE/sizeof(int); i++) > p[i] = i; > madvise((void *)p, SIZE, MADV_PAGEOUT); > if (!fork()) > _exit(0); > for (int i = 0; i < SIZE/sizeof(int); i++) > q = p[i]; > for (int i = 0; i < SIZE/sizeof(int); i++) > p[i] = i; > munmap(p, SIZE); > } > return 0; > } > > 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. > > 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). -- Cheers, David