On Mon, 27 Oct 2014 18:15:15 +0530, Er Krishna said:
1. In case of Paging and discontiguous memory allocation for a particular process, is it possible that all the segments say DS, SS, CS, Heap and all can be in different page frames. I am asking this for a particular segment (I know all the segments can be in different non contiguous pages seperately, but what about one segment in different non contiguous page frames). For example lets say stack segment (or data segment) which is in RAM requires 45 pages and in RAM 45 pages are not contiguous avilable, so can it be reside from page frame no 100-125 and then again in 140-160 ?
Yes. In fact, it *probably* ends up in frames 97, 215, 112, 438, etc etc - the kernel doesn't try too hard to keep them contiguous unless something specifically requests contiguous allocation.
2. In case of execution of fast interrupt handler when other interrupts are disable, if device controller generate interrupt then it will not reaches to CPU ? Is it loss of interrupts and not a good condition on system/driver ? Can we ignore it safely for our driver or we must not fall into this scenario ?
In general, losing interrrupts isn't a good idea. Whether it's acceptable for your driver or not will depend on your hardware, and basically depends on the answer to the question "What does your hardware do if it requests an interrupt, and it decides it needs another interrupt before the first one has been serviced?" Some devices are stupid, and totally lose the plot if this happens - others are smarter and can either coalesce the interrupts or just ask again after then first one has been serviced.
3. Inside kernel page fault should not happen, I was trying to understand it w.r.t copy from/to usr api. Say on usr address the particular page is not there and kernel wants to copy it in kernel address space? What will happen if we use memcpy rather than copy from usr, will the kernel crash due to page fault? how copy from usr prevents this? Is the page fault handler code always remains in RAM in case of Linux Kernel ?
Copy from user basically checks for the page's availability first, and requests a page in, and then waits for the page to arrive, to avoid the nasty mess of taking a page fault inside the kernel.
4. In case of read/write system call, a normal user program wants to acess some file or more specifically (direct or indirect block of particular inode). This user space process page table which is in RAM doesn't contain valid bit for particular pages which correspond to these blocks. If these blocks are not in page-cache, it will be bring ito RAM by vector i/o with the help of bio-vec data structures. Will this scenario triggers the page fault inside kernel ?
I'll leave this one for you to figure out. It's not that hard. (Hint - in the code path you describe, why would any of those steps cause a page fault?)