From patchwork Thu May 2 08:52:59 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: David Hildenbrand X-Patchwork-Id: 13651448 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 4DB07524B8 for ; Thu, 2 May 2024 08:53:11 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714639992; cv=none; b=huFo3NhF/Eik+M2wmfPrKnhB6bpK+p8s3F8Zlz2PHkGqOmKQTOv5VX2JgZdxyWkNyyZ4i6RXfihqdW3lv8RjGDd2cigzym3TKDnf2vPAor6G2em7bz+C1oj92EBT9PaJOGGCUq2mLMQDxD6j77RDFuf+0CgPNlkdkTWC34eLg/c= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1714639992; c=relaxed/simple; bh=2P0XQ3f/nErLPEazYra6MFwog6B2HmsLDNWBea6YiYQ=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=fYmxW8LHu/6NH4r58CouNylNYnXMtUErApGtD9J6ccljhx6r1TpnkvzES9qS2zyAGtgeujBJyOjTKn4GKQmVGuf+lQJ4fMt8mkzFW4iKkyI+jRAYbUE5Yl7sHoe5v/n6TAMh27aBgJUujKNq6Oi2ItGZzNF1DD2pFvvGvp4A3q8= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=bDgTxs/P; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="bDgTxs/P" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1714639990; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=T0uhogWZMK2FCWUvbV0NVhLRqNmBD8nexNysXl5gy1w=; b=bDgTxs/P2/76qrH/X++uUJ8cvSOdBxWxNU03Xk2kOBUNGlN4PcyekdWuBgWopvrtsTUkFq TLaXXxjICKFN+x+v94bL+nqjw2iz7vfCmWQ4DZkH/91qsgHkkotXX4td97pQxw/4nZW2pe EmdzUj3Sc35Y38FNX//UE/jidCs6u7E= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-681-rpnWT_B-O0-ApCVzbzjOLA-1; Thu, 02 May 2024 04:53:07 -0400 X-MC-Unique: rpnWT_B-O0-ApCVzbzjOLA-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 8D9B6811029; Thu, 2 May 2024 08:53:06 +0000 (UTC) Received: from t14s.redhat.com (unknown [10.39.193.224]) by smtp.corp.redhat.com (Postfix) with ESMTP id 8ED1A40ED2F; Thu, 2 May 2024 08:53:04 +0000 (UTC) From: David Hildenbrand To: linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, linux-kselftest@vger.kernel.org, David Hildenbrand , Andrew Morton , Muchun Song , Shuah Khan , Peter Xu Subject: [PATCH v1 2/2] mm/hugetlb: document why hugetlb uses folio_mapcount() for COW reuse decisions Date: Thu, 2 May 2024 10:52:59 +0200 Message-ID: <20240502085259.103784-3-david@redhat.com> In-Reply-To: <20240502085259.103784-1-david@redhat.com> References: <20240502085259.103784-1-david@redhat.com> Precedence: bulk X-Mailing-List: linux-kselftest@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 Let's document why hugetlb still uses folio_mapcount() and is prone to leaking memory between processes, for example using vmsplice() that still uses FOLL_GET. More details can be found in [1], especially around how hugetlb pages cannot really be overcommitted, and why we don't particularly care about these vmsplice() leaks for hugetlb -- in contrast to ordinary memory. [1] https://lore.kernel.org/all/8b42a24d-caf0-46ef-9e15-0f88d47d2f21@redhat.com/ Suggested-by: Peter Xu Signed-off-by: David Hildenbrand --- mm/hugetlb.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/mm/hugetlb.c b/mm/hugetlb.c index 417fc5cdb6eeb..a7efb350f5d07 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5963,6 +5963,13 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio, /* * If no-one else is actually using this page, we're the exclusive * owner and can reuse this page. + * + * Note that we don't rely on the (safer) folio refcount here, because + * copying the hugetlb folio when there are unexpected (temporary) + * folio references could harm simple fork()+exit() users when + * we run out of free hugetlb folios: we would have to kill processes + * in scenarios that used to work. As a side effect, there can still + * be leaks between processes, for example, with FOLL_GET users. */ if (folio_mapcount(old_folio) == 1 && folio_test_anon(old_folio)) { if (!PageAnonExclusive(&old_folio->page)) {