[7/9] docs: remove description of DISCONTIGMEM

From: Mike Rapoport <rppt@linux.ibm.com>

Remove description of DISCONTIGMEM from the "Memory Models" document and
update VM sysctl description so that it won't mention DISCONIGMEM.

Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
---
 Documentation/admin-guide/sysctl/vm.rst | 12 +++----
 Documentation/vm/memory-model.rst       | 45 ++-----------------------
 2 files changed, 8 insertions(+), 49 deletions(-)

On 02.06.21 12:53, Mike Rapoport wrote:
> From: Mike Rapoport <rppt@linux.ibm.com>
> 
> Remove description of DISCONTIGMEM from the "Memory Models" document and
> update VM sysctl description so that it won't mention DISCONIGMEM.
> 
> Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
> ---
>   Documentation/admin-guide/sysctl/vm.rst | 12 +++----
>   Documentation/vm/memory-model.rst       | 45 ++-----------------------
>   2 files changed, 8 insertions(+), 49 deletions(-)
> 
> diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst
> index 586cd4b86428..ddbd71d592e0 100644
> --- a/Documentation/admin-guide/sysctl/vm.rst
> +++ b/Documentation/admin-guide/sysctl/vm.rst
> @@ -936,12 +936,12 @@ allocations, THP and hugetlbfs pages.
>   
>   To make it sensible with respect to the watermark_scale_factor
>   parameter, the unit is in fractions of 10,000. The default value of
> -15,000 on !DISCONTIGMEM configurations means that up to 150% of the high
> -watermark will be reclaimed in the event of a pageblock being mixed due
> -to fragmentation. The level of reclaim is determined by the number of
> -fragmentation events that occurred in the recent past. If this value is
> -smaller than a pageblock then a pageblocks worth of pages will be reclaimed
> -(e.g.  2MB on 64-bit x86). A boost factor of 0 will disable the feature.
> +15,000 means that up to 150% of the high watermark will be reclaimed in the
> +event of a pageblock being mixed due to fragmentation. The level of reclaim
> +is determined by the number of fragmentation events that occurred in the
> +recent past. If this value is smaller than a pageblock then a pageblocks
> +worth of pages will be reclaimed (e.g.  2MB on 64-bit x86). A boost factor
> +of 0 will disable the feature.
>   
>   
>   watermark_scale_factor
> diff --git a/Documentation/vm/memory-model.rst b/Documentation/vm/memory-model.rst
> index ce398a7dc6cd..30e8fbed6914 100644
> --- a/Documentation/vm/memory-model.rst
> +++ b/Documentation/vm/memory-model.rst
> @@ -14,15 +14,11 @@ for the CPU. Then there could be several contiguous ranges at
>   completely distinct addresses. And, don't forget about NUMA, where
>   different memory banks are attached to different CPUs.
>   
> -Linux abstracts this diversity using one of the three memory models:
> -FLATMEM, DISCONTIGMEM and SPARSEMEM. Each architecture defines what
> +Linux abstracts this diversity using one of the two memory models:
> +FLATMEM and SPARSEMEM. Each architecture defines what
>   memory models it supports, what the default memory model is and
>   whether it is possible to manually override that default.
>   
> -.. note::
> -   At time of this writing, DISCONTIGMEM is considered deprecated,
> -   although it is still in use by several architectures.
> -
>   All the memory models track the status of physical page frames using
>   struct page arranged in one or more arrays.
>   
> @@ -63,43 +59,6 @@ straightforward: `PFN - ARCH_PFN_OFFSET` is an index to the
>   The `ARCH_PFN_OFFSET` defines the first page frame number for
>   systems with physical memory starting at address different from 0.
>   
> -DISCONTIGMEM
> -============
> -
> -The DISCONTIGMEM model treats the physical memory as a collection of
> -`nodes` similarly to how Linux NUMA support does. For each node Linux
> -constructs an independent memory management subsystem represented by
> -`struct pglist_data` (or `pg_data_t` for short). Among other
> -things, `pg_data_t` holds the `node_mem_map` array that maps
> -physical pages belonging to that node. The `node_start_pfn` field of
> -`pg_data_t` is the number of the first page frame belonging to that
> -node.
> -
> -The architecture setup code should call :c:func:`free_area_init_node` for
> -each node in the system to initialize the `pg_data_t` object and its
> -`node_mem_map`.
> -
> -Every `node_mem_map` behaves exactly as FLATMEM's `mem_map` -
> -every physical page frame in a node has a `struct page` entry in the
> -`node_mem_map` array. When DISCONTIGMEM is enabled, a portion of the
> -`flags` field of the `struct page` encodes the node number of the
> -node hosting that page.
> -
> -The conversion between a PFN and the `struct page` in the
> -DISCONTIGMEM model became slightly more complex as it has to determine
> -which node hosts the physical page and which `pg_data_t` object
> -holds the `struct page`.
> -
> -Architectures that support DISCONTIGMEM provide :c:func:`pfn_to_nid`
> -to convert PFN to the node number. The opposite conversion helper
> -:c:func:`page_to_nid` is generic as it uses the node number encoded in
> -page->flags.
> -
> -Once the node number is known, the PFN can be used to index
> -appropriate `node_mem_map` array to access the `struct page` and
> -the offset of the `struct page` from the `node_mem_map` plus
> -`node_start_pfn` is the PFN of that page.
> -
>   SPARSEMEM
>   =========
>   
> 

Reviewed-by: David Hildenbrand <david@redhat.com>