June 2021 - kernelnewbies@kernelnewbies.org - Kernelnewbies Mailing Lists

Getting backtrace of a process
by hemanth nandish June 15, 2021

June 15, 2021

I am trying to get create a custom error log of a process when it causes a segmentation fault. We can modify the* /proc/sys/kernel/core_pattern* file to pipe the core information to a handler binary. I am trying to use the *libunwind *library within this handler to also get the backtrace information but it is failing. As per my understanding, the *do_coredump* kills the faulty process inside *coredump_wait* function before invoking the userspace handler, This might be the reason the libunwind fails to gather the backtrace as it uses PTRACE calls. Is there any way to overcome this without modifying the kernel code?

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CMA: why DMA_ATTR_FORCE_CONTIGUOUS is not used in v4l2 or DRM cma helper
by ayaka June 11, 2021

June 11, 2021

Hello All I am looking for the CMA allocator in current kernel, when I check the videobuf2-dma-contig.c and drm_gem_cma_helper.c, they just call dma_alloc_wc()/dma_alloc_attrs() without any flag that request a contiguous pages. I could hardly find a driver use the flag DMA_ATTR_FORCE_CONTIGUOUS, what I need to do if I want to allocate a CMA and DMA capable buffer for a driver(not all the buffers, because it may have a iommu in a different direction) ? And anything I could that limit a driver that only allocate the pages for the DMA in a address base(for example only the low 4G memory) ? Sincerely ayaka

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SNDRV_PCM_IOCTL_READI_FRAMES returns EIO
by Muni Sekhar June 10, 2021

June 10, 2021

Hi all, I noticed recording(arecord) audio data on a USB sound card fails at SNDRV_PCM_IOCTL_READI_FRAMES is giving EIO. Can someone please clarify when the kernel returns EIO for Ioctl SNDRV_PCM_IOCTL_READI_FRAMES? -- Thanks, Sekhar

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Data present in vm_area_struct
by Deepanshu Kartikey June 7, 2021

June 7, 2021

How can I access the "data" present in vm_area_struct i.e virtual memory page. As I want to learn how the data is picked from the vm page add put it to the physical ram. Thanks

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Missing panic logs in /var/log/kern.log
by manty kuma June 7, 2021

June 7, 2021

I just triggered a panic, expecting that the logs will be visible in `/var/log/kern.log` after reboot, but there are no logs present there. Considering I have no access to the serial port, how do I know what went wrong? Is there anything we can do to make sure that the kernel log contains all the messages until before reboot? # sysctl kernel.panic kernel.panic = 30 If more information is needed, please let me know. Best Regards, Manty

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how to return unneeded init-data to kernel ?
by jim.cromie＠gmail.com June 3, 2021

June 3, 2021

hi all, When one builds with CONFIG_DYNAMIC_DEBUG=y one consumes extra memory for kernel data Ive managed to coax dyndbg into "not needing" about 30KiB in a __dyndbg* section, how can I return it to the kernel ? https://lore.kernel.org/lkml/20210529200029.205306-1-jim.cromie@gmail.com/ b4: dyndbg: 284 modules, 3013 entries and 11360 bytes in ddebug tables, 168728 bytes in __dyndbg section after: dyndbg: 2900 prdebugs in 277 modules, 12 KiB in ddebug tables, 90 KiB in __dyndbg section, 67 KiB in __dyndbg_sites section about 30Kib of the 67 is recoverable patchset, the short version: A- split __dyndbg in 2, isolate repeating module,file,function data. reach by new .site ptr. B- be safe if !.site C- work towards replacing .site with indexing on vectors: __dyndbgs & __dyndbg_sites c1 add ._index c2 use .index to get to __dyndbgs[0] ( then -> sites[._index] ) c3 insert header record at __dyndbgs[0] c4 rework header to differentiate from struct _ddebug, keep .site c5 drop .site, use indirection to get sites[._index] D- split _index into 2 jobs; _back (to [0]) and _map (to [N]) E- find duplicate site recs e1 - use just 1st of duplicate blocks e2 - in modules, move new site recs into front of sites vector e3 - in builtins, pack the whole sites vector, not just within the module-blocks So that leaves about 40% of __dyndbg_sites unneeded. How to return this to free memory ? FWIW, I tried krealloc on a module's chunk of memory (as attached by module:load_info()) it might have worked if that chunk was kallocd separately by elf-loader code. It evidently was not. PS: questions welcome here, reviews probably best at lkml

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