May 2017 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

where is __percpu_seg initialized?
by 落痕 March 28, 2018

March 28, 2018

Hi all: I knew the kernel implement the per-cpu variable via __percpu_seg. For example on x86 architecture, it's defined as: #ifdef CONFIG_X86_64 #define __percpu_seg gs #define __percpu_mov_op movq #else #define __percpu_seg fs #define __percpu_mov_op movl #endif But I'm curious where __percpu_seg is initialized?? I fail to find the relative code. Thanks.

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How to handle Hotplug with UIO userspace driver
by Mandeep Sandhu Sept. 28, 2017

Sept. 28, 2017

Hi All, We're working on a custom PCIe based hardware, which can be hotplugged into the system. We're using the UIO approach of having a minimal kernel driver which basically handles the interrupt and provides a mmap of the device memory to userspace and userspace driver which mmap's the provided memory and manages the device. We're using systemd and udev rules to spawn off a user process whenever the hardware is plugged into the system. Now my question is, how can we gracefully inform the userspace process if someone pulls out the hardware (sitting on a PCI slot). Even with full hotplug support, there can always be a case of "surprise removal", where someone yanks the board without going through the whole "attention button" routine. While the kernel driver has no problem with hot-removal, I see that if I "unregister" our UIO driver in response to the removal, then the mapping for the hardware's physical memory would become "invalid" (done by UIO on unregsiter). This could possibly cause the user-space process to crash due to illegal memory access. Is there a "standard" way to handle this scenario, i.e for hotpluggable hardware using UIO? Basically, we'd want to "unregister" the UIO device only after the userspace process is informed of the removal so that it could unmap/close the device and exit gracefully. We could try doing it using some systemd trick, where we put a dependency on one of the UIO created files (/dev/uioX), so that when the file is destroyed, systemd could kill/shutdown the process. Although, there's no guarantee that the userspace process won't try to access the mmaped space during that interval, so this isn't a reliable solution either. Any hints, thoughts? Thanks., -mandeep

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Is vnode number also limit system-wide number of open file?
by Yubin Ruan June 1, 2017

June 1, 2017

I notice that there is a unsigned long i_ino; in definition of `struct inode' [1], which is the virtual filesystem inode. Does that mean "inode number" and is it used for indexing in the system-wide inode table? If that is the case, would that limit the number of open file in Linux? I know there *is* such a limit, and superusers can adjust that by /proc/sys/fs/file-max. Currently I cannot raise that to too high, otherwise the system would crash, which I think is because I have limited memory. But, the point is, if I have lots of memory in my machine (say hunderds of Gigabytes), would the number of open file system-wide limited by the `i_ino' above? Since its type is "unsigned long", I guess I can only open 2^(sizeof(unsigned long)) file simultaneously? -- Yubin [1]: http://elixir.free-electrons.com/linux/latest/source/include/linux/fs.h#L575

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Update a newly-created '.config' file with some predefined values
by Roman Storozhenko May 31, 2017

May 31, 2017

Hello everybody, I have a host machine and I have a VM machine running CentOS 7 with 3.x. There is a kernel source tree on the host. I have made 'make menuconfig', populated all options that I want to have in my custom kernel and as a result got a '.config' file. Then I copied a centos-default distro config file from my VM's 'boot' catalog to my host's kernel source tree catalog. So now I have two files in my source tree: '.config' and '.config.old'. I am able to see difference between them using the following command: scripts/diffconfig .config.old .config | less But is there anyway to update values in '.config' file with the apropriate values from '.config.old' file? I just want to make sure that all modules will be included in a new custom kernel. Thanks in advance, Roman

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Re: cross compiling
by juan pedro meriño May 31, 2017

May 31, 2017

Toolchin for compiler linux?

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understanding of sysfs.
by Madhu K May 29, 2017

May 29, 2017

Hi All, why there is no character device entry present in /sys/class? where as block and net device entries are present. Not only in /sys/class, for that matter why there is no character device entries present in /sys file system. please help me to understand for what purpose char devices are kept away from /sys file system. Thanks Madhu

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Applying the pagecache monitoring patch
by Yun-Chih Chen May 27, 2017

May 27, 2017

Hi: I've been trying to apply the patch here: https://lkml.org/lkml/2011/7/18/326 It is great that we can have such fine-grained monitoring on the activity of kernel page cache, isn't it? The author said: My patches are based on the latest "linux-tip.git" tree and also the following 3 commits in "tip:tracing/mm" and a "pagecache object collections" patch. - dcac8cd: tracing/mm: add page frame snapshot trace - 1487a7a: tracing/mm: fix mapcount trace record field - eb46710: tracing/mm: rename 'trigger' file to 'dump_range' - http://lkml.org/lkml/2010/2/9/156 My understanding is: - He was working on the head of linux-tip: https://kernel.googlesource.com/pub/scm/linux/kernel/git/tip/tip/ - The HEAD had cherry-picked from three commits at branch tracing/mm at another repo: https://git.kernel.org/pub/scm/linux/kernel/git/mingo/tip.git/?h=tracing/mm - Finally, he had also applied a patch posted here: https://lkml.org/lkml/2010/2/9/156 What I did is: 1. Checkout to a commit near 2011/7/18 (the latest commit at the time the patch was sent) 2. Cherry-picked the three commits from tracing/mm 3. Apply the patch I failed at step 3. Anyone has any suggestion on this? Thanks, Yunchih

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Understand I/O path in linux kernel
by jitendra kumar khasdev May 27, 2017

May 27, 2017

Hi All, I am looking to understand I/O path in linux kernel. Here is following things I have tried to understand it, 1. try to understand vfs_write 2. then move into fs specific write call 3. I thought then after the call goes to I/O scheduler. 4. then low-level device driver eg. SCSI, ide 5. these drivers read/write sectors. These are above I/O path I understood at a very high level. Please correct me if somewhere I misunderstood. I want to get an understanding of how blocks get read and write from a disk. Please suggest me something that will help me to understand and visualize data path in linux kernel. --- Jitendra

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Does anyone have experience about linux kernel dev and test in QEMU?
by Douglas Su May 26, 2017

May 26, 2017

I used to develop and test kernel driver in virtual box with full Linux Debian installed. It works but not efficient due to that I have to install a full functional Linux distribution. Previously, in this mail list, I noted some friends said that to accelerate the development process by using QEMU, a lightweight and fast VM than QEMU. After some searching works, however, I barely find any reading material or tutorial about the details of using QEMU in kernel development, especially the driver development. Can anyone give a clue?

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Lock routine for /proc file system
by Kim, Jeong-Hwan May 26, 2017

May 26, 2017

Hi, everyone Is there locking routine for protecting /proc file system? Can I access the same /proc file (ex. /proc/cpuinfo) from several tasks without explicit lock routine? Thanks in advance J.Hwan Kim

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