Mulyadi Santosa wrote:
Dear Joe Knapka
[snip]
As we all know, using 3G/1G VM split (3G for user space, 1G for kernel
space), Linux kernel can only do the identity mapping on the first 896
MB of available RAM and the rest is considered as highmem.
Now let's assume we have a PC with 1GB RAM and we disable highmem
support in currently running Linux kernel. Also assume there is still
around 900 MB free RAM available to be used (it's currently consumed by
kernel code and data, also several user space daemons) and there is no
swap partition mounted at all.
When a task is keep allocating memory (and the page frames are really
allocated, so it's not just a matter of extending the size of existing
VMA region such as heap) and it reaches 896 MB mark, can it keep asking
for more RAM?
Merely allocting memory (eg via malloc() or brk()) is NOT what causes
physical
RAM to be allocated. Basically, RAM is allocated to user tasks only
during page-fault
processing.
A user task does not ask for RAM; it allocates memory in its VIRTUAL
address space.
It's true that when all physical RAM pages are in use, the kernel cannot
allocate any more
physical RAM, but it does not follow that user task requests for virtual
address space
will fail. The process of allocation involves ONLY extending a VMA; no
physical
page is required until the task actually accesses an address in the
newly-allocated
region.
The virtual address requested by a user task is either mapped into RAM
(in which case the
usual TLB stuff happens and the task proceeeds as usual); or it is NOT
mapped, in which case
the task experiences a page fault and sleeps pending the page being
mapped into RAM.
(In the case of newly-allocated VM, almost certainly the page is NOT
mapped.)
The kernel's VM sytem then sets about clearing out some space so that
the requested
page can be mapped. That process could succeed or fail, depending on the
available
swap space, and probably other things. But usually the kernel will be
able to evict
some other page from RAM and allocate the freed physical page to the
process that
got the page fault.
My understanding so far is: user space memory allocation is satisfied by
using free pages from ZONE_NORMAL pool. Since ZONE_NORMAL is ranging
from 16 MB to 896 MB, once it is all used, kernel memory allocator will
start looking in ZONE_HIGHMEM. But since we disable highmem support,
ZONE_HIGHMEM isn't exist. Therefore, the kernel can't satisfy user
space request beyond 896 MB, thus it failed. My other understanding is,
128 MB of RAM is simply wasted since it can't be addressed.
I'm pretty sure that space will be used by the boot-time fixed mappings
(grep the kernel
source for "fixmap"), but I could be wrong.
-- JK
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