April 2015 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Function Pointer Question
by Nicholas Krause April 2, 2015

April 2, 2015

Greetings All, I'm a little rough with how function pointers work in C. I assumed there similar to way C++ stores virtual functions internally in the vtable for virtual functions by the compiler and checks the table at runtime. Please let me know if I'm wrong in my understanding. Thanks, Nick -- Sent from my Android device with K-9 Mail. Please excuse my brevity.

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Questions about the zoned page frame allocator and fix mapped addresses
by Sunny Shah April 1, 2015

April 1, 2015

Hello, I have few more questions from my reading of "Understanding the Linux Kernel", chapter "Memory Management". - The book says, about releasing page frames to the per CPU cache - "no page frame is ever released to the cold cache: the kernel always assumes the freed page frame is hot with respect to the hardware cache". What is the reason for this decision ? - It is possible for a page to be in ZONE_NORMAL and yet have it's PG_reserved flag cleared. Is this correct ? - The function "fix_to_virt" for fix-mapped linear addresses does the following: return (0xfffff000UL - (idx << PAGE_SHIFT)); Why are the upper 4096 bytes not used, and the addressing starts from the top of the virtual address space - 4096 ? - The book says "each fix-mapped linear address maps one page frame of the physical memory". Shouldn't it be "maps one *physical location* of memory" rather than one page frame ? - My understanding is that the kernel page table entries for addresses > 896 MB would be empty and those addresses would be mapped using separate data structures used for temporary and permanent kernel mappings and non-contiguous page frame allocation. Is this wrong ? Thanks, Sunny

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what is the precise udev event that handles miscdevices?
by Robert P. J. Day April 1, 2015

April 1, 2015

i can trivially create misc character devices, and i know special device files for them will automatically show up under /dev, but what is the precise udev event/rule that comes into play for this? if i start to monitor udevadm with: $ udevadm monitor --kernel --udev --property and insert my misc char module, i see: KERNEL[5342.740294] add /module/mymisc (module) ACTION=add DEVPATH=/module/mymisc SEQNUM=3007 SUBSYSTEM=module KERNEL[5342.740528] add /devices/virtual/misc/mymisc (misc) ACTION=add DEVNAME=/dev/mymisc DEVPATH=/devices/virtual/misc/mymisc MAJOR=10 MINOR=55 SEQNUM=3008 SUBSYSTEM=misc UDEV [5342.741071] add /module/mymisc (module) ACTION=add DEVPATH=/module/mymisc SEQNUM=3007 SUBSYSTEM=module USEC_INITIALIZED=740419 UDEV [5342.741921] add /devices/virtual/misc/mymisc (misc) ACTION=add DEVNAME=/dev/mymisc DEVPATH=/devices/virtual/misc/mymisc MAJOR=10 MINOR=55 SEQNUM=3008 SUBSYSTEM=misc USEC_INITIALIZED=740750 so, yes, i can see the line "DEVNAME=/dev/mymisc", but is there something i can point at in the collection of udev rules that deals with that? or is that simply done internally? rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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coming shortly: series of simple kernel questions i should know the answers to
by Robert P. J. Day April 1, 2015

April 1, 2015

the lead-in: for the purposes of updating a pile of my existing kernel tutorials and courseware (and writing new material), i'm digging into the source to document numerous concepts from beginning to end, and i'm not surprised when i trip over all sorts of things that are fairly simple but i don't immediately know the answer to. so for the next while, i'm going to ask all sorts of questions to clarify numerous things, then i'll write up the final results and post them for public consumption at my web site so other people can take advantage of this. and i figured it would be useful to ask those questions on this list, as this list is full of clever people who probably know the answers. a lot of the early questions will be based on clarifying details of the driver model, as the current documentation under Documentation/driver-model/ is somewhat out of date so i desperately need to put together something far more current. first question(s) coming shortly ... rday -- ======================================================================== Robert P. J. Day Ottawa, Ontario, CANADA http://crashcourse.ca Twitter: http://twitter.com/rpjday LinkedIn: http://ca.linkedin.com/in/rpjday ========================================================================

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mmap text file
by Ssagarr Patil April 1, 2015

April 1, 2015

Hi, I am trying to achieve faster file writes using mmap. so I am trying the follwowing: /* open file */ while (loop) { /* get the current file size of output file */ filesize = lseek(outfd, 0, SEEK_END); /* Increase file size */ ftruncate(outfd, (off_t)filesize + (off_t)buf.size); /* mmap destination file */ if((dest = mmap(0, buf.size, PROT_WRITE, MAP_SHARED, outfd, filesize)) == (void *) -1) { printf( "Error mapping ouput file: %s: %d, %d\n", argv[1], i, filesize); perror("MMAP Failed\n"); goto error; } memcpy(dest, src, size); munmap(dest); } I am able to only mmap it once next time it fails to mmap, any pointers why it fails ? Regards, --Sagar

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