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book3s64/radix : Optimize vmemmap start alignment

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If we always align the vmemmap start to PAGE_SIZE, there is a
chance that we may end up allocating page-sized vmemmap backing
pages in RAM in the altmap not present case, because a PAGE_SIZE
aligned address is not PMD_SIZE-aligned.

In this patch, we are aligning the vmemmap start address to
PMD_SIZE if altmap is not present. This ensures that a PMD_SIZE
page is always allocated for the vmemmap mapping if altmap is
not present.

If altmap is present, Make sure we align the start vmemmap addr to
PAGE_SIZE so that we calculate the correct start_pfn in altmap
boundary check to decide whether we should use altmap or RAM based
backing memory allocation. Also the address need to be aligned for
set_pte operation. If the start addr is already PMD_SIZE aligned
and with in the altmap boundary then we will try to use a pmd size
altmap mapping else we go for page size  mapping.

So if altmap is present, we try to use the maximum number of
altmap pages; otherwise, we allocate a PMD_SIZE RAM page.

Signed-off-by: Donet Tom <donettom@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/895c4afd912c85d344a2065e348fac90529ed48f.1750593372.git.donettom@linux.ibm.com
This commit is contained in:
Donet Tom
2025-06-22 07:01:25 -05:00
committed by Madhavan Srinivasan
parent 58450938f7
commit f516479728
1 changed files with 18 additions and 11 deletions
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29
arch/powerpc/mm/book3s64/radix_pgtable.c
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@@ -1122,18 +1122,25 @@ int __meminit radix__vmemmap_populate(unsigned long start, unsigned long end, in
pte_t *pte;
/*
* Make sure we align the start vmemmap addr so that we calculate
* the correct start_pfn in altmap boundary check to decided whether
* we should use altmap or RAM based backing memory allocation. Also
* the address need to be aligned for set_pte operation.
* If the start addr is already PMD_SIZE aligned we will try to use
* a pmd mapping. We don't want to be too aggressive here beacause
* that will cause more allocations in RAM. So only if the namespace
* vmemmap start addr is PMD_SIZE aligned we will use PMD mapping.
* If altmap is present, Make sure we align the start vmemmap addr
* to PAGE_SIZE so that we calculate the correct start_pfn in
* altmap boundary check to decide whether we should use altmap or
* RAM based backing memory allocation. Also the address need to be
* aligned for set_pte operation. If the start addr is already
* PMD_SIZE aligned and with in the altmap boundary then we will
* try to use a pmd size altmap mapping else we go for page size
* mapping.
*
* If altmap is not present, align the vmemmap addr to PMD_SIZE and
* always allocate a PMD size page for vmemmap backing.
*
*/
start = ALIGN_DOWN(start, PAGE_SIZE);
if (altmap)
start = ALIGN_DOWN(start, PAGE_SIZE);
else
start = ALIGN_DOWN(start, PMD_SIZE);
for (addr = start; addr < end; addr = next) {
next = pmd_addr_end(addr, end);
@@ -1159,7 +1166,7 @@ int __meminit radix__vmemmap_populate(unsigned long start, unsigned long end, in
* in altmap block allocation failures, in which case
* we fallback to RAM for vmemmap allocation.
*/
if (!IS_ALIGNED(addr, PMD_SIZE) || (altmap &&
if (altmap && (!IS_ALIGNED(addr, PMD_SIZE) ||
altmap_cross_boundary(altmap, addr, PMD_SIZE))) {
/*
* make sure we don't create altmap mappings