On Mon, Jan 09, 2023 at 03:33:15PM +0200, Mike Rapoport wrote: > > Absolutely wonderfully written. > > Thanks to Mel :) > I should have known :) > > > +Each node may be divided up into a number of blocks called zones which > > > +represent ranges within memory. These ranges are usually determined by > > > +architectural constraints for accessing the physical memory. A zone is > > > +described by a ``struct zone_struct``, typedeffed to ``zone_t`` and each zone > > > +has one of the types described below. > > > > I don't think it's quite right to say 'may' be divided up into zones, as they > > absolutely will be so (and the entire phsyical memory allocator hinges on being > > zoned, even if trivially in UMA/single zone cases). > > Not necessarily. ZONE_DMA or ZONE_NORMAL may span the entire memory. I see what you mean, here again we get the confusion around zones as a term (And Willy has yet to propose a 'zolio' :), what I meant to say is that every byte of memory is in a zone, though a zone may span a node, multiple nodes or all nodes. > > > + > > > +`ZONE_DMA` and `ZONE_DMA32` > > > + represent memory suitable for DMA by peripheral devices that cannot > > > + access all of the addressable memory. Depending on the architecture, > > > + either of these zone types or even they both can be disabled at build > > > + time using ``CONFIG_ZONE_DMA`` and ``CONFIG_ZONE_DMA32`` configuration > > > + options. Some 64-bit platforms may need both zones as they support > > > + peripherals with different DMA addressing limitations. > > > > It might be worth pointing out ZONE_DMA spans an incredibly little range that > > probably won't matter for any peripherals this side of the cretaceous period, > > On RPi4 ZONE_DMA spans 1G, which is quite some part of the memory ;-) > Ah yeah that's another weirdness, my asahi laptop actually puts everything into ZONE_DMA so fair point. Arches do complicate things... (hence why I limit my scope to only one) > > > + > > > +`ZONE_NORMAL` > > > + is for normal memory that can be accessed by the kernel all the time. DMA > > > + operations can be performed on pages in this zone if the DMA devices support > > > + transfers to all addressable memory. ZONE_NORMAL is always enabled. > > > + > > > > Might be worth saying 'this is where memory ends up if not otherwise in another > > zone'. > > This may not be the case on !x86. Yeah again, I am being a fool because I keep burying in my mind the fact that my Asahi laptop literally doesn't do this... :) I think in 'principle' though it still is where things should go unless you just decide to have the first zone only? But in any case, I think then the original explanation is better. > > > > +`ZONE_HIGHMEM` > > > + is the part of the physical memory that is not covered by a permanent mapping > > > + in the kernel page tables. The memory in this zone is only accessible to the > > > + kernel using temporary mappings. This zone is available only some 32-bit > > > + architectures and is enabled with ``CONFIG_HIGHMEM``. > > > + > > > > I comment here only to say 'wow I am so glad I chose to only focus on 64-bit so > > I could side-step all the awkward discussion of high pages' :) > > > > > +The relation between node and zone extents is determined by the physical memory > > > +map reported by the firmware, architectural constraints for memory addressing > > > +and certain parameters in the kernel command line. > > > > Perhaps worth mentioning device tree here? Though perhaps encapsulated in the > > 'firmware' reference. > > It is :) Ack, and that makes sense > > > > +Node structure > > > +-------------- > > > + > > > +The struct pglist_data is declared in `include/linux/mmzone.h > > > +<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/mmzone.h>`_. > > > +Here we briefly describe fields of this structure: > > > > Perhaps worth saying 'The node structure' just to reiterate. > > Not sure I follow, can you phrase the entire sentence? > Sorry I wasn't clear here, I meant to say simply reiterate that the pglist_data struct is the one describing a node. > > > + > > > +General > > > +~~~~~~~ > > > + > > > +`node_zones` > > > + The zones for this node. Not all of the zones may be populated, but it is > > > + the full list. It is referenced by this node's node_zonelists as well as > > > + other node's node_zonelists. > > > > Perhaps worth describing what zonelists (and equally zonerefs) are here or > > above, and that this is the canonical place where zones reside. Maybe reference > > populated_zone() and for_each_populated_zone() in reference to the fact that not > > all here may be populated? > > I'd prefer to start simple and than add more content on top. > Absolutely, makes sense! > > > + > > > +`node_zonelists` The list of all zones in all nodes. This list defines the > > > + order of zones that allocations are preferred from. The `node_zonelists` is > > > + set up by build_zonelists() in mm/page_alloc.c during the initialization of > > > + core memory management structures. > > > + > > > +`nr_zones` > > > + Number of populated zones in this node. > > > + > > > +`node_mem_map` > > > + For UMA systems that use FLATMEM memory model the 0's node (and the only) > > > + `node_mem_map` is array of struct pages representing each physical frame. > > > + > > > +`node_page_ext` > > > + For UMA systems that use FLATMEM memory model the 0's (and the only) node > > > + `node_mem_map` is array of extensions of struct pages. Available only in the > > > + kernels built with ``CONFIG_PAGE_EXTENTION`` enabled. > > > + > > > +`node_start_pfn` > > > + The page frame number of the starting page frame in this node. > > > + > > > +`node_present_pages` > > > + Total number of physical pages present in this node. > > > + > > > +`node_spanned_pages` > > > + Total size of physical page range, including holes. > > > + > > > > I think it'd be useful to discuss briefly the meaning of managed, spanned and > > present pages in the context of zones. > > This will be a part of the Zones section. Makes sense again! Overall it's very good. Nitpicking here really!
