linux.git/mm/sparse.c, branch v7.1-rc2

mm/sparse: fix preinited section_mem_map clobbering on failure path

2026-04-18T07:10:52+00:00

sparse_init_nid() is careful to leave alone every section whose vmemmap
has already been set up by sparse_vmemmap_init_nid_early(); it only clears
section_mem_map for the rest:

        if (!preinited_vmemmap_section(ms))
                ms->section_mem_map = 0;

A leftover line after that conditional block

        ms->section_mem_map = 0;

was supposed to be deleted but was missed in the failure path, causing the
field to be overwritten for all sections when memory allocation fails,
effectively destroying the pre-initialization check.

Drop the stray assignment so that preinited sections retain their
already valid state.

Those pre-inited sections (HugeTLB pages) are not activated.  However,
such failures are extremely rare, so I don't see any major userspace
issues.

Link: https://lore.kernel.org/20260331113724.2080833-1-songmuchun@bytedance.com
Fixes: d65917c42373 ("mm/sparse: allow for alternate vmemmap section init at boot")
Signed-off-by: Muchun Song 
Acked-by: David Hildenbrand (Arm) 
Reviewed by: Donet Tom 
Cc: David Hildenbrand 
Cc: Frank van der Linden 
Cc: Liam Howlett 
Cc: Lorenzo Stoakes (Oracle) 
Cc: Michal Hocko 
Cc: Mike Rapoport 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Signed-off-by: Andrew Morton

mm/sparse: move memory hotplug bits to sparse-vmemmap.c

2026-04-05T20:53:33+00:00

Let's move all memory hoptplug related code to sparse-vmemmap.c.

We only have to expose sparse_index_init().  While at it, drop the
definition of sparse_index_init() for !CONFIG_SPARSEMEM, which is unused,
and place the declaration in internal.h.

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-15-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm/sparse: move __section_mark_present() to internal.h

2026-04-05T20:53:33+00:00

Let's prepare for moving memory hotplug handling from sparse.c to
sparse-vmemmap.c by moving __section_mark_present() to internal.h.

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-14-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Mike Rapoport (Microsoft) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm/sparse: move sparse_init_one_section() to internal.h

2026-04-05T20:53:33+00:00

While at it, convert the BUG_ON to a VM_WARN_ON_ONCE, avoid long lines,
and merge sparse_encode_mem_map() into its only caller
sparse_init_one_section().

Clarify the comment a bit, pointing at page_to_pfn().

[david@kernel.org: s/VM_WARN_ON/VM_WARN_ON_ONCE/]
  Link: https://lkml.kernel.org/r/6b04c1a1-74e7-42e8-8523-a40802e5dacc@kernel.org
Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-13-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm/sparse: drop set_section_nid() from sparse_add_section()

2026-04-05T20:53:32+00:00

CONFIG_MEMORY_HOTPLUG is CONFIG_SPARSEMEM_VMEMMAP-only.  And
CONFIG_SPARSEMEM_VMEMMAP implies that NODE_NOT_IN_PAGE_FLAGS cannot be
set: see include/linux/page-flags-layout.h

	...
	#elif defined(CONFIG_SPARSEMEM_VMEMMAP)
	#error "Vmemmap: No space for nodes field in page flags"
	...

Which implies that the node is always stored in page flags and
NODE_NOT_IN_PAGE_FLAGS cannot be set.  Therefore, set_section_nid() is a
NOP on CONFIG_SPARSEMEM_VMEMMAP.

So let's remove the set_section_nid() call to prepare for moving
CONFIG_MEMORY_HOTPLUG to mm/sparse-vmemmap.c

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-12-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm: prepare to move subsection_map_init() to mm/sparse-vmemmap.c

2026-04-05T20:53:32+00:00

We want to move subsection_map_init() to mm/sparse-vmemmap.c.

To prepare for getting rid of subsection_map_init() in mm/sparse.c
completely, use a static inline function for !CONFIG_SPARSEMEM_VMEMMAP.

While at it, move the declaration to internal.h and rename it to
"sparse_init_subsection_map()".

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-11-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm/sparse: remove CONFIG_MEMORY_HOTPLUG-specific usemap allocation handling

2026-04-05T20:53:32+00:00

In 2008, we added through commit 48c906823f39 ("memory hotplug: allocate
usemap on the section with pgdat") quite some complexity to try allocating
memory for the "usemap" (storing pageblock information per memory section)
for a memory section close to the memory of the "pgdat" of the node.

The goal was to make memory hotunplug of boot memory more likely to
succeed.  That commit also added some checks for circular dependencies
between two memory sections, whereby two memory sections would contain
each others usemap, turning both boot memory sections un-removable.

However, in 2010, commit a4322e1bad91 ("sparsemem: Put usemap for one node
together") started allocating the usemap for multiple memory sections on
the same node in one chunk, effectively grouping all usemap allocations of
the same node in a single memblock allocation.

We don't really give guarantees about memory hotunplug of boot memory, and
with the change in 2010, it is impossible in practice to get any circular
dependencies.

So let's simply remove this complexity.

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-10-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm/sparse: remove sparse_decode_mem_map()

2026-04-05T20:53:32+00:00

section_deactivate() applies to CONFIG_SPARSEMEM_VMEMMAP only.  So we can
just use pfn_to_page() (after making sure we have the start PFN of the
section), and remove sparse_decode_mem_map().

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-9-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm/sparse: remove !CONFIG_SPARSEMEM_VMEMMAP leftovers for CONFIG_MEMORY_HOTPLUG

2026-04-05T20:53:32+00:00

CONFIG_MEMORY_HOTPLUG now depends on CONFIG_SPARSEMEM_VMEMMAP.  So let's
remove the !CONFIG_SPARSEMEM_VMEMMAP leftovers that are dead code.

Adjust the comment above fill_subsection_map() accordingly.

Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-6-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton

mm/sparse: remove WARN_ONs from (online|offline)_mem_sections()

2026-04-05T20:53:31+00:00

We do not allow offlining of memory with memory holes, and always hotplug
memory without holes.

Consequently, we cannot end up onlining or offlining memory sections that
have holes (including invalid sections).  That's also why these WARN_ONs
never fired.

Let's remove the WARN_ONs along with the TODO regarding double-checking.
Link: https://lkml.kernel.org/r/20260320-sparsemem_cleanups-v2-3-096addc8800d@kernel.org
Signed-off-by: David Hildenbrand (Arm) 
Reviewed-by: Lorenzo Stoakes (Oracle) 
Reviewed-by: Mike Rapoport (Microsoft) 
Cc: Axel Rasmussen 
Cc: Liam Howlett 
Cc: Michal Hocko 
Cc: Oscar Salvador 
Cc: Sidhartha Kumar 
Cc: Suren Baghdasaryan 
Cc: Vlastimil Babka 
Cc: Wei Xu 
Cc: Yuanchu Xie 
Signed-off-by: Andrew Morton