alistair23-linux

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Gabriel Krisman Bertazi 1dab82dd20 um: ubd: Submit all data segments atomically [ Upstream commit `fc6b6a872d` ] Internally, UBD treats each physical IO segment as a separate command to be submitted in the execution pipe. If the pipe returns a transient error after a few segments have already been written, UBD will tell the block layer to requeue the request, but there is no way to reclaim the segments already submitted. When a new attempt to dispatch the request is done, those segments already submitted will get duplicated, causing the WARN_ON below in the best case, and potentially data corruption. In my system, running a UML instance with 2GB of RAM and a 50M UBD disk, I can reproduce the WARN_ON by simply running mkfs.fvat against the disk on a freshly booted system. There are a few ways to around this, like reducing the pressure on the pipe by reducing the queue depth, which almost eliminates the occurrence of the problem, increasing the pipe buffer size on the host system, or by limiting the request to one physical segment, which causes the block layer to submit way more requests to resolve a single operation. Instead, this patch modifies the format of a UBD command, such that all segments are sent through a single element in the communication pipe, turning the command submission atomic from the point of view of the block layer. The new format has a variable size, depending on the number of elements, and looks like this: +------------+-----------+-----------+------------ \| cmd_header \| segment 0 \| segment 1 \| segment ... +------------+-----------+-----------+------------ With this format, we push a pointer to cmd_header in the submission pipe. This has the advantage of reducing the memory footprint of executing a single request, since it allow us to merge some fields in the header. It is possible to reduce even further each segment memory footprint, by merging bitmap_words and cow_offset, for instance, but this is not the focus of this patch and is left as future work. One issue with the patch is that for a big number of segments, we now perform one big memory allocation instead of multiple small ones, but I wasn't able to trigger any real issues or -ENOMEM because of this change, that wouldn't be reproduced otherwise. This was tested using fio with the verify-crc32 option, and by running an ext4 filesystem over this UBD device. The original WARN_ON was: ------------[ cut here ]------------ WARNING: CPU: 0 PID: 0 at lib/refcount.c:28 refcount_warn_saturate+0x13f/0x141 refcount_t: underflow; use-after-free. Modules linked in: CPU: 0 PID: 0 Comm: swapper Not tainted 5.5.0-rc6-00002-g2a5bb2cf75c8 #346 Stack: 6084eed0 6063dc77 00000009 6084ef60 00000000 604b8d9f 6084eee0 6063dcbc 6084ef40 6006ab8d e013d780 1c00000000 Call Trace: [<600a0c1c>] ? printk+0x0/0x94 [<6004a888>] show_stack+0x13b/0x155 [<6063dc77>] ? dump_stack_print_info+0xdf/0xe8 [<604b8d9f>] ? refcount_warn_saturate+0x13f/0x141 [<6063dcbc>] dump_stack+0x2a/0x2c [<6006ab8d>] __warn+0x107/0x134 [<6008da6c>] ? wake_up_process+0x17/0x19 [<60487628>] ? blk_queue_max_discard_sectors+0x0/0xd [<6006b05f>] warn_slowpath_fmt+0xd1/0xdf [<6006af8e>] ? warn_slowpath_fmt+0x0/0xdf [<600acc14>] ? raw_read_seqcount_begin.constprop.0+0x0/0x15 [<600619ae>] ? os_nsecs+0x1d/0x2b [<604b8d9f>] refcount_warn_saturate+0x13f/0x141 [<6048bc8f>] refcount_sub_and_test.constprop.0+0x2f/0x37 [<6048c8de>] blk_mq_free_request+0xf1/0x10d [<6048ca06>] __blk_mq_end_request+0x10c/0x114 [<6005ac0f>] ubd_intr+0xb5/0x169 [<600a1a37>] __handle_irq_event_percpu+0x6b/0x17e [<600a1b70>] handle_irq_event_percpu+0x26/0x69 [<600a1bd9>] handle_irq_event+0x26/0x34 [<600a1bb3>] ? handle_irq_event+0x0/0x34 [<600a5186>] ? unmask_irq+0x0/0x37 [<600a57e6>] handle_edge_irq+0xbc/0xd6 [<600a131a>] generic_handle_irq+0x21/0x29 [<60048f6e>] do_IRQ+0x39/0x54 [...] ---[ end trace c6e7444e55386c0f ]--- Cc: Christopher Obbard <chris.obbard@collabora.com> Reported-by: Martyn Welch <martyn@collabora.com> Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com> Tested-by: Christopher Obbard <chris.obbard@collabora.com> Acked-by: Anton Ivanov <anton.ivanov@cambridgegreys.com> Signed-off-by: Richard Weinberger <richard@nod.at> Signed-off-by: Sasha Levin <sashal@kernel.org>		2021-01-06 14:48:40 +01:00
Documentation	x86/CPU/AMD: Save AMD NodeId as cpu_die_id	2020-12-30 11:51:47 +01:00
LICENSES	LICENSES: Rename other to deprecated	2019-05-03 06:34:32 -06:00
arch	um: ubd: Submit all data segments atomically	2021-01-06 14:48:40 +01:00
block	scsi: block: Fix a race in the runtime power management code	2021-01-06 14:48:37 +01:00
certs	PKCS#7: Refactor verify_pkcs7_signature()	2019-08-05 18:40:18 -04:00
crypto	crypto: ecdh - avoid unaligned accesses in ecdh_set_secret()	2020-12-30 11:51:35 +01:00
drivers	i3c master: fix missing destroy_workqueue() on error in i3c_master_register	2021-01-06 14:48:40 +01:00
fs	fs/namespace.c: WARN if mnt_count has become negative	2021-01-06 14:48:40 +01:00
include	of: fix linker-section match-table corruption	2021-01-06 14:48:37 +01:00
init	initramfs: fix clang build failure	2020-12-30 11:51:30 +01:00
ipc	ipc/util.c: sysvipc_find_ipc() incorrectly updates position index	2020-05-20 08:20:16 +02:00
kernel	module: delay kobject uevent until after module init call	2021-01-06 14:48:40 +01:00
lib	lib/syscall: fix syscall registers retrieval on 32-bit platforms	2020-12-11 13:23:32 +01:00
mm	mm: don't wake kswapd prematurely when watermark boosting is disabled	2020-12-30 11:51:27 +01:00
net	net/sched: sch_taprio: reset child qdiscs before freeing them	2021-01-06 14:48:35 +01:00
samples	samples: bpf: Fix lwt_len_hist reusing previous BPF map	2020-12-30 11:51:12 +01:00
scripts	kconfig: fix return value of do_error_if()	2020-12-30 11:51:29 +01:00
security	ima: Don't modify file descriptor mode on the fly	2020-12-30 11:51:39 +01:00
sound	ALSA: rawmidi: Access runtime->avail always in spinlock	2021-01-06 14:48:38 +01:00
tools	tools headers UAPI: Sync linux/const.h with the kernel headers	2021-01-06 14:48:37 +01:00
usr	initramfs: restore default compression behavior	2020-04-08 09:08:38 +02:00
virt	KVM: arm64: vgic-v3: Drop the reporting of GICR_TYPER.Last for userspace	2020-12-02 08:49:46 +01:00
.clang-format	clang-format: Update with the latest for_each macro list	2019-08-31 10:00:51 +02:00
.cocciconfig	scripts: add Linux .cocciconfig for coccinelle	2016-07-22 12:13:39 +02:00
.get_maintainer.ignore	Opt out of scripts/get_maintainer.pl	2019-05-16 10:53:40 -07:00
.gitattributes	.gitattributes: set git diff driver for C source code files	2016-10-07 18:46:30 -07:00
.gitignore	Modules updates for v5.4	2019-09-22 10:34:46 -07:00
.mailmap	ARM: SoC fixes	2019-11-10 13:41:59 -08:00
COPYING	COPYING: use the new text with points to the license files	2018-03-23 12:41:45 -06:00
CREDITS	MAINTAINERS: Remove Simon as Renesas SoC Co-Maintainer	2019-10-10 08:12:51 -07:00
Kbuild	kbuild: do not descend to ./Kbuild when cleaning	2019-08-21 21:03:58 +09:00
Kconfig	docs: kbuild: convert docs to ReST and rename to *.rst	2019-06-14 14:21:21 -06:00
MAINTAINERS	Documentation/llvm: add documentation on building w/ Clang/LLVM	2020-08-26 10:40:46 +02:00
Makefile	Linux 5.4.86	2020-12-30 11:51:48 +01:00
README	Drop all 00-INDEX files from Documentation/	2018-09-09 15:08:58 -06:00

README

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.