On 2025/2/17 22:30, David Hildenbrand wrote:
On 17.02.25 11:34, Zhenhua Huang wrote:
On 2025/2/17 17:44, David Hildenbrand wrote:
On 17.02.25 10:29, Zhenhua Huang wrote:
On the arm64 platform with 4K base page config, SECTION_SIZE_BITS is
set
to 27, making one section 128M. The related page struct which vmemmap
points to is 2M then.
Commit c1cc1552616d ("arm64: MMU initialisation") optimizes the
vmemmap to populate at the PMD section level which was suitable
initially since hot plug granule is always one section(128M). However,
commit ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
introduced a 2M(SUBSECTION_SIZE) hot plug granule, which disrupted the
existing arm64 assumptions.
The first problem is that if start or end is not aligned to a section
boundary, such as when a subsection is hot added, populating the entire
section is wasteful.
The Next problem is if we hotplug something that spans part of 128 MiB
section (subsections, let's call it memblock1), and then hotplug
something
that spans another part of a 128 MiB section(subsections, let's call it
memblock2), and subsequently unplug memblock1, vmemmap_free() will
clear
the entire PMD entry which also supports memblock2 even though
memblock2
is still active.
Assuming hotplug/unplug sizes are guaranteed to be symmetric. Do the
fix similar to x86-64: populate to pages levels if start/end is not
aligned
with section boundary.
Signed-off-by: Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx>
---
arch/arm64/mm/mmu.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index b4df5bc5b1b8..eec1666da368 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -1178,7 +1178,8 @@ int __meminit vmemmap_populate(unsigned long
start, unsigned long end, int node,
{
WARN_ON((start < VMEMMAP_START) || (end > VMEMMAP_END));
- if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES))
+ if (!IS_ENABLED(CONFIG_ARM64_4K_PAGES) ||
+ (end - start < PAGES_PER_SECTION * sizeof(struct page)))
return vmemmap_populate_basepages(start, end, node, altmap);
else
return vmemmap_populate_hugepages(start, end, node, altmap);
Yes, this does mimic what x86 does. That handling does look weird,
because it
doesn't care about any address alignments, only about the size, which is
odd.
I wonder if we could do better and move this handling
into vmemmap_populate_hugepages(), where we already have a fallback
to vmemmap_populate_basepages().
Hi David,
I had the same doubt initially.
After going through the codes, I noticed for vmemmap_populate(), the
arguments "start" and "end" passed down should already be within one
section.
early path:
for_each_present_section_nr
__populate_section_memmap
..
vmemmap_populate()
hotplug path:
__add_pages
section_activate
vmemmap_populate()
Therefore.. focusing only on the size seems OK to me, and fall back
solution below appears unnecessary?
Ah, in that case it is fine. Might make sense to document/enforce that
somehow for the time being ...
Shall I document and WARN_ON if the size exceeds? like:
WARN_ON(end - start > PAGES_PER_SECTION * sizeof(struct page))
Since vmemmap_populate() is implemented per architecture, the change
should apply for other architectures as well. However I have no setup to
test it on...
Therefore, May I implement it only for arm64 now ?
Additionally, from previous discussion, the change is worth
backporting(apologies for missing to CC stable kernel in this version).
Keeping it for arm64 should simplify for backporting. WDYT?
+/*
+ * Try to populate PMDs, but fallback to populating base pages when
ranges
+ * would only partially cover a PMD.
+ */
int __meminit vmemmap_populate_hugepages(unsigned long start,
unsigned
long end,
int node, struct vmem_altmap
*altmap)
{
@@ -313,6 +317,9 @@ int __meminit vmemmap_populate_hugepages(unsigned
long start, unsigned long end,
for (addr = start; addr < end; addr = next) {
This for loop appears to be redundant for arm64 as well, as above
mentioned, a single call to pmd_addr_end() should suffice.
Right, that was what was confusing me in the first place.
next = pmd_addr_end(addr, end);
+ if (!IS_ALIGNED(addr, PMD_SIZE) || !IS_ALIGNED(next,
PMD_SIZE))
+ goto fallback;
+
pgd = vmemmap_pgd_populate(addr, node);
if (!pgd)
return -ENOMEM;
@@ -346,6 +353,7 @@ int __meminit vmemmap_populate_hugepages(unsigned
long start, unsigned long end,
}
} else if (vmemmap_check_pmd(pmd, node, addr, next))
continue;
+fallback:
if (vmemmap_populate_basepages(addr, next, node,
altmap))
return -ENOMEM;
It seems we have no chance to call populate_basepages here?
Can you elaborate?
It's invoked within vmemmap_populate_hugepages(), which is called by
vmemmap_populate(). This implies that we are always performing a whole
section hotplug?
However, since it's common code used by other architectures like x86,
RISC-V and LoongArch, it is still necessary to review the code for these
architectures as well. At the very least, it's not a BUG :)
