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 760312D94A0; Tue, 12 May 2026 06:14:26 +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=1778566466; cv=none; b=V4tOXcPVnGdtAUi4Nag7jcscM1Q1dk7LSENAB4pxdnxl0BUkPiNp/4bqAfnyVQN67S0nSyCBIG3HHAik8082Z1VGAE+yg1+i1Id83/3hATyGjwQD6xWwNkDEz1HgCLq1LL1SpIYwpQbAUFOOa7o4XMwfCAbRBR6aesLtDX3ieB4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778566466; c=relaxed/simple; bh=0EZtptM8iTZEc9KkhIGOPS4y0SLEEYo00bdW3QFajZA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=oFUvKWeZZorIeEXU4t78WcAE0s3UDMC8pjkEd9N2GFqMqOgn/KkmFv1HQRCK+BlIqL8/QmQz0EzW30zrZxayUU5ukLFClCNTSxaZjtRLXSn0yD6GpLf6KYJeeLhkaZczQ+rglxLw0h/Q1HQNtT2HzOmexO8RhexHSIuf5nd6gWc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=HDehqxco; 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="HDehqxco" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 8E579C2BCB8; Tue, 12 May 2026 06:14:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778566464; bh=0EZtptM8iTZEc9KkhIGOPS4y0SLEEYo00bdW3QFajZA=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=HDehqxcoOny8+q9R/EQrECENhPrcPyoKzzgP48p0XVXIeAHZK7kpMVnvB44IoPzBo t6peJordL6ZByfnTUShrHNma+rjCqHTXfn9dvabV+92O0EfkBkI8KUoBKymYSOty0d 0DAxql17MW5mjR1tyRF5NcCKv0QZCAUpwjGe6tslOAClLsNNSGbJ6CB+qQ2vBal5uB +th+LAgu3k/fmOJc1Du1SrOEBn/5tIRmvvsIdh9cXGYHQ35ks0K9D8FMEwuUphn9nQ EJfLzffgxpbdRb43CYfTDmjNKAn9dLdq4RyT8l+6poo9Dk2F5WGu15s/fBUx90mvT6 o0aHe7Qa+VN7Q== Message-ID: Date: Tue, 12 May 2026 08:14:20 +0200 Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 0/5] mm: support zswap-backed anonymous large folio swapin To: Yosry Ahmed , fujunjie Cc: Andrew Morton , Chris Li , Kairui Song , Johannes Weiner , Nhat Pham , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, Jonathan Corbet , Ryan Roberts , Barry Song , Baolin Wang , Chengming Zhou , Baoquan He , Lorenzo Stoakes References: From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/12/26 00:13, Yosry Ahmed wrote: > On Fri, May 08, 2026 at 08:18:29PM +0000, fujunjie wrote: >> Hi, >> >> This RFC explores anonymous large folio swapin when a contiguous swap >> range is backed consistently by zswap. >> >> Large folio swapout to zswap is already supported by storing each base >> page in the folio as a separate zswap entry. The anonymous synchronous >> swapin path has remained order-0 once zswap has ever been enabled: >> zswap_load() rejected large folios, and alloc_swap_folio() avoided large >> folio allocation to protect against mixed backend ranges. >> >> This RFC keeps the scope intentionally conservative. It does not try to >> read one large folio from mixed zswap and disk backends, and it does not >> change shmem swapin. Shmem still has its existing zswap fallback and is >> left for later discussion. For anonymous swapin, the backend rule is made >> explicit: >> >> - a range fully absent from zswap can keep using the disk backend >> - a range fully present in zswap can be decompressed into a large folio >> - a mixed zswap/non-zswap range falls back to order-0 swapin >> >> The series adds a zswap range query helper, teaches zswap_load() to >> decompress all-zswap large folios one base page at a time, accounts mTHP >> swpin for zswap-loaded large folios, retries synchronous large-folio >> insertion races with order-0 swapin, and removes the anonymous >> zswap-never-enabled restriction once mixed ranges are filtered. >> >> I tested the series with a full bzImage build using CONFIG_ZSWAP=y, >> CONFIG_ZRAM=y, CONFIG_MEMCG=y and CONFIG_THP_SWAP=y. >> >> The QEMU/KVM runs covered both the fully-zswap path and the mixed-backend >> fallback path. In the all-zswap run, a 512MiB anonymous mapping was faulted >> as 8192 64KiB groups, reclaimed into zswap, and faulted back. Reclaim >> reported mthp64_zswpout=8192 and zswpout=131072. Refault then reported >> mthp64_swpin=8192 and zswpin=131072, and pagemap/kpageflags showed 8192 >> order-4 THP groups in the mapping. >> >> In the mixed-backend run, the workload used a 64MiB anonymous mapping >> split into 1024 64KiB groups. After shrinker debugfs wrote back exactly >> one zswap base-page entry, refault left 1023 order-4 THP groups and one >> order-0 mixed group. The kernel stats matched that shape: >> mthp64_swpin=1023, zswpin=16383 and zswpwb=1. >> >> CONFIG_SHRINKER_DEBUG is only a test aid for making that one zswap >> writeback deterministic; it is not required by the implementation. >> >> Nhat Pham's active Virtual Swap Space series is adjacent work. It moves >> swap cache and zswap entry state into a virtual swap descriptor, and lists >> mixed backing THP swapin as a future use case. This RFC is independent and >> works with the current swap/zswap infrastructure, but may need rebasing if >> VSS lands first. >> >> Feedback would be especially helpful on: >> >> 1. whether it makes sense to support all-zswap large folio swapin first, >> while keeping mixed zswap/disk ranges on the order-0 fallback path > > I think so, yes, but based on my read of the code this RFC only affects > synchornous swapin, which is more-or-less zram+zswap. This is an > uncommon setup outside of testing. BLK_FEAT_SYNCHRONOUS is also set for pmem and brd devices I think, but that's also pretty uncommon I assume. Well, maybe if your hypervisor provides you with an emulated NVDIMM to use as swap backend ... maybe. I thought there were other ways to get BLK_FEAT_SYNCHRONOUS set, but I don't see other usage. So seeing it for zswap is pretty rare I assume. -- Cheers, David