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kernel: demand_paging: allow reserving page frames

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This adds the kconfig to allow reserving a number of page frames
which do not count towards free memory. This is to ensure that
there are enough page frames available for paging code and data.
Or else, it would be possible to exhaust all page frames via
anonymous memory mappings.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>
This commit is contained in:
Daniel Leung 2021-07-22 15:08:28 -07:00 • committed by Christopher Friedt
commit 2dfae4a0f7
2 changed files with 19 additions and 0 deletions
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11
arch/Kconfig
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@ -701,6 +701,17 @@ config DEMAND_PAGING_ALLOW_IRQ
runs with interrupts disabled for the entire operation. However,
ISRs may also page fault.
config DEMAND_PAGING_PAGE_FRAMES_RESERVE
int "Number of page frames reserved for paging"
default 32 if !LINKER_GENERIC_SECTIONS_PRESENT_AT_BOOT
default 0
help
This sets the number of page frames that will be reserved for
paging that do not count towards free memory. This is to
ensure that there are some page frames available for paging
code and data. Otherwise, it would be possible to exhaust
all page frames via anonymous memory mappings.
config DEMAND_PAGING_STATS
bool "Gather Demand Paging Statistics"
help

8
kernel/mmu.c
View file

@ -624,7 +624,15 @@ size_t k_mem_free_get(void)
__ASSERT(page_frames_initialized, "%s called too early", __func__);
key = k_spin_lock(&z_mm_lock);
#ifdef CONFIG_DEMAND_PAGING
if (z_free_page_count > CONFIG_DEMAND_PAGING_PAGE_FRAMES_RESERVE) {
ret = z_free_page_count - CONFIG_DEMAND_PAGING_PAGE_FRAMES_RESERVE;
} else {
ret = 0;
}
#else
ret = z_free_page_count;
#endif
k_spin_unlock(&z_mm_lock, key);
return ret * (size_t)CONFIG_MMU_PAGE_SIZE;