From nobody Thu Oct 9 16:36:01 2025 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 AA6402EF645 for ; Tue, 17 Jun 2025 14:27:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750170462; cv=none; b=ODOwSgc6JnAydRNiewoHcHhZday7j5/vOpaPuANBZRaMo2cskAxqEinBYoWVkitK3L1uRUgmwjYJ1sBnh9Lxhb9zhxsnrw4s8TEuncUJ2mfSXTzkjDMKce1fQNlh/VLQjs6xgmRJW24iO3dKMm+1wdQQ3m85Tmlc6ucxMrVnVzc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1750170462; c=relaxed/simple; bh=3GABNhTx9HkRcQf2tK/eLDKY0ldtUL3DdytTiwqE/Ug=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=Pf8C8TUbWyRiHw5wek+oJ6zMPk+iIpMHGZJu4UaKwJK+g/iKkrUfUddifxNDnUaY99WY2ssD0SUZfM7wAK3ajyR23tm0bCgL9SSaMS4QS/vaw7Aht5qquSw3WLSWqlATWJ2ck48+EFyipVzWumQpqcwa8a1pL6NzWcX/jLOe/5c= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine 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=fCe0oVV/; arc=none smtp.client-ip=170.10.133.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine 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="fCe0oVV/" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1750170459; 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=KDlgrgXxAg21YzMqYW6Qh6Z/vBEV3Xdk3QnBC6RuzMQ=; b=fCe0oVV/uSWtX+4S+TvDvYG1RG5CWtZC3xezeQwAV+tHoiTlka3wDgq+ChlavWgHpLJ3QG zkzHpbRa0zy06GgRqjKroDWaNF1san1HbhMBfSluvemxiK0F4U1NCoTBYddCvQ3VPmAVmL Mv7i3rx67J/P6IMF38+dKrvgcQH9klM= Received: from mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (ec2-35-165-154-97.us-west-2.compute.amazonaws.com [35.165.154.97]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-530-XSWUeR9EPlir_gCcV2Jq5w-1; Tue, 17 Jun 2025 10:27:36 -0400 X-MC-Unique: XSWUeR9EPlir_gCcV2Jq5w-1 X-Mimecast-MFC-AGG-ID: XSWUeR9EPlir_gCcV2Jq5w_1750170455 Received: from mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.12]) (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 mx-prod-mc-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 0889D1809C8D; Tue, 17 Jun 2025 14:27:35 +0000 (UTC) Received: from fedora.redhat.com (unknown [10.22.80.174]) by mx-prod-int-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 8585619560A3; Tue, 17 Jun 2025 14:27:33 +0000 (UTC) From: Luiz Capitulino To: david@redhat.com, willy@infradead.org Cc: akpm@linux-foundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lcapitulino@gmail.com, shivankg@amd.com Subject: [RFC 2/3] proc: kpagecount: use snapshot_page() Date: Tue, 17 Jun 2025 10:27:09 -0400 Message-ID: <35c0a0255d209db0de3d4881b956dc0581399c46.1750170418.git.luizcap@redhat.com> In-Reply-To: References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.0 on 10.30.177.12 Content-Type: text/plain; charset="utf-8" Currently, the call to folio_precise_page_mapcount() from kpage_read() can race with a folio split. When the race happens we trigger a VM_BUG_ON_FOLIO() in folio_entire_mapcount() (see splat below). This commit fixes this race by using snapshot_page() so that we retreive the folio mapcount using a folio snapshot. Splat: [ 2356.558576] page: refcount:1 mapcount:1 mapping:0000000000000000 index:0= xffff85200 pfn:0x6f7c00 [ 2356.558748] memcg:ffff000651775780 [ 2356.558763] anon flags: 0xafffff60020838(uptodate|dirty|lru|owner_2|swap= backed|node=3D1|zone=3D2|lastcpupid=3D0xfffff) [ 2356.558796] raw: 00afffff60020838 fffffdffdb5d0048 fffffdffdadf7fc8 ffff= 00064c1629c1 [ 2356.558817] raw: 0000000ffff85200 0000000000000000 0000000100000000 ffff= 000651775780 [ 2356.558839] page dumped because: VM_BUG_ON_FOLIO(!folio_test_large(folio= )) [ 2356.558882] ------------[ cut here ]------------ [ 2356.558897] kernel BUG at ./include/linux/mm.h:1103! [ 2356.558982] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP [ 2356.564729] CPU: 8 UID: 0 PID: 1864 Comm: folio-split-rac Tainted: G S = W 6.15.0+ #3 PREEMPT(voluntary) [ 2356.566196] Tainted: [S]=3DCPU_OUT_OF_SPEC, [W]=3DWARN [ 2356.566814] Hardware name: Red Hat KVM, BIOS edk2-20241117-3.el9 11/17/2= 024 [ 2356.567684] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE= =3D--) [ 2356.568563] pc : kpage_read.constprop.0+0x26c/0x290 [ 2356.569605] lr : kpage_read.constprop.0+0x26c/0x290 [ 2356.569992] sp : ffff80008fb739b0 [ 2356.570263] x29: ffff80008fb739b0 x28: ffff00064aa69580 x27: 00000000ff0= 00000 [ 2356.570842] x26: 0000fffffffffff8 x25: ffff00064aa69580 x24: ffff80008fb= 73ae0 [ 2356.571411] x23: 0000000000000001 x22: 0000ffff86c6e8b8 x21: 00000000000= 00008 [ 2356.571978] x20: 00000000006f7c00 x19: 0000ffff86c6e8b8 x18: 00000000000= 00000 [ 2356.572581] x17: 3630303066666666 x16: 0000000000000003 x15: 00000000000= 01000 [ 2356.573217] x14: 00000000ffffffff x13: 0000000000000004 x12: 00aaaaaa00a= aaaaa [ 2356.577674] x11: 0000000000000000 x10: 00aaaaaa00aaaaaa x9 : ffffbf3afca= 6c300 [ 2356.578332] x8 : 0000000000000002 x7 : 0000000000000001 x6 : 00000000000= 00001 [ 2356.578984] x5 : ffff000c79812408 x4 : 0000000000000000 x3 : 00000000000= 00000 [ 2356.579635] x2 : 0000000000000000 x1 : ffff00064aa69580 x0 : 00000000000= 0003e [ 2356.580286] Call trace: [ 2356.580524] kpage_read.constprop.0+0x26c/0x290 (P) [ 2356.580982] kpagecount_read+0x28/0x40 [ 2356.581336] proc_reg_read+0x38/0x100 [ 2356.581681] vfs_read+0xcc/0x320 [ 2356.581992] ksys_read+0x74/0x118 [ 2356.582306] __arm64_sys_read+0x24/0x38 [ 2356.582668] invoke_syscall+0x70/0x100 [ 2356.583022] el0_svc_common.constprop.0+0x48/0xf8 [ 2356.583456] do_el0_svc+0x28/0x40 [ 2356.583930] el0_svc+0x38/0x118 [ 2356.584328] el0t_64_sync_handler+0x144/0x168 [ 2356.584883] el0t_64_sync+0x19c/0x1a0 [ 2356.585350] Code: aa0103e0 9003a541 91082021 97f813fc (d4210000) [ 2356.586130] ---[ end trace 0000000000000000 ]--- [ 2356.587377] note: folio-split-rac[1864] exited with irqs disabled [ 2356.588050] note: folio-split-rac[1864] exited with preempt_count 1 Signed-off-by: Luiz Capitulino --- fs/proc/page.c | 21 +++++++++++++++++---- 1 file changed, 17 insertions(+), 4 deletions(-) diff --git a/fs/proc/page.c b/fs/proc/page.c index 999af26c7298..936f8bbe5a6f 100644 --- a/fs/proc/page.c +++ b/fs/proc/page.c @@ -43,6 +43,22 @@ static inline unsigned long get_max_dump_pfn(void) #endif } =20 +static u64 get_kpage_count(const struct page *page) +{ + struct page_snapshot ps; + u64 ret; + + snapshot_page(&ps, page); + + if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) + ret =3D folio_precise_page_mapcount(&ps.folio_snapshot, + &ps.page_snapshot); + else + ret =3D folio_average_page_mapcount(&ps.folio_snapshot); + + return ret; +} + static ssize_t kpage_read(struct file *file, char __user *buf, size_t count, loff_t *ppos, enum kpage_operation op) @@ -75,10 +91,7 @@ static ssize_t kpage_read(struct file *file, char __user= *buf, info =3D stable_page_flags(page); break; case KPAGE_COUNT: - if (IS_ENABLED(CONFIG_PAGE_MAPCOUNT)) - info =3D folio_precise_page_mapcount(page_folio(page), page); - else - info =3D folio_average_page_mapcount(page_folio(page)); + info =3D get_kpage_count(page); break; case KPAGE_CGROUP: info =3D page_cgroup_ino(page); --=20 2.49.0