The guaranteed contiguous memory allocator

In the distant past, Dan Magenheimer introduced the concept of transcendent memory — memory that is not directly addressable, but which can be used opportunistically by the kernel for caching or other purposes. Most of the transcendent-memory work has since gone unused and been removed from the kernel, but the idea persists, and this patch series makes use of it to provide guaranteed CMA.

Specifically, the patch set includes a subsystem called "cleancache", which is a concept that was proposed by Magenheimer in 2012. If the kernel has to dump a page of data, but would like to keep that data around if possible, it can put it into the cleancache, which will stash it aside somewhere. Should the need for that data arise, the kernel can copy it back out of the cleancache — if it is still there. Meanwhile, the page that initially contained that data can be reclaimed for other uses.

Guaranteed CMA then builds on cleancache by allocating a region of physically contiguous memory at boot, when such allocations are relatively easy. That memory is then turned into a cleancache and made available to the kernel. Whenever the memory-management system reclaims pages of file-backed memory, it can choose to place the data from those pages into the cleancache. Should that data be needed, an attempt will be made to retrieve it from the cleancache before rereading it from disk. The memory reserved for CMA is thus available to the kernel when not allocated to a CMA user, but in a restricted way.

At some point, some kernel subsystem will need a large, physically contiguous buffer. Requesting that buffer from the guaranteed CMA subsystem will result in an allocation from the reserved memory, after dropping any cached data that happens to be in the allocated region. This allocation can happen quickly, since that data has been cached with the explicit stipulation that it can be dropped at any time. This approach was proposed by Seongjae Park and Kim in 2014.

This new subsystem is integrated with the existing CMA API, so CMA users need not change to make use of it. The reserved region is set up by way of a devicetree property explicitly requesting the "guaranteed" behavior.

The end result is a version of CMA that is guaranteed to succeed as long as the total allocations do not exceed the size of the reserved area; existing CMA has a higher likelihood of failure. Since CMA usage is often restricted to a problematic device or two with known needs, sizing the reserved area for a specific system should be straightforward.

The other advantage of guaranteed CMA is latency; if the memory is available, it can be allocated quickly. CMA in current kernels may have to migrate data out of the allocated region first, which takes time. The downside is that the memory reserved for guaranteed CMA can only be used for data that can be dropped at will; that will increase the pressure on the rest of the memory in the system.

This patch series was posted just ahead of the 2025 Linux Storage, Filesystem, Memory-Management, and BPF Summit, where it is currently scheduled for a discussion in the memory-management track. There will probably not be a lot of comments on it ahead of that discussion. The patches are relatively small, though, and do not intrude into the memory-management subsystem on systems where CMA is not in use, so we might just see a transcendent-memory application actually go forward, some 15 years after the idea was first proposed.