The patch titled
Subject: mm: allow deferred page init for vmemmap only
has been removed from the -mm tree. Its filename was
mm-allow-deferred-page-init-for-vmemmap-only.patch
This patch was dropped because an alternative patch was merged
------------------------------------------------------
From: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Subject: mm: allow deferred page init for vmemmap only
It is unsafe to do virtual to physical translations before mm_init() is
called if struct page is needed in order to determine the memory section
number (see SECTION_IN_PAGE_FLAGS). This is because only in mm_init() we
initialize struct pages for all the allocated memory when deferred struct
pages are used.
My recent fix exposed this problem, because it greatly reduced number of
pages that are initialized before mm_init(), but the problem existed even
before my fix, as Fengguang Wu found.
Below is a more detailed explanation of the problem.
We initialize struct pages in four places:
1. Early in boot a small set of struct pages is initialized to fill
the first section, and lower zones.
2. During mm_init() we initialize "struct pages" for all the memory
that is allocated, i.e reserved in memblock.
3. Using on-demand logic when pages are allocated after mm_init call (when
memblock is finished)
4. After smp_init() when the rest free deferred pages are initialized.
The problem occurs if we try to do va to phys translation of a memory
between steps 1 and 2. Because we have not yet initialized struct pages
for all the reserved pages, it is inherently unsafe to do va to phys if
the translation itself requires access of "struct page" as in case of this
combination: CONFIG_SPARSE && !CONFIG_SPARSE_VMEMMAP
Here is a sample path, where translation is required, that occurs before
mm_init():
start_kernel()
trap_init()
setup_cpu_entry_areas()
setup_cpu_entry_area(cpu)
get_cpu_gdt_paddr(cpu)
per_cpu_ptr_to_phys(addr)
pcpu_addr_to_page(addr)
virt_to_page(addr)
pfn_to_page(__pa(addr) >> PAGE_SHIFT)
The problems are discussed in these threads:
http://lkml.kernel.org/r/20180418135300.inazvpxjxowogyge@xxxxxxxxxxxxxxxxxxxxxx
http://lkml.kernel.org/r/20180419013128.iurzouiqxvcnpbvz@xxxxxxxxxxxxxxxxxxxxxx
http://lkml.kernel.org/r/20180426202619.2768-1-pasha.tatashin@xxxxxxxxxx
My worry is that trap_init() is arch specific and we cannot guarantee
that arches won't do virt to phys in trap_init() in other places.
Therefore, I think a proper fix is simply allow
DEFERRED_STRUCT_PAGE_INIT when it is safe to do virt to phys without
accessing struct pages, which is with SPARSEMEM_VMEMMAP.
Link: http://lkml.kernel.org/r/20180509191713.23794-1-pasha.tatashin@xxxxxxxxxx
Fixes: 3a80a7fa7989 ("mm: meminit: initialise a subset of struct pages if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set")
Signed-off-by: Pavel Tatashin <pasha.tatashin@xxxxxxxxxx>
Reviewed-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: <steven.sistare@xxxxxxxxxx>
Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Cc: Dennis Zhou <dennisszhou@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/Kconfig | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff -puN mm/Kconfig~mm-allow-deferred-page-init-for-vmemmap-only mm/Kconfig
--- a/mm/Kconfig~mm-allow-deferred-page-init-for-vmemmap-only
+++ a/mm/Kconfig
@@ -635,7 +635,7 @@ config DEFERRED_STRUCT_PAGE_INIT
bool "Defer initialisation of struct pages to kthreads"
default n
depends on NO_BOOTMEM
- depends on !FLATMEM
+ depends on SPARSEMEM_VMEMMAP
help
Ordinarily all struct pages are initialised during early boot in a
single thread. On very large machines this can take a considerable
_
Patches currently in -mm which might be from pasha.tatashin@xxxxxxxxxx are
mm-dont-allow-deferred-pages-with-need_per_cpu_km.patch
sparc64-ng4-memset-32-bits-overflow.patch
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