From patchwork Sun May 28 21:31:05 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Filipe Manana X-Patchwork-Id: 9756557 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 17B51602BF for ; Wed, 31 May 2017 10:10:11 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 1295B262AE for ; Wed, 31 May 2017 10:10:11 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 05604283BA; Wed, 31 May 2017 10:10:11 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-6.9 required=2.0 tests=BAYES_00,RCVD_IN_DNSWL_HI autolearn=ham version=3.3.1 Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id AE5F2262AE for ; Wed, 31 May 2017 10:10:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751021AbdEaKKH (ORCPT ); Wed, 31 May 2017 06:10:07 -0400 Received: from mail.kernel.org ([198.145.29.99]:59116 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750995AbdEaKKG (ORCPT ); Wed, 31 May 2017 06:10:06 -0400 Received: from debian3.lan (bl12-226-64.dsl.telepac.pt [85.245.226.64]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 1FABB23A0B for ; Wed, 31 May 2017 10:10:04 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 1FABB23A0B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=fdmanana@kernel.org From: fdmanana@kernel.org To: linux-btrfs@vger.kernel.org Subject: [PATCH v2] Btrfs: fix invalid extent maps due to hole punching Date: Sun, 28 May 2017 22:31:05 +0100 Message-Id: <20170528213105.19249-1-fdmanana@kernel.org> X-Mailer: git-send-email 2.11.0 In-Reply-To: <20170528163153.31962-1-fdmanana@kernel.org> References: <20170528163153.31962-1-fdmanana@kernel.org> Sender: linux-btrfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-btrfs@vger.kernel.org X-Virus-Scanned: ClamAV using ClamSMTP From: Filipe Manana While punching a hole in a range that is not aligned with the sector size (currently the same as the page size) we can end up leaving an extent map in memory with a length that is smaller then the sector size, which is not expected and can lead to problems. This issue is easily detected after the patch from commit a7e3b975a0f9 ("Btrfs: fix reported number of inode blocks"), introduced in kernel 4.12-rc1, in a scenario like the following for example: $ mkfs.btrfs -f /dev/sdb $ mount /dev/sdb /mnt $ xfs_io -c "pwrite -S 0xaa -b 100K 0 100K" /mnt/foo $ xfs_io -c "fpunch 60K 90K" /mnt/foo $ xfs_io -c "pwrite -S 0xbb -b 100K 50K 100K" /mnt/foo $ xfs_io -c "pwrite -S 0xcc -b 50K 100K 50K" /mnt/foo $ umount /mnt After the unmount operation we can see several warnings emmitted due to underflows related to space reservation counters: [ 2837.443299] ------------[ cut here ]------------ [ 2837.447395] WARNING: CPU: 8 PID: 2474 at fs/btrfs/inode.c:9444 btrfs_destroy_inode+0xe8/0x27e [btrfs] [ 2837.452108] Modules linked in: dm_flakey dm_mod ppdev parport_pc psmouse parport sg pcspkr acpi_cpufreq tpm_tis tpm_tis_core i2c_piix4 i2c_core evdev tpm button se rio_raw sunrpc loop autofs4 ext4 crc16 jbd2 mbcache btrfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c crc32c_gene ric raid1 raid0 multipath linear md_mod sr_mod cdrom sd_mod ata_generic virtio_scsi ata_piix libata virtio_pci virtio_ring virtio e1000 scsi_mod floppy [ 2837.458389] CPU: 8 PID: 2474 Comm: umount Tainted: G W 4.10.0-rc8-btrfs-next-43+ #1 [ 2837.459754] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014 [ 2837.462379] Call Trace: [ 2837.462379] dump_stack+0x68/0x92 [ 2837.462379] __warn+0xc2/0xdd [ 2837.462379] warn_slowpath_null+0x1d/0x1f [ 2837.462379] btrfs_destroy_inode+0xe8/0x27e [btrfs] [ 2837.462379] destroy_inode+0x3d/0x55 [ 2837.462379] evict+0x177/0x17e [ 2837.462379] dispose_list+0x50/0x71 [ 2837.462379] evict_inodes+0x132/0x141 [ 2837.462379] generic_shutdown_super+0x3f/0xeb [ 2837.462379] kill_anon_super+0x12/0x1c [ 2837.462379] btrfs_kill_super+0x16/0x21 [btrfs] [ 2837.462379] deactivate_locked_super+0x30/0x68 [ 2837.462379] deactivate_super+0x36/0x39 [ 2837.462379] cleanup_mnt+0x58/0x76 [ 2837.462379] __cleanup_mnt+0x12/0x14 [ 2837.462379] task_work_run+0x77/0x9b [ 2837.462379] prepare_exit_to_usermode+0x9d/0xc5 [ 2837.462379] syscall_return_slowpath+0x196/0x1b9 [ 2837.462379] entry_SYSCALL_64_fastpath+0xab/0xad [ 2837.462379] RIP: 0033:0x7f3ef3e6b9a7 [ 2837.462379] RSP: 002b:00007ffdd0d8de58 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 [ 2837.462379] RAX: 0000000000000000 RBX: 0000556f76a39060 RCX: 00007f3ef3e6b9a7 [ 2837.462379] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000556f76a3f910 [ 2837.462379] RBP: 0000556f76a3f910 R08: 0000556f76a3e670 R09: 0000000000000015 [ 2837.462379] R10: 00000000000006b4 R11: 0000000000000246 R12: 00007f3ef436ce64 [ 2837.462379] R13: 0000000000000000 R14: 0000556f76a39240 R15: 00007ffdd0d8e0e0 [ 2837.519355] ---[ end trace e79345fe24b30b8d ]--- [ 2837.596256] ------------[ cut here ]------------ [ 2837.597625] WARNING: CPU: 8 PID: 2474 at fs/btrfs/extent-tree.c:5699 btrfs_free_block_groups+0x246/0x3eb [btrfs] [ 2837.603547] Modules linked in: dm_flakey dm_mod ppdev parport_pc psmouse parport sg pcspkr acpi_cpufreq tpm_tis tpm_tis_core i2c_piix4 i2c_core evdev tpm button serio_raw sunrpc loop autofs4 ext4 crc16 jbd2 mbcache btrfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c crc32c_generic raid1 raid0 multipath linear md_mod sr_mod cdrom sd_mod ata_generic virtio_scsi ata_piix libata virtio_pci virtio_ring virtio e1000 scsi_mod floppy [ 2837.659372] CPU: 8 PID: 2474 Comm: umount Tainted: G W 4.10.0-rc8-btrfs-next-43+ #1 [ 2837.663359] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014 [ 2837.663359] Call Trace: [ 2837.663359] dump_stack+0x68/0x92 [ 2837.663359] __warn+0xc2/0xdd [ 2837.663359] warn_slowpath_null+0x1d/0x1f [ 2837.663359] btrfs_free_block_groups+0x246/0x3eb [btrfs] [ 2837.663359] close_ctree+0x1dd/0x2e1 [btrfs] [ 2837.663359] ? evict_inodes+0x132/0x141 [ 2837.663359] btrfs_put_super+0x15/0x17 [btrfs] [ 2837.663359] generic_shutdown_super+0x6a/0xeb [ 2837.663359] kill_anon_super+0x12/0x1c [ 2837.663359] btrfs_kill_super+0x16/0x21 [btrfs] [ 2837.663359] deactivate_locked_super+0x30/0x68 [ 2837.663359] deactivate_super+0x36/0x39 [ 2837.663359] cleanup_mnt+0x58/0x76 [ 2837.663359] __cleanup_mnt+0x12/0x14 [ 2837.663359] task_work_run+0x77/0x9b [ 2837.663359] prepare_exit_to_usermode+0x9d/0xc5 [ 2837.663359] syscall_return_slowpath+0x196/0x1b9 [ 2837.663359] entry_SYSCALL_64_fastpath+0xab/0xad [ 2837.663359] RIP: 0033:0x7f3ef3e6b9a7 [ 2837.663359] RSP: 002b:00007ffdd0d8de58 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 [ 2837.663359] RAX: 0000000000000000 RBX: 0000556f76a39060 RCX: 00007f3ef3e6b9a7 [ 2837.663359] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000556f76a3f910 [ 2837.663359] RBP: 0000556f76a3f910 R08: 0000556f76a3e670 R09: 0000000000000015 [ 2837.663359] R10: 00000000000006b4 R11: 0000000000000246 R12: 00007f3ef436ce64 [ 2837.663359] R13: 0000000000000000 R14: 0000556f76a39240 R15: 00007ffdd0d8e0e0 [ 2837.739445] ---[ end trace e79345fe24b30b8e ]--- [ 2837.745595] ------------[ cut here ]------------ [ 2837.746412] WARNING: CPU: 8 PID: 2474 at fs/btrfs/extent-tree.c:5700 btrfs_free_block_groups+0x261/0x3eb [btrfs] [ 2837.747955] Modules linked in: dm_flakey dm_mod ppdev parport_pc psmouse parport sg pcspkr acpi_cpufreq tpm_tis tpm_tis_core i2c_piix4 i2c_core evdev tpm button serio_raw sunrpc loop autofs4 ext4 crc16 jbd2 mbcache btrfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c crc32c_generic raid1 raid0 multipath linear md_mod sr_mod cdrom sd_mod ata_generic virtio_scsi ata_piix libata virtio_pci virtio_ring virtio e1000 scsi_mod floppy [ 2837.755395] CPU: 8 PID: 2474 Comm: umount Tainted: G W 4.10.0-rc8-btrfs-next-43+ #1 [ 2837.756769] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014 [ 2837.758526] Call Trace: [ 2837.758925] dump_stack+0x68/0x92 [ 2837.759383] __warn+0xc2/0xdd [ 2837.759383] warn_slowpath_null+0x1d/0x1f [ 2837.759383] btrfs_free_block_groups+0x261/0x3eb [btrfs] [ 2837.759383] close_ctree+0x1dd/0x2e1 [btrfs] [ 2837.759383] ? evict_inodes+0x132/0x141 [ 2837.759383] btrfs_put_super+0x15/0x17 [btrfs] [ 2837.759383] generic_shutdown_super+0x6a/0xeb [ 2837.759383] kill_anon_super+0x12/0x1c [ 2837.759383] btrfs_kill_super+0x16/0x21 [btrfs] [ 2837.759383] deactivate_locked_super+0x30/0x68 [ 2837.759383] deactivate_super+0x36/0x39 [ 2837.759383] cleanup_mnt+0x58/0x76 [ 2837.759383] __cleanup_mnt+0x12/0x14 [ 2837.759383] task_work_run+0x77/0x9b [ 2837.759383] prepare_exit_to_usermode+0x9d/0xc5 [ 2837.759383] syscall_return_slowpath+0x196/0x1b9 [ 2837.759383] entry_SYSCALL_64_fastpath+0xab/0xad [ 2837.759383] RIP: 0033:0x7f3ef3e6b9a7 [ 2837.759383] RSP: 002b:00007ffdd0d8de58 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 [ 2837.759383] RAX: 0000000000000000 RBX: 0000556f76a39060 RCX: 00007f3ef3e6b9a7 [ 2837.759383] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000556f76a3f910 [ 2837.759383] RBP: 0000556f76a3f910 R08: 0000556f76a3e670 R09: 0000000000000015 [ 2837.759383] R10: 00000000000006b4 R11: 0000000000000246 R12: 00007f3ef436ce64 [ 2837.759383] R13: 0000000000000000 R14: 0000556f76a39240 R15: 00007ffdd0d8e0e0 [ 2837.777063] ---[ end trace e79345fe24b30b8f ]--- [ 2837.778235] ------------[ cut here ]------------ [ 2837.778856] WARNING: CPU: 8 PID: 2474 at fs/btrfs/extent-tree.c:9825 btrfs_free_block_groups+0x348/0x3eb [btrfs] [ 2837.791385] Modules linked in: dm_flakey dm_mod ppdev parport_pc psmouse parport sg pcspkr acpi_cpufreq tpm_tis tpm_tis_core i2c_piix4 i2c_core evdev tpm button serio_raw sunrpc loop autofs4 ext4 crc16 jbd2 mbcache btrfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c crc32c_generic raid1 raid0 multipath linear md_mod sr_mod cdrom sd_mod ata_generic virtio_scsi ata_piix libata virtio_pci virtio_ring virtio e1000 scsi_mod floppy [ 2837.797711] CPU: 8 PID: 2474 Comm: umount Tainted: G W 4.10.0-rc8-btrfs-next-43+ #1 [ 2837.798594] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014 [ 2837.800118] Call Trace: [ 2837.800515] dump_stack+0x68/0x92 [ 2837.801015] __warn+0xc2/0xdd [ 2837.801471] warn_slowpath_null+0x1d/0x1f [ 2837.801698] btrfs_free_block_groups+0x348/0x3eb [btrfs] [ 2837.801698] close_ctree+0x1dd/0x2e1 [btrfs] [ 2837.801698] ? evict_inodes+0x132/0x141 [ 2837.801698] btrfs_put_super+0x15/0x17 [btrfs] [ 2837.801698] generic_shutdown_super+0x6a/0xeb [ 2837.801698] kill_anon_super+0x12/0x1c [ 2837.801698] btrfs_kill_super+0x16/0x21 [btrfs] [ 2837.801698] deactivate_locked_super+0x30/0x68 [ 2837.801698] deactivate_super+0x36/0x39 [ 2837.801698] cleanup_mnt+0x58/0x76 [ 2837.801698] __cleanup_mnt+0x12/0x14 [ 2837.801698] task_work_run+0x77/0x9b [ 2837.801698] prepare_exit_to_usermode+0x9d/0xc5 [ 2837.801698] syscall_return_slowpath+0x196/0x1b9 [ 2837.801698] entry_SYSCALL_64_fastpath+0xab/0xad [ 2837.801698] RIP: 0033:0x7f3ef3e6b9a7 [ 2837.801698] RSP: 002b:00007ffdd0d8de58 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 [ 2837.801698] RAX: 0000000000000000 RBX: 0000556f76a39060 RCX: 00007f3ef3e6b9a7 [ 2837.801698] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000556f76a3f910 [ 2837.801698] RBP: 0000556f76a3f910 R08: 0000556f76a3e670 R09: 0000000000000015 [ 2837.801698] R10: 00000000000006b4 R11: 0000000000000246 R12: 00007f3ef436ce64 [ 2837.801698] R13: 0000000000000000 R14: 0000556f76a39240 R15: 00007ffdd0d8e0e0 [ 2837.818441] ---[ end trace e79345fe24b30b90 ]--- [ 2837.818991] BTRFS info (device sdc): space_info 1 has 7974912 free, is not full [ 2837.819830] BTRFS info (device sdc): space_info total=8388608, used=417792, pinned=0, reserved=0, may_use=18446744073709547520, readonly=0 [ 2837.821227] ------------[ cut here ]------------ [ 2837.821897] WARNING: CPU: 8 PID: 2474 at fs/btrfs/extent-tree.c:9825 btrfs_free_block_groups+0x348/0x3eb [btrfs] [ 2837.823331] Modules linked in: dm_flakey dm_mod ppdev parport_pc psmouse parport sg pcspkr acpi_cpufreq tpm_tis tpm_tis_core i2c_piix4 i2c_core evdev tpm button serio_raw sunrpc loop autofs4 ext4 crc16 jbd2 mbcache btrfs raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c crc32c_generic raid1 raid0 multipath linear md_mod sr_mod cdrom sd_mod ata_generic virtio_scsi ata_piix libata virtio_pci virtio_ring virtio e1000 scsi_mod floppy [ 2837.829575] CPU: 8 PID: 2474 Comm: umount Tainted: G W 4.10.0-rc8-btrfs-next-43+ #1 [ 2837.830767] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.9.1-0-gb3ef39f-prebuilt.qemu-project.org 04/01/2014 [ 2837.832407] Call Trace: [ 2837.832820] dump_stack+0x68/0x92 [ 2837.833336] __warn+0xc2/0xdd [ 2837.833561] warn_slowpath_null+0x1d/0x1f [ 2837.833561] btrfs_free_block_groups+0x348/0x3eb [btrfs] [ 2837.833561] close_ctree+0x1dd/0x2e1 [btrfs] [ 2837.833561] ? evict_inodes+0x132/0x141 [ 2837.833561] btrfs_put_super+0x15/0x17 [btrfs] [ 2837.833561] generic_shutdown_super+0x6a/0xeb [ 2837.833561] kill_anon_super+0x12/0x1c [ 2837.833561] btrfs_kill_super+0x16/0x21 [btrfs] [ 2837.833561] deactivate_locked_super+0x30/0x68 [ 2837.833561] deactivate_super+0x36/0x39 [ 2837.833561] cleanup_mnt+0x58/0x76 [ 2837.833561] __cleanup_mnt+0x12/0x14 [ 2837.833561] task_work_run+0x77/0x9b [ 2837.833561] prepare_exit_to_usermode+0x9d/0xc5 [ 2837.833561] syscall_return_slowpath+0x196/0x1b9 [ 2837.833561] entry_SYSCALL_64_fastpath+0xab/0xad [ 2837.833561] RIP: 0033:0x7f3ef3e6b9a7 [ 2837.833561] RSP: 002b:00007ffdd0d8de58 EFLAGS: 00000246 ORIG_RAX: 00000000000000a6 [ 2837.833561] RAX: 0000000000000000 RBX: 0000556f76a39060 RCX: 00007f3ef3e6b9a7 [ 2837.833561] RDX: 0000000000000001 RSI: 0000000000000000 RDI: 0000556f76a3f910 [ 2837.833561] RBP: 0000556f76a3f910 R08: 0000556f76a3e670 R09: 0000000000000015 [ 2837.833561] R10: 00000000000006b4 R11: 0000000000000246 R12: 00007f3ef436ce64 [ 2837.833561] R13: 0000000000000000 R14: 0000556f76a39240 R15: 00007ffdd0d8e0e0 [ 2837.858288] ---[ end trace e79345fe24b30b91 ]--- [ 2837.858829] BTRFS info (device sdc): space_info 4 has 1073328128 free, is not full [ 2837.859721] BTRFS info (device sdc): space_info total=1073741824, used=28672, pinned=0, reserved=0, may_use=319488, readonly=65536 What happens in the above example is the following: 1) When punching the hole, at btrfs_punch_hole(), the variable tail_len is set to 2048 (as tail_start is 148Kb + 1 and offset + len is 150Kb). This results in the creation of an extent map with a length of 2Kb starting at file offset 148Kb, through find_first_non_hole() -> btrfs_get_extent(). 2) The second write (first write after the hole punch operation), sets the range [50Kb, 152Kb[ to delalloc. 3) The third write, at btrfs_find_new_delalloc_bytes(), sees the extent map covering the range [148Kb, 150Kb[ and ends up calling set_extent_bit() for the same range, which results in splitting an existing extent state record, covering the range [148Kb, 152Kb[ into two 2Kb extent state records, covering the ranges [148Kb, 150Kb[ and [150Kb, 152Kb[. 4) Finally at lock_and_cleanup_extent_if_need(), immediately after calling btrfs_find_new_delalloc_bytes() we clear the delalloc bit from the range [100Kb, 152Kb[ which results in the btrfs_clear_bit_hook() callback being invoked against the two 2Kb extent state records that cover the ranges [148Kb, 150Kb[ and [150Kb, 152Kb[. When called against the first 2Kb extent state, it calls btrfs_delalloc_release_metadata() with a length argument of 2048 bytes. That function rounds up the length to a sector size aligned length, so it ends up considering a length of 4096 bytes, and then calls calc_csum_metadata_size() which results in decrementing the inode's csum_bytes counter by 4096 bytes, so after it stays a value of 0 bytes. Then the same happens when btrfs_clear_bit_hook() is called against the second extent state that has a length of 2Kb, covering the range [150Kb, 152Kb[, the length is rounded up to 4096 and calc_csum_metadata_size() ends up being called to decrement 4096 bytes from the inode's csum_bytes counter, which at that time has a value of 0, leading to an underflow, which is exactly what triggers the first warning, at btrfs_destroy_inode(). All the other warnings relate to several space accounting counters that underflow as well due to similar reasons. So fix the hole punching operation to make sure it never creates extent maps with a length that is not aligned to the sector size, as this breaks all assumptions and it's a land mine. Fixes: d77815461f04 ("btrfs: Avoid trucating page or punching hole in a already existed hole.") Cc: Signed-off-by: Filipe Manana --- V2: Rebased on latest for-linus-4.12 branch from Chris, so that it applies cleanly. fs/btrfs/file.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c index da1096eb1a40..928fe290e834 100644 --- a/fs/btrfs/file.c +++ b/fs/btrfs/file.c @@ -2390,10 +2390,12 @@ static int fill_holes(struct btrfs_trans_handle *trans, */ static int find_first_non_hole(struct inode *inode, u64 *start, u64 *len) { + struct btrfs_fs_info *fs_info = btrfs_sb(inode->i_sb); struct extent_map *em; int ret = 0; - em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, *start, *len, 0); + em = btrfs_get_extent(BTRFS_I(inode), NULL, 0, *start, + round_up(*len, fs_info->sectorsize), 0); if (IS_ERR(em)) return PTR_ERR(em);