On Mon, Jan 08, 2024 at 12:15:05PM -0800, Sukadev Bhattiprolu wrote: > > On 1/5/2024 4:05 PM, Roman Gushchin wrote: > > I'm not sure there is a "one size fits all" solution here. > agree - that's why we are thinking a configurable cma utilization would be > useful. > > There are two distinctive cases: > > 1) A relatively small cma area used for a specific purpose. This is how cma > > was used until recently. And it was barely used by the kernel for non-cma > > allocations. > > 2) A relatively large cma area which is used to allocate gigantic hugepages > > and as an anti-fragmentation mechanism in general (basically as a movable > > zone). In this case it might be preferable to use cma for movable > > allocations, because the space for non-movable allocations might be limited. > > > > I see two options here: > > 1) introduce per-cma area flags which will define the usage policy > Could you please elaborate on this - how would we use the per-cma flags > when allocating pages? I mean potentially we can add some per-cma area configuration options which will define the "priority" of using the memory from this cma area. > > 2) redesign the page allocator to better take care of fragmentation at 1Gb scale > > > > The latter is obviously not a small endeavour. > > The fundamentally missing piece is a notion of an anti-fragmentation cost. > > E.g. how much work does it makes sense to put into page migration > > before "polluting" a new large block of memory with an unmovable folio. > > Stepping back, we are trying to solve for a situation where system: > - has lot of movable allocs in zone normal > - has lot of idle memory in CMA region > - but is low on memory for unmovable allocs, leading to oom-kills > > On devices where cma region is mostly idle, allocating movable pages from > the cma region would have lesser overhead? It's not that easy: imagine booting up a small system with a cma area reserved for some hardware-related operations. This is pretty much what cma was initially designed. How to not fill the cma area up with the page cache? Thanks!
