On Tue, Sep 11, 2018 at 11:55:55AM -0600, Jonathan Corbet wrote: > Sorry for being so slow to get to this...it fell into a dark crack in my > rickety email folder hierarchy. I do have one question... > > On Fri, 17 Aug 2018 17:47:16 +0300 > Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx> wrote: > > > + ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory > > + will be directly accessible by the kernel or the hardware and > > + implies that the data is movable. > > + > > + ``GFP_HIGHUSER`` means that the allocated memory is not movable, > > + but it is not required to be directly accessible by the kernel or > > + the hardware. An example may be a hardware allocation that maps > > + data directly into userspace but has no addressing limitations. > > + > > + ``GFP_USER`` means that the allocated memory is not movable and it > > + must be directly accessible by the kernel or the hardware. It is > > + typically used by hardware for buffers that are mapped to > > + userspace (e.g. graphics) that hardware still must DMA to. > > I realize that this is copied from elsewhere, but still...as I understand > it, the "HIGH" part means that the allocation can be satisfied from high > memory, nothing more. So...it's irrelevant on 64-bit machines to start > with, right? And it has nothing to do with DMA, I would think. That would > be handled by the DMA infrastructure and, perhaps, the DMA* zones. Right? > > I ask because high memory is an artifact of how things are laid out on > 32-bit systems; hardware can often DMA quite easily into memory that the > kernel sees as "high". So, to me, this description seems kind of > confusing; I wouldn't mention hardware at all. But maybe I'm missing > something? Well, I've amended the original text from gfp.h in attempt to make it more "user friendly". The GFP_HIGHUSER became really confusing :) I think that we can drop mentions of hardware from GFP_HIGHUSER_MOVABLE and GFP_USER, but it makes sense to leave the example in the GFP_HIGHUSER description. How about: ``GFP_HIGHUSER_MOVABLE`` does not require that allocated memory will be directly accessible by the kernel and implies that the data is movable. ``GFP_HIGHUSER`` means that the allocated memory is not movable, but it is not required to be directly accessible by the kernel. An example may be a hardware allocation that maps data directly into userspace but has no addressing limitations. ``GFP_USER`` means that the allocated memory is not movable and it must be directly accessible by the kernel > Thanks, > > jon > -- Sincerely yours, Mike.