The on-demand paging logic is obsolete and incomplete as well as
incorrect for the current architecture. It was an experiment only for
the LPC3131 when I was running out of a small SRAM with dynamic paging
from SPI flash. It really should be removed.
True on-demand paging as you envision should be implemented in the
context of the kernel build mode (which did not exist in the time frame
of the LPC3131) which already supports dynamic paging of the heap, but
not the stack or instruction space.
There is also:
https://cwiki.apache.org/confluence/display/NUTTX/On-Demand+Paging
On 1/24/2024 2:14 PM, Tiago Medicci Serrano wrote:
Hi!
The topic is *on-demand paging and "lazy loading" on NuttX*.
Well, the on-demand paging article
<https://nuttx.apache.org/docs/latest/components/paging.html> (and `
CONFIG_PAGING
<https://github.com/apache/incubator-nuttx/blob/90f24ec29d0731cdda4b1db14ec73a87d88c50bf/arch/Kconfig#L888>`)
seems to have implemented the dynamic loading of instruction sections
(*.text) from a non-volatile media to internal RAM (able to execute
instructions). This was done mainly for LPC31xx and stated that it isn't
complete yet (a kind of experiment) and it does contain a page fault
handling logic.
On the other hand, one could use the Shared Memory Support
<https://nuttx.apache.org/docs/latest/components/mm/shm.html> mechanism of
paging allocation to implement on-demand paging. It even states that:
NOTE: An improved implementation my perform a “lazy” back up of the
physical memory, i.e., do not allocate the physical memory until the memory
is required, for example, when a page fault occurs when a application tries
to allocate the memory.
But lacks the "page fault" mechanism to provide a newly mapped page for the
task that caused the exception.
I may be missing something about this subject, but have you thought about
it? The idea is to allocate bigger virtual memory to the tasks' stack (with
few mapped pages) and treat page fault exceptions when they occur (and
freeing physical memory when it isn't needed anymore, making it possible to
be mapped to other pages if needed). How far are we from providing that?
Any thoughts about it?
Best regards,
