September 2016 - kernelnewbies@kernelnewbies.org

Power saving in case of CPU hotplugin
by Debdipta Ghosh Sept. 7, 2016

Sept. 7, 2016

HEllo all, I was going through a linux well known concept, hot plugin of cpu cores: https://www.kernel.org/doc/Documentation/cpu-hotplug.txt Is it more power saving than highest C-State and P-State? If we make one cpu offline, it disappears from the OS as it is not present, Is that means that CPU is powered off? Thanks, Debdipta

2 1

Vmalloc
by Er Krishna Sept. 6, 2016

Sept. 6, 2016

> Hi all > > I have one confusion, so asking this question for understanding: > > Vmalloc return address are virtually contiguous or not ? > > Basically vmalloc return non contiguous memory from ram which may not be one to one mapped in process kernel address space . > > Is my above understanding correct? > > Thanks > Krishna

2 1

Ioreamp vs mmap
by Er Krishna Sept. 6, 2016

Sept. 6, 2016

> Hi all > > I am asking this question for understanding purpose: > > Please help me here: > > What are the core difference between mmap and ioreamp ? > > In which scenario one can be used for other and when it can not be ? > > Thanks > Krishna

2 1

Linux Security Module listsecurity (and security xattr hooks) only one called?
by Pasquier, Thomas Sept. 6, 2016

Sept. 6, 2016

Hello everyone, In security/security.c (see here:https://github.com/torval ds/linux/blob/835c92d43b29eb354abdbd5475308a474d7efdfa/security/security.c) Looking at this: int security_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer_size) { if (unlikely(IS_PRIVATE(inode))) return 0; return call_int_hook(inode_listsecurity, 0, inode, buffer, buffer_size); } One would expect all stacked security module to be called. However, when looking at this: #define call_int_hook(FUNC, IRC, ...) ({ \ int RC = IRC; \ do { \ struct security_hook_list *P; \ \ list_for_each_entry(P, &security_hook_heads.FUNC, list) { \ RC = P->hook.FUNC(__VA_ARGS__); \ if (RC != 0) \ break; \ } \ } while (0); \ RC; \ }) If one of the module return a non-zero value, the other are not called. That means that the list of security of xattr is not built (or rather that it only contains the first module being called), as the lenght of inserted element is returned. Similarly for setsecurity and getsecurity, it seems that only one module will be called as if the given module does not support the security xattr, it returns -EOPNOTSUPP (instead I believe the next module being called, until one supporting the attribute is met). I am aware that LSM stacking is not fully supported yet: https://lwn.net/Articles/635771/ https://lwn.net/Articles/697259/ Here fix setsecurity and getsecurity: https://github. com/cschaufler/smack-next/blob/next/security/security.c; but listsecurity would appear to still not behave as I would expect. However, I may be missing something. What is the rational here? I could change the code to support multiple security xattr, but don't want to do so before I am sure I understand why it was done like this. Any help is welcome. Thanks. Thomas

3 2

Request Chinese mail list and FrontPage
by Hao Lee Sept. 6, 2016

Sept. 6, 2016

Hi, Sorry to bother you. I'm a kernel newbie from China, so it will be better if we could have a new mail list to communicate in chinese. If you don't mind, could you create a mail list called kernel-zh(a)kernelnewbies.org and create a FrontPage for us. We would appreciate it very much. If you need to specify someone to edit our own FrontPage, I can help. My username is HaoLee, thanks a lot. Thanks, Hao Lee

5 6

Q: dma requires copying into dma buffer - so what's the benefit ?
by Ran Shalit Sept. 6, 2016

Sept. 6, 2016

Hello, There is something I don't understand about dma, when doing memory to memory dma, it requires to cllocate dma buffer (for example with dma_alloc_coherent), than for each transfer we need to copy the buffer to the allocated memory and than trigger dma transaction. So, if it requires additional memcpy for each transaction , what's the benefit of using dma ? without dma: 1. copy buffer (memcpy) from source to destination with dma: 1. copy buffer (memcpy) from source to dma buffer 2. trigger dma transaction (which shall copy the buffer eventually to destination buffer) Regards, Ran

2 1

Creating sysfs brightness for every led channels
by Raul Piper Sept. 5, 2016

Sept. 5, 2016

Hi, I am trying to send the brightness value ( for 5 channels) by echo "val1val2val3val4val5" > brightness , But seems like even if I change the brightness type to enum or int in my driver ,I will never be able to send the brightness string because led class defines the brightness value as enum. *extern void led_set_brightness(struct led_classdev *led_cdev, enum led_brightness brightness);* Only possible way I am finding is to create attributes for each channel like brightness,default trigger,max_brightness and any user defined attributes, but i am not getting how to do this. Can some on please comment or post the link to the example driver. Do i have to call led_class_register for every channel and I have to define every channel in the device tree? Thanks in advance ! Regards, Rp

1 0

Network unreachable error when sending mail from mutt
by Gargi Sharma Sept. 5, 2016

Sept. 5, 2016

Hi all, I'm getting a "SMTP server problem Network is unreachable" "Error sending message, child exited 69 (Service unavailable.). " when trying to send a mail using mutt. When I used telnet smtp.gmail.com 587 or change the port to 465 I get Unable to connect to remote host: Network is unreachable. On trying openssl s_client -starttls smtp -crlf -connect smtp.gmail.com:587 I get --> connect: Connection timed out connect:errno=110 How do I solve this? Is there a problem with the firewall?

2 1

Linux Security Module listsecurity (and security xattr hooks) only one called?
by Pasquier, Thomas Sept. 5, 2016

Sept. 5, 2016

In security/security.c (see here: https://github.com/torvalds/linux/blob/835c92d43b29eb354abdbd5475308a474d7e… ) Looking at this: int security_inode_listsecurity(struct inode *inode, char *buffer, size_t buffer_size) { if (unlikely(IS_PRIVATE(inode))) return 0; return call_int_hook(inode_listsecurity, 0, inode, buffer, buffer_size); } One would expect all stacked security module to be called. However, when looking at this: #define call_int_hook(FUNC, IRC, ...) ({ \ int RC = IRC; \ do { \ struct security_hook_list *P; \ \ list_for_each_entry(P, &security_hook_heads.FUNC, list) { \ RC = P->hook.FUNC(__VA_ARGS__); \ if (RC != 0) \ break; \ } \ } while (0); \ RC; \ }) If one of the module return a non-zero value, the other are not called. That means that the list of security of xattr is not built (or rather that it only contains the first module being called), as the lenght of inserted element is returned. Similarly for setsecurity and getsecurity, it seems that only one module will be called as if the given module does not support the security xattr, it returns -EOPNOTSUPP (instead I believe the next module being called, until one supporting the attribute is met). What is the rational here? I could change the code to support multiple security xattr, but don't want to do so before I understand why it was done like this. Any help is welcome.

1 0

How is vmap_area inserted and can map_areas overlap?
by Peng Gao Sept. 4, 2016

Sept. 4, 2016

I am browsing the code static void __insert_vmap_area(struct vmap_area *va) { struct rb_node **p = &vmap_area_root.rb_node; struct rb_node *parent = NULL; struct rb_node *tmp; while (*p) { struct vmap_area *tmp_va; parent = *p; tmp_va = rb_entry(parent, struct vmap_area, rb_node); if (va->va_start < tmp_va->va_end) p = &(*p)->rb_left; else if (va->va_end > tmp_va->va_start) p = &(*p)->rb_right; else BUG(); } rb_link_node(&va->rb_node, parent, p); rb_insert_color(&va->rb_node, &vmap_area_root); /* address-sort this list */ tmp = rb_prev(&va->rb_node); if (tmp) { struct vmap_area *prev; prev = rb_entry(tmp, struct vmap_area, rb_node); list_add_rcu(&va->list, &prev->list); } else list_add_rcu(&va->list, &vmap_area_list); } As far as I can know map_area is a range for the vmalloc to alloc non-contigious memory, which confuses me is that why not use va->va_start > tmp_va->va_start and va->va_end < tmp_va->va_end to insert a new vmap_area. Can a virtual address mapping overlap? One more question In alloc_map_area, I found this part of code. /* from the starting point, walk areas until a suitable hole is found */ while (addr + size > first->va_start && addr + size <= vend) { if (addr + cached_hole_size < first->va_start) /* cached_hole_size */ cached_hole_size = first->va_start - addr; addr = ALIGN(first->va_end, align); if (addr + size < addr) goto overflow; if (list_is_last(&first->list, &vmap_area_list)) goto found; first = list_next_entry(first, list); } It seems like just inert a map_area at the end of list, what dose suitable hole means? Why not add this? if(addr+size < first->va_start) goto notound

1 2