On Thursday 30 August 2007 23:50:21 Valdis.Kletnieks@xxxxxx wrote:The "why" is easy: Having many mappings is expensive,
On Thu, 30 Aug 2007 23:41:09 +0200, Clemens Kolbitsch said:
On Thursday 30 August 2007 23:34:52 you wrote:There's an LSM exit point for mmap, you could perhaps do something there.
On Thu, 30 Aug 2007, Clemens Kolbitsch wrote:the thing is that they are not. the kernel chooses to REPLACE my mapping.
is there no way to tell the kernel, that a certain mapping must notI don't seem to get what is the issue here. Your mapping is not
be removed, no matter what (except of course an explicit call to
sys_unmap, of course)?
removed, only the VMAs are merged together into one larger VMA if they
have neighbouring address ranges and compatible protection bits. See
vma_merge().
consider the user-space code:
mmap(0xaaaa0000, 0x3000, MAP_FIXED, ...);
mmap(0xaaaa1000, 0x4000, MAP_FIXED, ...);
here, the second call to mmap will shorten the first mapping to 0x1000
bytes and create one big vma with size 0x5000 bytes.
is there a way to tell it that the second mmap MUST fail?
What are you trying to achieve by forcing the second one to fail?
puh... that is a good question :-)
I'm writing my master's thesis on a new model of memory protection and need to have every memory mapping in userspace duplicated. I also have kind of a second PGD/PTD that allows finding this mirrored mapping.
However, as the number of original mappings grows, I suddenly have the problem that the kernel tries to allocate a new mapping and picks the address of a mirrored memory page, which it shouldn't.
Honestly, I don't understand why it does so,