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include/linux/mmzone.h: add documentation for pfn_valid() 

Patch series "arm64: drop pfn_valid_within() and simplify pfn_valid()", v4.

These patches aim to remove CONFIG_HOLES_IN_ZONE and essentially hardwire
pfn_valid_within() to 1.

The idea is to mark NOMAP pages as reserved in the memory map and restore
the intended semantics of pfn_valid() to designate availability of struct
page for a pfn.

With this the core mm will be able to cope with the fact that it cannot
use NOMAP pages and the holes created by NOMAP ranges within MAX_ORDER
blocks will be treated correctly even without the need for
pfn_valid_within.

This patch (of 4):

Add comment describing the semantics of pfn_valid() that clarifies that
pfn_valid() only checks for availability of a memory map entry (i.e.
struct page) for a PFN rather than availability of usable memory backing
that PFN.

The most "generic" version of pfn_valid() used by the configurations with
SPARSEMEM enabled resides in include/linux/mmzone.h so this is the most
suitable place for documentation about semantics of pfn_valid().

Link: https://lkml.kernel.org/r/20210511100550.28178-1-rppt@kernel.org
Link: https://lkml.kernel.org/r/20210511100550.28178-2-rppt@kernel.org
Signed-off-by: Mike Rapoport <rppt@linux.ibm.com>
Suggested-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Mike Rapoport 2021-06-30 18:51:13 -07:00 committed by Linus Torvalds
parent 269fbe72cd
commit 51c656aef6
1 changed files with 11 additions and 0 deletions
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11
include/linux/mmzone.h
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@ -1445,6 +1445,17 @@ static inline int pfn_section_valid(struct mem_section *ms, unsigned long pfn)
#endif
#ifndef CONFIG_HAVE_ARCH_PFN_VALID
/**
* pfn_valid - check if there is a valid memory map entry for a PFN
* @pfn: the page frame number to check
*
* Check if there is a valid memory map entry aka struct page for the @pfn.
* Note, that availability of the memory map entry does not imply that
* there is actual usable memory at that @pfn. The struct page may
* represent a hole or an unusable page frame.
*
* Return: 1 for PFNs that have memory map entries and 0 otherwise
*/
static inline int pfn_valid(unsigned long pfn)
{
struct mem_section *ms;