March 2014 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Storing GUIDs in linux [filesystems]
by Sankar P March 27, 2014

March 27, 2014

Hi, http://en.wikipedia.org/wiki/Globally_unique_identifier is one way to uniquely identify users. It is used by Windows and Active Directory too. The size of a GUID is 128 bits. I want to store the GUID of an user on my experimental filesystem. The size of the "uid" in UNIX is just 16 bits. So what is the way used by the Linux [filesystems] to store the GUID in linux to uniquely identify an user ? Were there any proposals in the past, to increase the size of the unix uid to match that of a GUID ? -- Sankar P http://psankar.blogspot.com

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nf_conntrack: table full .. leads to reboot
by Pietro Paolini March 26, 2014

March 26, 2014

Hello, I am doing some investigation regarding an error I am experiencing using my linux embedded device as router in my home network. If I try to push the number the connections to around 20K I will se a lot of messages from the kernel saying : nf_conntrack: table full, dropping packet. nf_conntrack: table full, dropping packet. nf_conntrack: table full, dropping packet. nf_conntrack: table full, dropping packet. nf_conntrack: table full, dropping packet. nf_conntrack: table full, dropping packet. nf_conntrack: table full, dropping packet. Actually this is fine for me since the number of allowed conntrack is /proc/sys/net/netfilter/nf_conntrack_max is around 7844, the strange thing is that the device will reboot after a while - actually my watchdog reboots the device since is not able to allocate memory - I have a try with slabtop running and I have seen that but monitoring the /proc/slabinfo: 3968 nf_conntrack_c0d4eacc 4392 size-8192 6360 skbuff_head_cache 15356 size-4096 31808 size-1024 Well size-1024 is quite high, do you know where is that from and how is that related to the packets coming in ? Even point me at the right folder in the source code could be great since I am really lost here. Thanks in advance, Pietro.

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some questions on sparc reference memory management (SRMMU)
by Chan Kim March 26, 2014

March 26, 2014

Hi, folks, While I was trying to open a shell on the LCD, I found printk from inside the kernel and printf from user side busybox use the same frame buffer driver (and eventually bit blitting functions) as my previous email said. I later found the reason printf doesn't work in my case is that the page table is not correctly setup for context 1 which the bit blitting function is in while accessing the frame buffer memory for printf. When doing the printk, the context is 0. I used srmmu_swprobe function to check the page table entry for the frame buffer address. I can get the physical address corresponding to the virtual address.(by the way, I think the address for pixel data is a dma address which should bypass cache, and it still needs virtual-physical address translation..) When srmmu_swprobe probes the page table, it bypasses the mmu translation of course. The context table base and current context value are kept in MMU registers (in 0x100 and 0x200 each) What I found : when in context 0 and context 1 context table address was the same 0x60540800. and then I just found while blitting the printk text, the current context value is 0 and all the entries in the context table is 0x6054001.(I checked for context 0,1,2 entries). They are all the same! But while doing printf, (current context was 1), all the entries in the context table was 0x6054a41. (also checked for context 0,1,2) in context 0, the level 0 table starts at 0x60540000 and for context 1 it starts at 0x6054a400. (first level table has 1024 bytes, so the pointer is interpreted as pointing 1024 byte chunks, so right 4 bits are masked off and it is 4 bit left shifted,= 256 words) Q1 : I previously thought the context table should remain the same and be added an entry as new context is added but why has the pointer for context 0 been changed after busybox loading? Q2 : and in context 1 case,(where I can't access the frame buffer), when I read the address pointed to by the context 1 pointer, it reads zero. so something is wrong. It should contain either PTE or PTD. When does the OS setup the page table? I guess it does so while loading a program using sys_execve. somebody please confirm.. where in the code should I look? Q3 : when reading the value pointed to by the context table entry, the code below is used. (I'm using a sparc processor designed in our team.) ptr = (pgd & SRMMU_PTD_PMASK) << 4; // SRMMU_PTD_PMASK is 0xffff fff0 in our case. ptr += ((((vaddr) >> SNAKE_PGD_SH) & SNAKE_PGD_M) * 4); // SNAKE_PGD_SH is 24 and SNAKE_PGD_M is 0xff in our case pmd = SNAKE_BYPASS_LOAD_PA(ptr); // pmd reads 0 when in context 1 (read bypasses mmu translation here) in the second line, the 8 bit index from the virtual address is multiplied by 4 to make it point to 1024 byte chunks. (The first level table is of size 1024 by the SPARC reference MMU). But in the first line, why is it 4 bit left shfited? This may take any alignment, but is there any reason? I know I have to read more (and some part again) about the linux memory management but hope to hear some explanations or hints. best regards, Chan

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System shutdown sequence
by Rahul Bedarkar March 26, 2014

March 26, 2014

Hi, When system is going for shutdown, shutdown handler of each device driver is called. Does kernel in this sequence calls remove handler ? I believe that remove is not called if driver is built-in. Is my understanding correct ? Thanks, Rahul

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[Audio Driver Low power state]
by Gomathi Kumar March 26, 2014

March 26, 2014

Hi all, I am new to kernel work and am trying to understand the low power state of audio driver. I am using ALC262 codec, hda_intel and 3.10 kernel code. In this codec, d3_stop_clk flag is not set. So in the code flow I was able to understand the following. if (!codec->pm_down_notified && !bus->power_keep_link_on && (state & AC_PWRST_CLK_STOP_OK)) this particular condition in hda_power_work function fails, as power_keep_link_on flag is directly based on d3_stop_clk flag. codec->d3_stop_clk = snd_hda_codec_get_supported_ps(codec, fg, AC_PWRST_CLKSTOP); I modified the if condition to not check the power_keep_link_on flag. After this change the audio is consuming lesser power, but I am not able to see it going to suspend/runtime suspend. *Please clarify what is happening when I modify the if condition and how audio device is consuming low power?* *Also please help me understand, the significance of flags EPSS and D3_STOP_CLK and if they can be set by software code. * Thanks, Gomathi

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Selecting a Linux Kernel Bug
by Ashwin Jha March 26, 2014

March 26, 2014

Hi All, I am a first year graduate student at Indian Statistical Institute, Kolkata, India. I have to do a small OS assignment. As I always wanted to contribute to Linux, I am thinking of working on a Linux kernel bug or a new feature as part of my assignment. I have never worked on Linux kernel before. So, I need some help for selecting a bug or a feature that can be resolved in a month's time. The time period is not strict but desired. What I really want is a problem that will help me in building a good understanding of Linux kernel. -- Regards, Ashwin Jha

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regarding ext3 and RDF sesisons
by sham pavman March 25, 2014

March 25, 2014

Hi all, I'm creating a RDF session and using ext3 as my FS type. What i've noticed is that once the synchronization has been initiated between R1 and R2 i am unable to mount the device on R2 site. I always get a "Bad superblock error" . However if the RDF session is stopped or split then i'm able to mount the same device on R2 site. What i've so far found out is that when i try to mount (even with -r and -noload options). I see that the Journaling still seems to be active and during mount dmesg throws up error saying that "Journal recovery failed" . I dont have any debug tools to even follow the mount syscall. So i'm having to go through the code. But i'm not able to get the call stack once the mount syscall is called. Can anyone please help me find any docs or can you tell me how the call flow begins after the mount syscall is called. I'm aware of the VFS layer inbetween but i want to know the exact starting point to start tracing from. Thanks

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Managing kernel-updates in rpm
by Saket Sinha March 25, 2014

March 25, 2014

Hi, We have a rpm which installs a linux kernel driver. Now this driver has some kernel-level dependencies especially Development Tools (kernel-headers etc) such that they depend on running kernel version. Now in the rpm we provide drivers for different kernel versions since the user can update the kernel. In the rpm's SPEC file, we match the running kernel version with our driver for that particular version and install it. If the user has updated its kernel to such a version whose driver is not present in the RPM, the installation fails. We tell user in the error message to update the kernel to a supported version. Now this is very cumbersome and we plan to replace it with installing a yum plugin through our rpm which allows user to update the kernel level dependencies. Is this a right approach? Regards, Saket Sinha

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Vmalloc Information Needed
by Anup Buchke March 25, 2014

March 25, 2014

For a user/kernel configuration of 3/1GB and (0-16M DMA , 16-896 - Low , 896 - 1024 - High ) *Q:* Is amount of memory allocated to Vmalloc limited to 128MB? I read an article which said to increase the Vmalloc allocation size we need to either *1)* Change user/kernel distribution like 2.75/1.25 which proportionately increase the address space available to High memory *2)* For a fixed user/kernel distribution reduce the low memory like 16-800 thereby increasing the size available for high memory. In either of the case the memory for vmalloc is limited to user/kernel distribution . i.e. the amount of virtual address space available in high memory. I am a beginner, so please let me know if my understanding is correct..and rectify in-case of wrong.? Thanks, Anup Buchke.

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Re: How Kernel stack is used in case of different processor mode in ARM architecture?
by Arun KS March 25, 2014

March 25, 2014

On Tue, Mar 25, 2014 at 4:40 PM, Arun KS <getarunks(a)gmail.com> wrote: > Hello Rahul, > > On Tue, Mar 25, 2014 at 4:10 PM, Rahul Garg <rahul.lnmiit(a)gmail.com> wrote: >> Hi Arun, >> >> Lines from Robert Love : >> >> Early in the 2.6 kernel process, an option was added to reduce the >> stack size from two >> pages down to one, providing only a 4KB stack on 32-bit systems.This >> reduced memory >> pressure because every process on the system previously needed two >> pages of contiguous, >> nonswappable kernel memory.To cope with the reduced stack size, >> interrupt handlers >> were given their own stack, one stack per processor, one page in >> size.This stack is referred >> to as the interrupt stack.Although the total size of the interrupt >> stack is half that of the >> original shared stack, the average stack space available is greater >> because interrupt handlers >> get the full page of memory to themselves. > > Kernel stack size is architecture dependent. Some architecture uses > CONFIG_4KSTACKS to choose 4K stacks. > > Where as in arm, it is always 8KB. > > File:arch/arm/include/asm/thread_info.h > #define THREAD_SIZE_ORDER 1 > #define THREAD_SIZE 8192 > #define THREAD_START_SP (THREAD_SIZE - 8) > >> >> >> So with these lines, it is clear that interrupt stack is used by >> interrupt handlers. So can you please re-confirm your answer ? > On ARM when there is an irq, the processor switches to irq mode. But > the kernel switches to svc mode immediately and uses SVC stack, ie the > stack of the current process. > > Why do you believe my be? :-) > If you want re-confirmation, go through arch/arm/kernel/entry-armv.S > > Thanks, > Arun >> >> And one more thing, as you mentioned only interrupt, undefined and >> abort have stack, So how nested interrupt is handled because for that >> we need System mode stack ? Interrupts are no more nested in linux kernel, https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/kern… But nested exceptions(data, prefetch aborts etc) can still happen. And, what made you believe we need system mode to support nesting? What difference does it make if it is svc mode? Thanks, Arun > ARM linux never use system mode for anything. > >> >> Regards >> Rahul >> >> On Tue, Mar 25, 2014 at 3:06 PM, Arun KS <getarunks(a)gmail.com> wrote: >>> On Tue, Mar 25, 2014 at 2:58 PM, Arun KS <getarunks(a)gmail.com> wrote: >>>> Hello Rahul, >>>> >>>> On Tue, Mar 25, 2014 at 6:29 AM, Rahul Garg <rahul.lnmiit(a)gmail.com> wrote: >>>>> As I understand every process have a user stack and kernel stack. >>>> True. >>>> >>>>> Apart from that there is a stack for every mode in ARM achitecture. So >>>> This is wrong. >>>> Only irq, abort and undefined modes have stacks in linux. That too is >>>> very limited, 3 bytes per mode per cpu. >>>> Have a look at arch/arm/kernel/irq.c >>> Sorry. Wrong file, its at arch/arm/kernel/setup.c >>> >>> Thanks, >>> Arun >>>> struct stack { >>>> u32 irq[3]; >>>> u32 abt[3]; >>>> u32 und[3]; >>>> } ____cacheline_aligned; >>>> >>>> kernel runs in SVC mode and the stack used belong to the kernel stack >>>> of the current task. >>>> Even irq, abort and undefined exception handlers use kernel stack of >>>> current task. All the exception >>>> handlers switch to SVC mode at a very early stage and use kernel >>>> stack. Those 3 bytes are used >>>> as stack just during the transition phase(for example transition from >>>> irq to svc mode during and interrupt). >>>> >>>> Thanks, >>>> Arun >>>>> I want to know How different stack and stack pointer works in ARM >>>>> modes? Also when this kernel stack associated with the process will be >>>>> used ? >>>>> >>>>> _______________________________________________ >>>>> Kernelnewbies mailing list >>>>> Kernelnewbies(a)kernelnewbies.org >>>>> http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies

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