main memory to user MMU, as in the "normal" one, is basically a big table of page addresses, no ?
so the kernel can (and does) write in it what ever it wants and it shows in the user process

but the IO MMU (of which i see there are more kinds) can't ?

problem i'm thinking of is device DMA buffers, that have to have a flat piece of memory
popular solution it seems is to reserve a piece of memory at boot for when drivers need it

so why can't the kernel just stop a process or two, copy from the blocks of memory it needs to some other then just modify the page table to point to the other thing

if i got it right (in contrast to absolutely wrong), would there be concurrency problems ?

also on the IO MMU thing
if i have a process that maps shared memory with another process then copies its contents to a DMA'd buffer
can i just tell the (theoretical kernel) to change the table in the IO MMU to point to the other memory page ?

thx for answers
PS don't spare me, i wonder how much i understand wrong

Not sure what you are asking. Are you making an OS and want to know how to handle memory paging?

the first part is something that was brought up as a problem for gpu drivers that need large flat pieces in physical memory
in linux more specifically

the IO MMU part is for a program i'm making that gets DMA'd pages from the kernel, that are to the sound card
also in linux (alsa mmap'ed ring buffer; aka "zero-copy", that is DMA)

but i'm asking is it possible to do on "modern" x86 computers
and if so how complicated would it be

Is it possible to do what? Memory paging? DMA? Asking the kernel for particular page mappings?

ye, sry
its two questions
linux is the goal, but i'm asking in general

first one is basically run time physical memory defragging
if you need a big piece of contiguous physical memory and there is none due to fragmentation
can you find a piece then check what process owns the page after it then copy that page somewhere else and change the processes page table to point to the new one
in a multicpu environment, to make it more complicated

other one is can you change the DMA... physical RAM pointer i guess while the device is running, and how hard would that be

Yes, it is possible. Although I'm not sure which OS kernels would support such a thing. Check your OS to see how it manages things. Yes it is possible. But it depends upon your device, its driver and its capabilities in general. And, once again, it also depends upon your OS support.

As to how difficult it would be, well, once again it depends upon many factors, but my guess would be it is not going to be easy for you unless you are very familiar with how everything is interacting.

linux doesn't, i think kolibrios doesn't and i'm pretty sure none of the other can

would be nice thou, that a kernel can get a big DMA buffer without reserving memory at boot
in linux it would have to be in memory management (has two now, one for devices and one general) and in process scheduler and idk where else

probably not something i can do, but still interesting to me

edit:
https://lwn.net/images/pdf/LDD3/ch15.pdf
and
http://oreilly.com/catalog/linuxkernel/chapter/ch10.html
lots of reading...