[PATCH v3] powerpc/hugetlb: Disable gigantic hugepages if fadump is active

[Christophe Leroy]

Le 25/01/2025 à 11:49, Sourabh Jain a écrit :
> 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(). hugepages_supported() returns false if fadump is
> active.

Maybe you could explain how that's done. As far as I can see there is no 
powerpc specific version of hugepages_supported(), so it relies on 
hpage_shift being 0, but I was not able to quickly spot where that is done.

> 
> Returning early from arch_hugetlb_valid_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: Christophe Leroy <christophe.leroy at csgroup.eu>
> 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>
> Signed-off-by: Sourabh Jain <sourabhjain at linux.ibm.com>

Reviewed-by: Christophe Leroy <christophe.leroy at csgroup.eu>

> ---
> 
> 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:
>   - Do not modify the initialization of the shift variable
> 
> ---
>   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))