[PATCH v4] powerpc/hugetlb: Disable gigantic hugepages if fadump is active

[Sourabh Jain]

The fadump kernel boots with limited memory solely to collect the kernel
core dump. Having gigantic hugepages in the fadump kernel is of no use.
Many times, the fadump kernel encounters OOM (Out of Memory) issues if
gigantic hugepages are allocated.

To address this, disable gigantic hugepages if fadump is active by
returning early from arch_hugetlb_valid_size() using
hugepages_supported(). When fadump is active, the global variable
hugetlb_disabled is set to true, which is later used by the
PowerPC-specific hugepages_supported() function to determine hugepage
support.

Returning early from arch_hugetlb_vali_size() not only disables
gigantic hugepages but also avoids unnecessary hstate initialization for
every hugepage size supported by the platform.

kernel logs related to hugepages with this patch included:
kernel argument passed: hugepagesz=1G hugepages=1

First kernel: gigantic hugepage got allocated
==============================================

dmesg | grep -i "hugetlb"
-------------------------
HugeTLB: registered 1.00 GiB page size, pre-allocated 1 pages
HugeTLB: 0 KiB vmemmap can be freed for a 1.00 GiB page
HugeTLB: registered 2.00 MiB page size, pre-allocated 0 pages
HugeTLB: 0 KiB vmemmap can be freed for a 2.00 MiB page

$ cat /proc/meminfo | grep -i "hugetlb"
-------------------------------------
Hugetlb:         1048576 kB

Fadump kernel: gigantic hugepage not allocated
===============================================

dmesg | grep -i "hugetlb"
-------------------------
[    0.000000] HugeTLB: unsupported hugepagesz=1G
[    0.000000] HugeTLB: hugepages=1 does not follow a valid hugepagesz, ignoring
[    0.706375] HugeTLB support is disabled!
[    0.773530] hugetlbfs: disabling because there are no supported hugepage sizes

$ cat /proc/meminfo | grep -i "hugetlb"
----------------------------------
<Nothing>

Cc: Hari Bathini <hbathini at linux.ibm.com>
Cc: Madhavan Srinivasan <maddy at linux.ibm.com>
Cc: Mahesh Salgaonkar <mahesh at linux.ibm.com>
Cc: Michael Ellerman <mpe at ellerman.id.au>
Cc: Ritesh Harjani (IBM)" <ritesh.list at gmail.com>
Reviewed-by: Christophe Leroy <christophe.leroy at csgroup.eu>
Signed-off-by: Sourabh Jain <sourabhjain at linux.ibm.com>
---
Changelog:

v1:
https://lore.kernel.org/all/20250121150419.1342794-1-sourabhjain@linux.ibm.com/

v2:
https://lore.kernel.org/all/20250124103220.111303-1-sourabhjain@linux.ibm.com/
 - disable gigantic hugepage in arch code, arch_hugetlb_valid_size()

v3:
https://lore.kernel.org/all/20250125104928.88881-1-sourabhjain@linux.ibm.com/
 - Do not modify the initialization of the shift variable

v4:
- Update commit message to include how hugepages_supported() detects
  hugepages support when fadump is active
- Add Reviewed-by tag
- NO functional change

---
 arch/powerpc/mm/hugetlbpage.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index 6b043180220a..88cfd182db4e 100644
--- a/arch/powerpc/mm/hugetlbpage.c
+++ b/arch/powerpc/mm/hugetlbpage.c
@@ -138,6 +138,9 @@ bool __init arch_hugetlb_valid_size(unsigned long size)
 	int shift = __ffs(size);
 	int mmu_psize;
 
+	if (!hugepages_supported())
+		return false;
+
 	/* Check that it is a page size supported by the hardware and
 	 * that it fits within pagetable and slice limits. */
 	if (size <= PAGE_SIZE || !is_power_of_2(size))
-- 
2.48.1