Hi Rajat, So kernel virtual memory is always directly and permanently mapped and never has to fault? Is this for performance or is it because the kernel can't handle its own faults (kernel doesn't want to take responsibility for its own faults!). Also I would be grateful if you could describe in a sentence or two, how this copy from user to kernel space happens? my guess - it looks up the process's mm_struct and gets the physical location of its pages whether on disk or in physically memory, and then makes a copy of it to kernel space? wouldn't this be slow if the user memory is still on disk? Also at the time copy_from_user is called, it seems the memory would be uptodate anyway and going to disk wouldn't be required. The user obviously stored something in the memory and the processor would have segfaulted already? thanks, -Joel On Tue, Oct 20, 2009 at 4:08 AM, Rajat Jain <Rajat.Jain@xxxxxxxxxxxx> wrote: > > Hi, > >> Thank you for your reply. >> it's interesting, my modified kernel image is run ok on my >> hardware(arm926ejs). i test mounting ramfs and nfs, they are all ok. >> are they occasional? >> >> sorry, i don't comprehend your explanation about it >> In my opinion, if it's possible that the content of >> parameters isn't in memory at the time of the call, the >> "sys_mount" can't get them also. >> >> could u explain it in detail? Thanks > > OK. So here is it. Not all memory used by user space actually needs to > be in RAM all the time. It may be swapped out to disk since the actual > memory in use in a system is much more than its RAM size. When a piece > of memory that is currently swapped out on disk needs to be accessed, it > needs to be brought back into RAM memory - this is done by the page > fault handler. But consider that the Disk IO is a very slow process, and > thus it is relatively a very huge time for the kernel. For this reason, > any memory that is accessed by the kernel needs to be locked down in RAM > so that it cannot be swapped out. > > Secondly, the 4GB virtual address space is split up into user space and > kernel space code (3G/1G split generally). User space cannot access > kernel space virtual addresses and vice versa. Thus the user space > pointer cannot be dereferenced in the kernel. > > Thus, any user data that needs to be accessed firstly needs to be copied > into kernel address space. This done generally by copy_from_user() > function or its varians that sys_mount() uses: > > exact_copy_from_user((void *)page, data, size); > strncpy_from_user(page, filename, len); > > Now, how you code works comes as a surprize to me though... > > Thanks, > > Rajat > > -- > To unsubscribe from this list: send an email with > "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx > Please read the FAQ at http://kernelnewbies.org/FAQ > > -- To unsubscribe from this list: send an email with "unsubscribe kernelnewbies" to ecartis@xxxxxxxxxxxx Please read the FAQ at http://kernelnewbies.org/FAQ
