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 48F79393DC0 for ; Mon, 22 Jun 2026 08:58:25 +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=1782118707; cv=none; b=hRnICtWAmwZE0LILOCiOqyIVOK7wGgD56Ah1FMyWHl7rE/PWx4Qx9ok85sc53jIs4DPFOtGVEvC/lcTnLUW7USj1sSjUAUZaOV8u+Ch5HKySPs8l8sgnmiSJ3fVh1LCmQ6MkgzvEXFUoxYA7E01IQSr+4MbDPDlDpexQ5dVTHkY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782118707; c=relaxed/simple; bh=m+wgHo0pfqvSAWZyliuIBq1EkViiqzcC2a+nt7PImEE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=S7l6hwWtIOK+Ww5tqIG35L1Q5g4oPzv5XwkZLcQWKVRrupNF7gOPKA92dOT+sU/KOM5mlxpEOJohnUb4GbgjIR6VeTmdH6OGRmEbHtLRBskblRqaje2j2zJxRl9yg/s6S1XTN1DR62pCuca3njnxo3AaOPKJrGvYMWp20OdcD9s= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Qg7LegUu; 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="Qg7LegUu" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6A7651F00A3D; Mon, 22 Jun 2026 08:58:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782118705; bh=1axEUDSKUCZxXWYIuAFylYrZQLRQGoLVwSR5dVqSN1g=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Qg7LegUu+eE/t5ZVmf123u++iukFhC3v6Y0SR+Zf/z+Gf8dQvLeMP5Yl15J54JXtF 7CuCD7p5Ohy5AQkm9SZZ8ckeqHHMoV6i+3QD4yNCt+aUJMIdQNOfhNVvXITkMLOibq gBkVFCqpLX9EY/Y1dEITKk299MSqD9fs7K8bC/NesR/jHSan4dlhCKvvjnEoffOLnY SknsBq0f10PADghliYZdWWlMT0M3hANV3uILg3NpJ3g3uXimncVYJ1L+FR9DA5XAo7 1jqFRXVGUp04GilEjCFB4/2tJz1lpmlJ8+m/BS8yUmw+SCuMaKmC4yv7kMrQnkQlMp GhGViHi6ddDmQ== Message-ID: <4aa8350e-712f-4380-b3bf-2ff06cf2a35d@kernel.org> Date: Mon, 22 Jun 2026 10:58:20 +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: [RFC PATCH] mm: Avoiding split large folios if swap has no space To: "Barry Song (Xiaomi)" Cc: akpm@linux-foundation.org, axelrasmussen@google.com, baolin.wang@linux.alibaba.com, dev.jain@arm.com, kasong@tencent.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, npache@redhat.com, qi.zheng@linux.dev, ryan.roberts@arm.com, shakeel.butt@linux.dev, weixugc@google.com, yuanchu@google.com, zhaonanzhe@xiaomi.com, ziy@nvidia.com References: <5790c4a4-d502-4180-82f5-47de5809a4fe@kernel.org> <20260620081017.89085-1-baohua@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 6/20/26 10:10, Barry Song (Xiaomi) wrote: > On Fri, Jun 19, 2026 at 10:04 PM David Hildenbrand (Arm) wrote: > [...] >>> /* >>> * The page can not be swapped. >>> * >>> @@ -1280,6 +1289,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, >>> >>> if (!folio_test_large(folio)) >>> goto activate_locked_split; >>> + if (!__can_reclaim_anon_pages(memcg, sc)) >>> + goto activate_locked_split; >> >> Why are we even trying to allocate swap space if we cannot reclaim such pages? >> Makes we wonder whether we would want to have that check earlier, before the >> folio_alloc_swap(). >> >> Any downsides? > > I don't think there are any obvious downsides there. One issue is that > the memcg may not be passed from reclaim_pages(), so memcg would > always be NULL. However, the folio could still belong to a memcg > whose swap quota has been exhausted. In that case, my > __can_reclaim_anon_pages() will fail when checking whether we can > swap out. But switching to folio_memcg() also seems awkward. > > So I feel Kairui’s suggestion [1] might be the best approach. In > folio_alloc_swap(), we return -EAGAIN to tell vmscan.c that > we can split the folio and retry the swap-out. > only when there are sufficient swap slots and sufficient memcg swap > quota do we return -EAGAIN, allowing vmscan to perform a split. > > diff --git a/mm/swapfile.c b/mm/swapfile.c > index 78b49b0658ad..62e2c506ccae 100644 > --- a/mm/swapfile.c > +++ b/mm/swapfile.c > @@ -1755,6 +1755,9 @@ int folio_alloc_swap(struct folio *folio) > VM_WARN_ON_ONCE(1); > return -EINVAL; > } > + > + if (get_nr_swap_pages() < (1 << order)) > + return -ENOMEM; I guess we could be clearer with the return value: -> !get_nr_swap_pages() -> -ENOSPC / -ENOMEM (no space at all) -> get_nr_swap_pages() < (1 << order) -> -E2BIG (there is some space, but not for the full thing) But now I wonder whether we would also want to check "is there any free swap space", not just "is there any swap". Essentially, try returning -E2BIG if there is the chance to swap out after split, and -ENOSPC / -ENOMEM if a split wouldn't help. > } > > again: > @@ -1769,11 +1772,13 @@ int folio_alloc_swap(struct folio *folio) > } > > /* Need to call this even if allocation failed, for MEMCG_SWAP_FAIL. */ > - if (unlikely(mem_cgroup_try_charge_swap(folio))) > + if (unlikely(mem_cgroup_try_charge_swap(folio))) { > swap_cache_del_folio(folio); > + return -ENOMEM; Here we wouldn't have the information whether we could charge after a split. So that would require a rework to signal this more cleanly to the caller. > + } > > if (unlikely(!folio_test_swapcache(folio))) > - return -ENOMEM; > + return -EAGAIN; > > return 0; > } > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 299b5d9e8836..63e8578454ea 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -1257,6 +1257,8 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, > */ > if (folio_test_anon(folio) && folio_test_swapbacked(folio) && > !folio_test_swapcache(folio)) { > + int ret; > + > if (!(sc->gfp_mask & __GFP_IO)) > goto keep_locked; > if (folio_maybe_dma_pinned(folio)) > @@ -1275,10 +1277,10 @@ static unsigned int shrink_folio_list(struct list_head *folio_list, > split_folio_to_list(folio, folio_list)) > goto activate_locked; > } > - if (folio_alloc_swap(folio)) { > + if ((ret = folio_alloc_swap(folio))) { I prefer doing the assignment outside the conditional. > int __maybe_unused order = folio_order(folio); > > - if (!folio_test_large(folio)) > + if (!folio_test_large(folio) || ret != -EAGAIN) > goto activate_locked_split; > /* Fallback to swap normal pages */ > if (split_folio_to_list(folio, folio_list)) > > What’s your view on this, David? I guess returning from folio_alloc_swap() whether a split could allow for swapout (e.g., -E2BIG) would be reasonable. To catch all the cases where it makes a difference: * No free swap space (split won't work) * Some free swap space (split would work) * Sufficient free swap space, but fragmented (split would work) * No memcg space (split won't work) * Some memcg space (split would work) -- Cheers, David