LKML Summary Podcast

Audio: COMING SOON

For the weekend of June 13th 2010, I’m Jon Masters with a summary of the past week’s LKML traffic.

In today’s issue: Linux 2.6.35-rc3, ext3, floppy, Kconfig, Memory Corruption, and Suspend Blockers.

Linux 2.6.35-rc3. Linus Torvalds announced the latest RC of 2.6.35 on Friday, June 11th 2010 at 8:01pm Best Coast Time (PDT). Quoting Linus, “So I’ve been hardnosed now for a week – perhaps over so – and hopefully that means that 2.6.35-rc3 will be better than -rc2 was. Not only do we have a number of regressions handled, we don’t have that silly memory corruptor that bit so many people with -rc2 and confused people with its many varied forms of bugs it seemed to take, depending on just what random memory it happened to corrupt. One effect of being strict is that this is likely the smallest -rc3 we’ve had in a long long time.” Linus has been on a bit of a crusade in recent times to reduce the churn post merge-window, which is what he means in describing himself as being “hardnosed” to take patches.

Ext3. Jeff Merkey posted a thread entitled “EXT3 File System Corruption in 2.6.34″, in which – in between his usual extraneous use of 4-letter words and outright offensiveness – he raised an issue that he was seeing on some systems running ext3 and recovering from a power failure. His workload raised an older concern that has been addressed before: file payload corruption following a power failure. The filesystem recovered its journal, but the content of the file was replaced with random other data (not either the original data, or the new data, as might have been expected). Jeff and Eric Sandeen had a discussion about the need to mount with data=ordered vs. the fact that Linus had previously changed the default such that the kernel config option CONFIG_EXT3_DEFAULTS_TO_ORDERED is no longer the default. Ted T’so noted that in lieu of explicit use of syncing within apps, everything is a tradeoff. Users could always mount with “-o sync” to guarantee things hit the disk, “but then the performance will be horrible”.

Floppy. Stephen Hemminger noted a number of issues with the very legacy floppy driver. Linus wondered why anyone really cared about floppies any more, but he agreed that oopsing was always bad. Stephen says he has a few fixes for the immediate issues that he will target for 2.6.36 (most of these were generated under virtualization without using real hardware), and both he and Linus seem quite interested in converting floppy over to threaded irqs, amongst other things. As a super special offer, after reading an online report on a website selling floppy drives (in which one purchaser noted that 3 out of 5 floppy drives had failed within 45 days) Linus even agreed to invest a cool $7.99 to pay for a real floppy disk drive for anyone twisted enough to really want to fix that code, while he himself will not “touch it…with a 10-foot pole”.

Kconfig. Vegard Nossum mailed to let everyone know that he has been accepted into this year’s Google Summer of Code (GSoC) program and will spend the summer working on integrating a “proper boolean constraint satisfiability solver into the configuration editors (menuconfig, etc.) in order to allow partial/incomplete configuration specifications. In short, this means that the user can choose to not specify a particular value for some config options, but let the system deduce their values”. There was some talk of the libzypp library used by Novell’s zypper boolean SAT enabled package manager, though James Bottomley noted that the difference here is that while sometimes package combinations result in an unsatisfiable install, kernel configuration should always be resolvable int a set of valid config options – a checker can be used to flag up broken Kconfig combinations and fix them. This conversation actually began a few weeks back, but had not previously been covered here.

Memory Corruption. It has been well known for some time that certain firmware (PC BIOS software) can be particularly prone to cause low memory (less than 1MB) corruption during suspend or other state transitions, including a mere call into a BIOS-provided System Management Mode (SMM) function. Yuhong Bao noted that the problem was also known to Microsoft, and he pointed to a Microsoft WHDC article covering the problem (and the fact that Windows 7 does not typically make use of low memory for this reason) in a posting entitled “Windows side agrees that lowmem corruption is a problem too”. Ingo Molnar noted that in one corruption case he saw, there was an on screen graphical bitmap that had been spewed all over the low memory. Peter Anvin suggested to just not make use of the first few pages (64K) of “low memory” region at all in order to avoid the increasing number of systems that corrupt it taking down the kernel and causing a system crash. He said that the rest of the lowest 1MB could be kept for ZONE_DMA use only “or something”.

Suspend Blockers. Ingo Molnar, replying to a separate message from Ted T’so, begun a lengthy thread entitled “suspend blockers & Android integration”. Google’s Android platform implements suspend blockers as a means to do what is implied in the naming: provide a means for applications to prevent the system from entering a suspend state. In the case of Android, this is apparently because it will attempt to suspend even with running applications in many cases where it has not been explicitly told not to, to save power. Google’s feature is interesting, but there have been some obstacles to it going into the upstream kernel, due it’s semi-invasive nature (and the objection of some to adding such features to the kernel). Ingo objected to Ted’s comment that “hundreds of engineering hours have been made trying to accomodate the demands of LKML — and LKML has said no to suspend blockers/wakelocks”, noting the many possible features that might have been added to the kernel if engineering time was the sole driving factor, rather than a pursuit of a more perfect option. Ingo believes “Linux is an engineering effort that has literally cost about ten thousand man years. That’s about a 85 million man hours. It takes effort to keep that kind of work valuable!”. Linux Weekly News had a lengthy article on this topic, so I’m going to suggest you subscribe and read that for more detail.

In today’s miscellaneous items:

*). Linus Walleij posted a patch implementing MTDparts style partition table specification via kernel command line parameter. The rationale here is that not all embedded systems use standard partition tables, as was already the case with MTD partitions. This extends the MTD concept to cover regular block devices on some newer embedded devices.

*). Yanmin Zhang reported a hang in btrfs related to calling sync() at the end of a test cycle. The kernel would ultimately output a hangcheck warning.

*). Some online controller reset patches for megaraid_sas from Bo Yang.

*). Jari Ruusu reported a module reference counting issue affecting block device mount and unmount that was caused by a bug introduced in a patch from Tejun Heo. Al Viro rediscovered the problem (bd_start_claiming grabs an extra reference that is never released), which Tejun had already fixed and was queued up for inclusion in a later kernel tree.

*). Patrick J. LoPresti posted a patch for a NULL pointer dereference bug he thought he had found in the device mapper multi-path code, and wondered aloud whether the Coverity folks were still running their nightly checks (he thought this was something that should have been flagged up by Coverity automatically in the course of such checks). Last time I heard, Coverity had had some issues with automated nightly runs but scan.coverity.com seems to still contain various data pertaining to linux-2.6.

*). Alan Olsen (whose name I had to extract from whois records because his email client is not configured to include it) noted a modpost segfault that Krzysztof Halasa tracked down to a faulty offset calculation in reading module license data. It was wondered why this had been ok with previous GCC releases, but in any case was urgently suggested for 2.6.35.

In today’s announcements:

*). Karel Zak announced the latest RC version of the util-linux-ng package is available at ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/v2.18/ The release candidate removes the rdev, ramsize, vidmode, and rootflags legacy utilities, adds a new libmount library for utlities such as mount (though the library API is not yet officially stable), and adds other commands cuh as findmnt, fsfreeze, swaplable, and so forth.

*). Robert P J Day announced that he has created an “online beginner’s kernel programming course” for $39, the details of which are available at http://www.crashcourse.ca Robert notes on that site that he will require a course text in the form of Robert Love’s upcoming third edition of “Linux Kernel Development”. That seems reasonable – this author already has a copy on pre-order, and it will be officially released on July 5th.

The latest kernel release is 2.6.35-rc3.

That’s a summary of today’s Linux Kernel Mailing List traffic, for further information visit www.kernel.org. I’m Jon Masters.