From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id CBC2D1519AC for ; Wed, 1 Oct 2025 04:23:34 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759292617; cv=none; b=sCzhyeXTXL158EkL1SiBeGwi9IWUaOCeL60hhszXjPYp191efq9Nq6HD4Nhl9zq8v4+QoYIr08BQ9AQ8X6AgsY1sqpsD0G6d8SD0VWyNCcNtBm5MPlZv1FQlQhRbBX50VrQ7aV54N3+TGk7wfuLGJrXo9MOfeof5POsI7p1wicI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759292617; c=relaxed/simple; bh=v+lbESnAe2ivpXOa54AKZRDPwIkwui+2ZYmZ+zkEMtg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=hHZ4qgQbomsjwoIzKUNqGK3QkSwobtUWuX093FicqwvWcVGqGIcQfdvKY+CBQ520lMw0UwldcTu/mZwPn7Mt4+U0xhZxCgsn9aLZeB6kRoW6eCEVrNy8OJ5i0sEsQwLRkhoJbVXq/jyr+C9hGnNiZRN2bfkWcyobJ1CE8F6yF9A= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id D6D8F16F2; Tue, 30 Sep 2025 21:23:25 -0700 (PDT) Received: from [10.164.18.53] (MacBook-Pro.blr.arm.com [10.164.18.53]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id B29623F66E; Tue, 30 Sep 2025 21:23:29 -0700 (PDT) Message-ID: <746c4dd3-2285-4353-9e15-a0a2fbd4e6b5@arm.com> Date: Wed, 1 Oct 2025 09:53:26 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [v2 PATCH] mm: hugetlb: avoid soft lockup when mprotect to large memory area To: Yang Shi , muchun.song@linux.dev, osalvador@suse.de, david@redhat.com, akpm@linux-foundation.org, catalin.marinas@arm.com, will@kernel.org, anshuman.khandual@arm.com, carl@os.amperecomputing.com, cl@gentwo.org Cc: linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20250929202402.1663290-1-yang@os.amperecomputing.com> <6cde8290-3aa2-411c-bf29-eb91a99e33a5@os.amperecomputing.com> Content-Language: en-US From: Dev Jain In-Reply-To: <6cde8290-3aa2-411c-bf29-eb91a99e33a5@os.amperecomputing.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 30/09/25 11:38 pm, Yang Shi wrote: > > > On 9/29/25 10:26 PM, Dev Jain wrote: >> >> 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 >>> Reviewed-by: Christoph Lameter (Ampere) >>> Reviewed-by: Catalin Marinas >>> Acked-by: David Hildenbrand >>> Acked-by: Oscar Salvador >>> Reviewed-by: Anshuman Khandual >>> Signed-off-by: Yang Shi >>> --- >>> 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 > > Thank you. > >> >> 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(). > > Thanks for raising this. I did think about it. But I didn't convince > myself because shared pmd should be not that common IMHO (If I'm > wrong, please feel free to correct me). At least PMD can't be shared > if the memory is tagged IIRC. So I'd like to keep the patch minimal > for now and defer adding cond_resched() until it is hit by some real > life workload. If we have large swathes of hugetlb memory like in your workload, and it is MAP_SHARED, then there should be high chances of sharing the PMD. Although, I incorrectly observed that we are clearing a page there - we are only clearing the pud entry which is 8 bytes. So yes a soft lockup should be highly unlikely. But since cond_resched() is cheap (I assume this is the case since it is liberally sprinkled all over the codebase) I think we should be consistent. Probably not an immediate concern and not a matter of this patch. > > Yang > >