Ramdisk for > 1GB / >2 GB

Ramdisk for > 1GB / >2 GB

[Manfred Spraul]

I've written a ramdisk driver that can use physical, unmapped memory. I've
posted a beta version this morning to linux-kernel.
Basically, it is a kernel patch that manages the memory (alloc_hugemem(),
free_hugemem()), and a block device driver that can use this memory.

I'm new in the Linux MM, perhaps you could help me on these questions:

1) SMP:
I use a spinlock for every ramdisk, and one page for each drive as a window
to the physical memory. Since only 1 processor uses this page, I can use
__flush_tlb_one( == INVLPG only on the local processor) without any further
synchronization.
Is that stable on SMP, and do you think that this parallel enough?
Linus suggested using one 4MB pte for each processor, but I think that this
would be to much overhead.
Another idea would be using a hash table (eg. 32 spinlocks, 32 pages) that
is shared by all processors.

2) I can't make the driver a module because I use 'pgt_offset_k()' to
traverse the page tables, but init_mm is not exported.
Is there any other way how I can find the pte_t that belongs to my page?

3) Is more than 2 GB memory a problem that only applies to the i386
architecture, or is there demand for that on PowerPC, Sparc32?

Regards,
    Manfred

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Re: Ramdisk for > 1GB / >2 GB

[Eric W. Biederman]

>>>>> "MS" == Manfred Spraul <masp0008@stud.uni-sb.de> writes:

MS> I've written a ramdisk driver that can use physical, unmapped memory. I've
MS> posted a beta version this morning to linux-kernel.
MS> Basically, it is a kernel patch that manages the memory (alloc_hugemem(),
MS> free_hugemem()), and a block device driver that can use this memory.

MS> I'm new in the Linux MM, perhaps you could help me on these questions:

MS> 1) SMP:
MS> I use a spinlock for every ramdisk, and one page for each drive as a window
MS> to the physical memory. Since only 1 processor uses this page, I can use
MS> __flush_tlb_one( == INVLPG only on the local processor) without any further
MS> synchronization.

Sounds good.  But it's not my area of expertise.

MS> Is that stable on SMP, and do you think that this parallel enough?

MS> Linus suggested using one 4MB pte for each processor, but I think that this
MS> would be to much overhead.
MS> Another idea would be using a hash table (eg. 32 spinlocks, 32 pages) that
MS> is shared by all processors.

Except for quantity of address space consumed a 4MB pte should be equal to a
4k pte.

MS> 3) Is more than 2 GB memory a problem that only applies to the i386
MS> architecture, or is there demand for that on PowerPC, Sparc32?

It's a problem for 32bit architectures.  Most of the RISC processors (I believe)
have 64bit extensions so it's less of an issue there.

Eric
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Re: Ramdisk for > 1GB / >2 GB

[ralf]

On Mon, Feb 01, 1999 at 10:25:54PM +0100, Manfred Spraul wrote:

> 3) Is more than 2 GB memory a problem that only applies to the i386
> architecture, or is there demand for that on PowerPC, Sparc32?

The limit is even lower, 512mb for the MIPS32 kernel.  This was so far not
a problem since the so far supported machines had a lower maximum but the
problem now showed up big time on the radar, so there will be a MIPS64
kernel with a 1TB maximum soon to solve the problem in the not far future.

  Ralf
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