On 11.07.23 06:48, Aneesh Kumar K.V wrote:
Add a new kconfig option that can be selected if we want to allow
pageblock alignment by reserving pages in the vmemmap altmap area.
This implies we will be reserving some pages for every memoryblock
This also allows the memmap on memory feature to be widely useful
with different memory block size values.
"reserving pages" is a nice way of saying "wasting memory". :) Let's
spell that out.
I think we have to find a better name for this, and I think we should
have a toggle similar to memory_hotplug.memmap_on_memory. This should be
an admin decision, not some kernel config option.
memory_hotplug.force_memmap_on_memory
"Enable the memmap on memory feature even if it could result in memory
waste due to memmap size limitations. For example, if the memmap for a
memory block requires 1 MiB, but the pageblock size is 2 MiB, 1 MiB
of hotplugged memory will be wasted. Note that there are still cases
where the feature cannot be enforced: for example, if the memmap is
smaller than a single page, or if the architecture does not support the
forced mode in all configurations."
Thoughts?
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
---
mm/Kconfig | 9 +++++++
mm/memory_hotplug.c | 59 +++++++++++++++++++++++++++++++++++++--------
2 files changed, 58 insertions(+), 10 deletions(-)
diff --git a/mm/Kconfig b/mm/Kconfig
index 932349271e28..88a1472b2086 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -570,6 +570,15 @@ config MHP_MEMMAP_ON_MEMORY
depends on MEMORY_HOTPLUG && SPARSEMEM_VMEMMAP
depends on ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
+config MHP_RESERVE_PAGES_MEMMAP_ON_MEMORY
+ bool "Allow Reserving pages for page block aligment"
+ depends on MHP_MEMMAP_ON_MEMORY
+ help
+ This option allows memmap on memory feature to be more useful
+ with different memory block sizes. This is achieved by marking some pages
+ in each memory block as reserved so that we can get page-block alignment
+ for the remaining pages.
+
endif # MEMORY_HOTPLUG
config ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
diff --git a/mm/memory_hotplug.c b/mm/memory_hotplug.c
index 07c99b0cc371..f36aec1f7626 100644
--- a/mm/memory_hotplug.c
+++ b/mm/memory_hotplug.c
@@ -1252,15 +1252,17 @@ static inline bool arch_supports_memmap_on_memory(unsigned long size)
{
unsigned long nr_vmemmap_pages = size >> PAGE_SHIFT;
unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page);
- unsigned long remaining_size = size - vmemmap_size;
- return IS_ALIGNED(vmemmap_size, PMD_SIZE) &&
- IS_ALIGNED(remaining_size, (pageblock_nr_pages << PAGE_SHIFT));
+ return IS_ALIGNED(vmemmap_size, PMD_SIZE);
}
#endif
static bool mhp_supports_memmap_on_memory(unsigned long size)
{
+ unsigned long nr_vmemmap_pages = size >> PAGE_SHIFT;
+ unsigned long vmemmap_size = nr_vmemmap_pages * sizeof(struct page);
+ unsigned long remaining_size = size - vmemmap_size;
+
/*
* Besides having arch support and the feature enabled at runtime, we
* need a few more assumptions to hold true:
@@ -1287,9 +1289,30 @@ static bool mhp_supports_memmap_on_memory(unsigned long size)
* altmap as an alternative source of memory, and we do not exactly
* populate a single PMD.
*/
- return mhp_memmap_on_memory() &&
- size == memory_block_size_bytes() &&
- arch_supports_memmap_on_memory(size);
+ if (!mhp_memmap_on_memory() || size != memory_block_size_bytes())
+ return false;
+ /*
+ * Without page reservation remaining pages should be pageblock aligned.
+ */
+ if (!IS_ENABLED(CONFIG_MHP_RESERVE_PAGES_MEMMAP_ON_MEMORY) &&
+ !IS_ALIGNED(remaining_size, (pageblock_nr_pages << PAGE_SHIFT)))
+ return false;
+
+ return arch_supports_memmap_on_memory(size);
+}
+
+static inline unsigned long memory_block_align_base(unsigned long size)
+{
+ if (IS_ENABLED(CONFIG_MHP_RESERVE_PAGES_MEMMAP_ON_MEMORY)) {
+ unsigned long align;
+ unsigned long nr_vmemmap_pages = size >> PAGE_SHIFT;
+ unsigned long vmemmap_size;
+
+ vmemmap_size = (nr_vmemmap_pages * sizeof(struct page)) >> PAGE_SHIFT;
+ align = pageblock_align(vmemmap_size) - vmemmap_size;
We should probably have a helper to calculate
a) the unaligned vmemmap size, for example used in
arch_supports_memmap_on_memory()
b) the pageblock-aligned vmemmap size.
--
Cheers,
David / dhildenb
