Some Kernel Questions/Confusions [Pls help]

[Er Krishna]

Hi Valdis,

Many thanks for the mail and quick answers. Regarding the point 4 I know
there won't be any page fault otherwise its going to be a complete mess for
the kernel. But since I have the confusion due to wrong understanding and I
want to clear my concepts I am putting my inline comments. Pls confirm and
correct my understanding by doing inline reply. Sorry in advance if my
question/understanding is wrong.

On Mon, Oct 27, 2014 at 6:35 PM, <Valdis.Kletnieks at vt.edu> wrote:

> 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.


I think this scenario will call for page fault and page fault handler
should be invoked.
I believe Page fault handler code will be inside the kernel and it will try
to fill the page
frame with  that particular file data say with the particular block.

Eventually this page fault handler code will do below mentioned Vector i/o
to put the
blocks into page frame.Once done Page table will be updated with valid bit
entry and
then user process can acess the data.

In this whole scenario, this page fault is a kind of  user space page fault
and not a kernel space
page fault, but the page fault handler will execute its handler in kernel
space.


> 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?)
>

Normal block driver Scsi or Sata fetch the particular sector/Block from
Disk
with the help of disk controller r/w command and put that block into the
page
frame (with the help of Bio structure of request queue) from where CPU
acess it.
In this scenario there is no page fault happens.

Can Cpu directly acess the data bypassing the page cache/RAM ?


Best Regards,
Krishna
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