Re: [v2 PATCH] mm: hugetlb: avoid soft lockup when mprotect to large memory area

[Dev Jain <dev.jain@arm.com>]

On 30/09/25 1:54 am, Yang Shi wrote:
> When calling mprotect() to a large hugetlb memory area in our customer's
> workload (~300GB hugetlb memory), soft lockup was observed:
>
> watchdog: BUG: soft lockup - CPU#98 stuck for 23s! [t2_new_sysv:126916]
>
> CPU: 98 PID: 126916 Comm: t2_new_sysv Kdump: loaded Not tainted 6.17-rc7
> Hardware name: GIGACOMPUTING R2A3-T40-AAV1/Jefferson CIO, BIOS 5.4.4.1 07/15/2025
> pstate: 20400009 (nzCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> pc : mte_clear_page_tags+0x14/0x24
> lr : mte_sync_tags+0x1c0/0x240
> sp : ffff80003150bb80
> x29: ffff80003150bb80 x28: ffff00739e9705a8 x27: 0000ffd2d6a00000
> x26: 0000ff8e4bc00000 x25: 00e80046cde00f45 x24: 0000000000022458
> x23: 0000000000000000 x22: 0000000000000004 x21: 000000011b380000
> x20: ffff000000000000 x19: 000000011b379f40 x18: 0000000000000000
> x17: 0000000000000000 x16: 0000000000000000 x15: 0000000000000000
> x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000000
> x11: 0000000000000000 x10: 0000000000000000 x9 : ffffc875e0aa5e2c
> x8 : 0000000000000000 x7 : 0000000000000000 x6 : 0000000000000000
> x5 : fffffc01ce7a5c00 x4 : 00000000046cde00 x3 : fffffc0000000000
> x2 : 0000000000000004 x1 : 0000000000000040 x0 : ffff0046cde7c000
>
> Call trace:
>    mte_clear_page_tags+0x14/0x24
>    set_huge_pte_at+0x25c/0x280
>    hugetlb_change_protection+0x220/0x430
>    change_protection+0x5c/0x8c
>    mprotect_fixup+0x10c/0x294
>    do_mprotect_pkey.constprop.0+0x2e0/0x3d4
>    __arm64_sys_mprotect+0x24/0x44
>    invoke_syscall+0x50/0x160
>    el0_svc_common+0x48/0x144
>    do_el0_svc+0x30/0xe0
>    el0_svc+0x30/0xf0
>    el0t_64_sync_handler+0xc4/0x148
>    el0t_64_sync+0x1a4/0x1a8
>
> Soft lockup is not triggered with THP or base page because there is
> cond_resched() called for each PMD size.
>
> Although the soft lockup was triggered by MTE, it should be not MTE
> specific. The other processing which takes long time in the loop may
> trigger soft lockup too.
>
> So add cond_resched() for hugetlb to avoid soft lockup.
>
> Fixes: 8f860591ffb2 ("[PATCH] Enable mprotect on huge pages")
> Tested-by: Carl Worth <carl@os.amperecomputing.com>
> Reviewed-by: Christoph Lameter (Ampere) <cl@gentwo.org>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Acked-by: Oscar Salvador <osalvador@suse.de>
> Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Signed-off-by: Yang Shi <yang@os.amperecomputing.com>
> ---
> v2: - Made the subject and commit message less MTE specific and fixed
>        the fixes tag.
>      - Collected all R-bs and A-bs.
>
>   mm/hugetlb.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index cb5c4e79e0b8..fe6606d91b31 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -7242,6 +7242,8 @@ long hugetlb_change_protection(struct vm_area_struct *vma,
>   						psize);
>   		}
>   		spin_unlock(ptl);
> +
> +		cond_resched();
>   	}
>   	/*
>   	 * Must flush TLB before releasing i_mmap_rwsem: x86's huge_pmd_unshare

Reviewed-by: Dev Jain <dev.jain@arm.com>

Does it make sense to also do cond_resched() in the huge_pmd_unshare() branch?
That also amounts to clearing a page. And I can see for example, zap_huge_pmd()
and change_huge_pmd() consume a cond_resched().