From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-174.mta0.migadu.com (out-174.mta0.migadu.com [91.218.175.174]) (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 99E243EF0BD for ; Fri, 24 Jul 2026 09:59:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.174 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784887191; cv=none; b=tcVfDdZtPvcAQNFs5j2MxQYyOYOGWtyjvgPYpR/O5HxrW9NSNORUuCCS6DTJQvE/rayzBZFALs4/Qtn4SdlSa0+XkoTK7D+w5X6BpzgO0YccvZKF/ni7V1aBBDhv/+qDDFdulBf8w3SlHHITXv/yNaVRDJwjpeeFEx1or6bI+N8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784887191; c=relaxed/simple; bh=/euDC0udAcgKDh0z6jB4c7KDJybUyTNgm51n0X0U6v4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MbXUiaQ0/oI9r12r3mQ4H5DPlgCDHo4wpkiwagJ9Gfyd0DIQ/EFJBAqDVd7s5K8/guFqflkoGFsrSiVRBYjbBCa1oUzawecFb+bi/mtpExkRrlpL+Q2qsHygIpBS6+06H5rQvlND1CiaxsAvkj4Xnv396O85XxJ+9gR6zq3cBHg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=k0HxrWGF; arc=none smtp.client-ip=91.218.175.174 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="k0HxrWGF" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1784887185; h=from:from: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; bh=ViiLl9ZFF0UEdOn7wLcmnHat7EVRXfbwbFG0wZYEi10=; b=k0HxrWGFl2ltc7kB+r1juMoWhA3cgV5epPavsYp57+2JiERhX+D4oeLdbHCEwKtwhy6ele jMSgTe4dzXRGXhKy+j7N+gv3IFru1lL8m4dFAlzICSatZDbo9F9VTeGpWxh9yi5+lcdLCW 7TcoyNw+C5AkTZM54vbVq6/RiE2eWZw= Date: Fri, 24 Jul 2026 10:59:40 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH v5 04/11] mm: zswap: add range lookup for large-folio swapin To: Yosry Ahmed Cc: Alexandre Ghiti , Andrew Morton , david@kernel.org, chrisl@kernel.org, kasong@tencent.com, ljs@kernel.org, ziy@nvidia.com, linux-mm@kvack.org, ying.huang@linux.alibaba.com, Baoquan He , willy@infradead.org, youngjun.park@lge.com, hannes@cmpxchg.org, riel@surriel.com, shakeel.butt@linux.dev, kas@kernel.org, baohua@kernel.org, dev.jain@arm.com, baolin.wang@linux.alibaba.com, Nico Pache , "Liam R. Howlett" , ryan.roberts@arm.com, Vlastimil Babka , lance.yang@linux.dev, linux-kernel@vger.kernel.org, nphamcs@gmail.com, shikemeng@huaweicloud.com, kernel-team@meta.com, Alexandre Ghiti References: <20260722152043.2273289-1-usama.arif@linux.dev> <20260722152043.2273289-5-usama.arif@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 23/07/2026 17:42, Yosry Ahmed wrote: >>>> @@ -1595,13 +1611,19 @@ int zswap_load(struct folio *folio) >>>> return -ENOENT; >>>> >>>> /* >>>> - * Large folios should not be swapped in while zswap is being used, as >>>> - * they are not properly handled. Zswap does not properly load large >>>> - * folios, and a large folio may only be partially in zswap. >>>> + * A large folio reaches zswap_load() only when its whole range is >>>> + * expected to be on disk: PMD swap-entry consumers split before >>>> + * calling into PMD-order swapin whenever any slot is still in zswap. >>>> + * Confirm the range is entirely absent from zswap and return -ENOENT >>>> + * so the caller reads it from disk; if a slot is unexpectedly still in >>>> + * zswap, fail the read rather than return partially-initialized data. >>>> */ >>>> - if (WARN_ON_ONCE(folio_test_large(folio))) { >>>> - folio_unlock(folio); >>>> - return -EINVAL; >>>> + if (folio_test_large(folio)) { >>>> + if (zswap_is_present(swp, folio_nr_pages(folio))) { >>> >>> Is dropping the warning here intentional (for the folio_test_large() && >>> zswap_is_present() case)? >>> >> >> >> Yes, so we can end up in a race, which should be handled gracefully. >> >> For example, lets say we have zswap writeback enabled, which means we >> can end up in a state where we have a PMD swap entry and 511 of the 512 >> slots have been written to disk, but 1 slot (slot X) is still in zswap. >> >> We can then have the following race: >> >> >> CPU A: PMD swap-in CPU B: split-PTE swap-in >> ------------------ ------------------------ >> >> Checks swap cache: empty >> >> Faults slot X >> Adds order-0 folio F >> zswap_load(F): >> removes X from zswap >> marks F dirty >> >> Checks zswap range: empty >> A is preempted >> >> Unmaps/reclaims F >> zswap_store(F): >> puts X back in zswap >> Removes F from swap cache >> >> A resumes >> Allocates large swap-cache folio G >> zswap_load(G) finds X in zswap > > Why don't we check the zswap range after allocating a folio in the > swap cache? I am assuming at this point we have the folio locked and > the result should be stable? > Yes that makes sense. The folio is locked and the result will be stable. How about something like below? diff --git a/mm/swap_state.c b/mm/swap_state.c index 1f08fb522036..41f168377cb1 100644 --- a/mm/swap_state.c +++ b/mm/swap_state.c @@ -12,6 +12,7 @@ #include #include #include +#include #include #include #include @@ -500,6 +501,17 @@ static struct folio *__swap_cache_alloc(struct swap_cluster_info *ci, __folio_set_locked(folio); __folio_set_swapbacked(folio); __swap_cache_do_add_folio(ci, folio, entry); + /* + * Reject mixed zswap/disk backing before starting the read. + */ + if (order && zswap_is_present(entry, nr_pages)) { + __swap_cache_do_del_folio(ci, folio, entry, shadow); + spin_unlock(&ci->lock); + folio_unlock(folio); + /* nr_pages refs from swap cache, 1 from allocation */ + folio_put_refs(folio, nr_pages + 1); + return ERR_PTR(-EBUSY); + } spin_unlock(&ci->lock); if (mem_cgroup_swapin_charge_folio(folio, memcg_id, >> >> The correct action would be to reject the PMD order read and fall back >> to per-page loading. >> >> I am bit torn about what to do for zswap here. The series is quite big >> already. Alexandre is looking at adding support for PMD swap, but will >> send patches once this series gets merged. My initial versions 1 >> and 2, basically stopped installing PMD swap entry if zswap was ever enabled. >> From v3, I used Alexandre's suggestion to do zswap_is_present() test >> so that PMD swap entry can keep on working. >> >> Both of these paths are temporary till Alex sends his series. I >> do feel my initial version was simpler but will basically stop >> working if zswap is ever enabled. Do you have any suggestions on >> what your preference is for zswap? > > Even with PMD zswap load support, we still have to deal with the case > where some order-0 pages are in zswap and some are on disk, right? I > don't see how that would eliminate the case you mentioned above. Ack, yeah writeback will still mess with things. I will keep the current implementation.