December 2012 - kernelnewbies@kernelnewbies.org

double exclamation mark meaning ?
by Shraddha Kamat Dec. 6, 2012

Dec. 6, 2012

Hi, # define likely(x) __builtin_expect(!!(x), 1) # define unlikely(x) __builtin_expect(!!(x), 0) I see these !! in the above expressions. Please explain me the meaning of those double exclamations. What is their significance ? -- Shraddha

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transfering pages from user space to user space
by Pablo Pessolani Dec. 6, 2012

Dec. 6, 2012

Hi: I am working on a project to copy (page aligned) the buffer content of one process to the buffer of other process. Now I resolved this issue using copy_page() but, analizing performance with different buffer sizes, the "copy_page" becames the critical time component and limiting factor. I read some articles about, COW, zero copy and page flipping and I wrote a kernel code to implement page "transfer" (only copy page references), but that code fail and hang. The kernel I use is 2.6.32. The pseudocode I think is: pte_t *src_pte, *dst_pte; src_pte = getpte(src_task); dst_pte = getpte(dst_task); free_pte(dst_pte); /*to free the page referenced by this PTE */ *dst_pte = *src_pte; /*copy source PTE to destination PTE */ And here is code added to the kernel: ------------------------------------------------------------------------------------------------------------- len = PAGE_SIZE; src_vma=find_vma_intersection(src_proc->p_task->mm,src_addr,src_addr+len); dst_vma=find_vma_intersection(dst_proc->p_task->mm,dst_addr,dst_addr+len); src_pgd=pgd_offset(src_proc->p_task->mm, (unsigned long) src_addr); src_pud=pud_offset(src_pgd,(unsigned long) src_addr); src_pmd=pmd_offset(src_pud,(unsigned long) src_addr); src_pte=pte_offset_map_lock(src_proc->p_task->mm, src_pmd,(unsigned long) src_addr, &src_ptl); dst_pgd=pgd_offset(dst_proc->p_task->mm, (unsigned long) dst_addr); dst_pud=pud_offset(dst_pgd,(unsigned long) dst_addr); dst_pmd=pmd_offset(dst_pud,(unsigned long) dst_addr); dst_pte=pte_offset_map_lock(dst_proc->p_task->mm, dst_pmd,(unsigned long) dst_addr, &dst_ptl); pte_free(dst_proc->p_task->mm, dst_pte); copy_pte_range(dst_proc->p_task->mm, src_proc->p_task->mm, dst_pmd, src_pmd, dst_vma, (unsigned long) dst_addr, ((unsigned long)dst_addr+PAGE_SIZE)); spin_unlock(src_ptl); spin_unlock(dst_ptl); ------------------------------------------------------------------------------------------------------------- There are a lots of kernel functions that are not well documented or they have been changed. Can anybody help me with this issue? Does COW (Copy On Write) will make the copy when the processes will modify their buffers? I am not suscribed to the mailing list, please CC: the answers to this email account. Thanks in Advance. I apologize for my basic English. Pablo Pessolani PD: once finishing with that code, some issues about page protection will be considered.

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(no subject)
by Niroj Pokhrel Dec. 5, 2012

Dec. 5, 2012

Hi, I'm trying to work on android audio (pcm_native.c) but got stuck in some parameters like start threshold, stop threshold, silence zone, silence threshold. Can anybody please elaborate on what they are each used for ?? -- Niroj Pokhrel Software Engineer, Samsung India Software Operations

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Max open file limit
by Vijay Chauhan Dec. 5, 2012

Dec. 5, 2012

Hello, How many files a process can open at a time? Is it configurable? I found following in the kernel code: .. .max_fds = NR_OPEN_DEFAULT, .. .. #define NR_OPEN_DEFAULT BITS_PER_LONG .. .. #ifdef __KERNEL__ #define BITS_PER_LONG 32 .. But I can open more than 32 files in my user space program. Thank you, Vijay

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Getting address of a symbol from kernel's symbol table.
by arif Dec. 3, 2012

Dec. 3, 2012

arif@khost:~/src/linux$ global -x ip_rcv_finish ip_rcv_finish 319 net/ipv4/ip_input.c static int ip_rcv_finish(struct sk_buff *skb) Now if i want to use this function i need to initialize a pointer to this function. To be able to do that i need the address of the function. I've seen that from user space i can read /proc/kallsyms to get an address of a symbol. Is their any similar mechanism exist where i can read the symbol table to extract a symbol's address from kernel space?

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What is modpost ?
by Pritam Bankar Dec. 3, 2012

Dec. 3, 2012

Hi guys, I want to confirm my understanding regarding modpost and Module.Symvers files. >From my understanding a. modpost is used to add versioning information to module. b. modpost is also used to create mod.c and Module.Symvers files per module c. then we compile all *.mod.c files and link it to .ko Is my understanding correct ? For what else purpose modpost is useful ? What is specific data in mod.c file ? Thanks, Pritam

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Qemu - Writing Plugins
by Rishi Agrawal Dec. 3, 2012

Dec. 3, 2012

Hi All, This may not be the right forum for this question but I am giving it a try. I am looking for a small "hello world" to writing qemu plugins. i am still googling it. Any kind of help is appreciated. -- Regards, Rishi Agrawal

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topic about old linux kernel version 0.11
by horse_rivers Dec. 3, 2012

Dec. 3, 2012

hi, I use bochs to boot linux 0.11 kernel .I put kernel image to floppy , when it run to mount root-file system , it prints "reset floppy called" hoe to continue with this issure? thanks!

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how to use kgtp in kernel debuging ?
by horse_rivers Dec. 3, 2012

Dec. 3, 2012

hi: I want to trace kernel of linux 1.0 version ,and now I have compile sucessfully a 1.0 kernel with source code . but how to use kgtp tools to trace kernel ? what is target side and what is host ? thanks!

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insmod: error inserting 'test2.ko': -1 Unknown symbol in module
by Pritam Bankar Dec. 3, 2012

Dec. 3, 2012

Hi, What is basic reason behind error "Unknown symbol in module" ? I have two modules test1.ko and test2.ko. test1.ko loads succesfully but test2.ko gives error insmod: error inserting 'test2.ko': -1 Unknown symbol in module dmesg shows all symbols in module as unknown. To give more details I have compiled both modules same way with same compiler options and with modpost, Module.Symvers files. Another point to mention is that both modules load perfectly on 2.6 kernel. But kernel on which I am trying is 3.0 series kernel. What can be entry point to look into this issue ? Thanks, Pritam

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