LWN: Comments on "The kernel's genpool subsystem"

The kernel's genpool subsystem

ganesh45in — Fri, 28 Jan 2022 12:22:04 +0000

Good article. Does this gen pool subsystem do defragmentation when we do a gen_pool_free? Lets say we have a 512 bytes of virtually contiguous memory added to the gen pool (memory allocated via vmalloc). Now we allocate 4 chunks of 128 bytes. Now, we free chunk 1 and 3 (0 and 2 still remain). Now we want to alloc 256 bytes of memory by calling gen_pool_alloc(256 bytes). We have a fragmented memory and there is no virtually contiguous 256 bytes of memory. Do we fail the alloc here?

The kernel's genpool subsystem

pH5 — Sat, 05 Aug 2017 12:14:10 +0000

Embedded SoCs sometimes have on-chip SRAM that can be used to run suspend code while the external DRAM is disabled, or as a small but fast temporary working memory for GPUs or VPUs. The SRAM driver for example attaches a genalloc pool to an SRAM device, providing it to drivers in other subsystems as a shared resource. The other drivers can then obtain the pool via a reference in the device tree.
The VIDEO_CODA driver for example uses memory from such a pool to let the hardware video encoder do motion estimation in SRAM.

The kernel's genpool subsystem

Paf — Fri, 04 Aug 2017 17:55:24 +0000

While others who work with hardware could presumably give you more (I do file systems...), the article touches on this:
"often that memory is located on a device somewhere".

So yes, I think it is primarily special hardware needs. Some (hardware) devices have all kinds of weird needs like that - Memory aligned on odd boundaries, etc, because of the limited/specific ways the hardware has of addressing memory.

But I would also be glad to hear more from someone who writes drivers...

The kernel's genpool subsystem

isido — Fri, 04 Aug 2017 06:13:51 +0000

I like these articles on introducing kernel internals, so thank you is in order. One question: what are the typical use-cases and reasons for using custom allocators? Are they related to hardware idiosyncracies or some more general issue?

The kernel's genpool subsystem

blackwood — Thu, 03 Aug 2017 22:56:19 +0000

I guess we could add drm_mm to the list of such allocators. That one even comes with pretty docs and all:

https://dri.freedesktop.org/docs/drm/gpu/drm-mm.html#drm-...

But it's more a build-your-own-allocator thing than the more readymade genalloc: All the structs are exposed and can be embedded into our own structs.

We've also have at least one driver (gma500) which uses the struct resource stuff to allocate/manage it's graphics bar with the vram.