From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id E957CCD342C for ; Wed, 6 May 2026 10:52:16 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 60DE66B0005; Wed, 6 May 2026 06:52:16 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 5E4D36B0088; Wed, 6 May 2026 06:52:16 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 522116B008A; Wed, 6 May 2026 06:52:16 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id 402196B0005 for ; Wed, 6 May 2026 06:52:16 -0400 (EDT) Received: from smtpin03.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay02.hostedemail.com (Postfix) with ESMTP id CE88412016B for ; Wed, 6 May 2026 10:52:15 +0000 (UTC) X-FDA: 84736680630.03.621B6D3 Received: from out-174.mta1.migadu.com (out-174.mta1.migadu.com [95.215.58.174]) by imf09.hostedemail.com (Postfix) with ESMTP id 9C50514000F for ; Wed, 6 May 2026 10:52:13 +0000 (UTC) Authentication-Results: imf09.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="kRr/BDzP"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf09.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.174 as permitted sender) smtp.mailfrom=lance.yang@linux.dev ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1778064734; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=zMpt19AKtKXEf6HWZe6U2H7Rk78vElmD03to1iCaQW0=; b=JNO9zw4TGlay3PqEuNTahuGqFeEJbIcGO4wylb4fiCIPnow0eX+Ac0zUwbRdbh8ry5sHLj nNtKvjao6941aroncKCtwDBVmSPm5heNL2AJdbgQaaII7Yu0dOSY4HEe+SgWGifi83xMzD Fydtliv3k9LSWc22IGuo5te62zWpAE8= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1778064734; a=rsa-sha256; cv=none; b=LD3cQpJVxOYxZhPla2TpQBSuR8yeJLNb/RIu3uVqsuDFX9UHIN/tBYM77T/aYRcohoThrh OMcx2COD2r++f4a7jiFQFmJs+oxUmVsjlTs9j1FPfvAGhy2Ntb+5UPYqd1Kp3JQP3MKK1Y SblloYERNffIGFk1TKUKCTwOWQqF8pg= ARC-Authentication-Results: i=1; imf09.hostedemail.com; dkim=pass header.d=linux.dev header.s=key1 header.b="kRr/BDzP"; dmarc=pass (policy=none) header.from=linux.dev; spf=pass (imf09.hostedemail.com: domain of lance.yang@linux.dev designates 95.215.58.174 as permitted sender) smtp.mailfrom=lance.yang@linux.dev X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1778064731; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=zMpt19AKtKXEf6HWZe6U2H7Rk78vElmD03to1iCaQW0=; b=kRr/BDzPfvHOut3Z5oRlb3TEmEcS+FurdMrniRy8OxU1SGMGBeUkPGaSEra56z5HxgJUX6 F7FR0vRd0mJzuTkApRYf4c2M+51HpXfFqFUomT/CpNHdi1qWExhIVEr+/Mo2QIp+KZLFS/ FFU/nS6HN5MXx4+fLxmrleTdauY2C7k= From: Lance Yang To: dev.jain@arm.com Cc: ye.liu@linux.dev, akpm@linux-foundation.org, david@kernel.org, ljs@kernel.org, xhao@linux.alibaba.com, liuye@kylinos.cn, ziy@nvidia.com, baolin.wang@linux.alibaba.com, liam@infradead.org, npache@redhat.com, ryan.roberts@arm.com, baohua@kernel.org, lance.yang@linux.dev, akpm@linux-foudation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v2] mm/khugepaged: clear MMF_VM_HUGEPAGE on mm_slot_alloc() failure Date: Wed, 6 May 2026 18:51:50 +0800 Message-Id: <20260506105150.21504-1-lance.yang@linux.dev> In-Reply-To: <3fb8c00d-d60d-4fec-81b1-e7633384a52d@arm.com> References: <3fb8c00d-d60d-4fec-81b1-e7633384a52d@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Migadu-Flow: FLOW_OUT X-Rspamd-Server: rspam11 X-Rspamd-Queue-Id: 9C50514000F X-Stat-Signature: ptm6p6ksdoy6qqstbg9y764rjsu51i4i X-Rspam-User: X-HE-Tag: 1778064733-438122 X-HE-Meta: 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 J6PXop7y xK5F6I1NLLtDQhxT0Rgn8m9n+URKznrPAt0VaHIIWrvpunEn9hE7XRkDucPzceyjomY5w+mPiSIqPxp6XulCMUUkVJe4kBnRoUX82az6feP1RYfGBjzSBfn15smktCYKU4il5Ia5Wat8pIkDu59gIoaiOBo6oM4/hnFUynxKx+Y+wtvPzrx9knZWP2Rbq1yVAbgRrJexbMD8T3p0lmaIaVNZ22gx64HSC0UMt0yW0KeB9qsQfB1flYiB5UPQjvx+RLFkj Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, May 06, 2026 at 12:16:35PM +0530, Dev Jain wrote: > > >On 06/05/26 6:51 am, Ye Liu wrote: >> From: Ye Liu >> >> __khugepaged_enter() sets MMF_VM_HUGEPAGE before allocating the >> corresponding mm_slot. If mm_slot_alloc() fails, the function >> returns with the flag set but without inserting the mm into the >> khugepaged tracking structures. >> >> This leaves the mm in an inconsistent state: it is marked as >> registered (MMF_VM_HUGEPAGE set), but will never be scanned by >> khugepaged. Future attempts to register the mm are skipped since >> khugepaged_enter_vma() checks the flag and returns early. >> >> Fix this by clearing MMF_VM_HUGEPAGE when mm_slot_alloc() fails, >> restoring the ability to retry registration later. >> >> Fixes: 16618670276a ("mm: khugepaged: avoid pointless allocation for struct mm_slot") >> Signed-off-by: Ye Liu >> --- >> Changes since v1: >> - Add Fixes tag as suggested by Dev Jain and Lance Yang >> >> mm/khugepaged.c | 4 +++- >> 1 file changed, 3 insertions(+), 1 deletion(-) >> >> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >> index 7d48d4fbd5f3..60ab7c1b61dd 100644 >> --- a/mm/khugepaged.c >> +++ b/mm/khugepaged.c >> @@ -559,8 +559,10 @@ void __khugepaged_enter(struct mm_struct *mm) >> return; >> >> slot = mm_slot_alloc(mm_slot_cache); >> - if (!slot) >> + if (!slot) { >> + mm_flags_clear(MMF_VM_HUGEPAGE, mm); >> return; >> + } > >Note that, a racing khugepaged_enter_vma() may back off >when it sees that MMF_VM_HUGEPAGE is set, but then the above >clears the flag after slot alloc failure. So we end up not >registering the mm with khugepaged. But I am sure no one >cares, we are in much big trouble if slot alloc is failing. Right. A racing khugepaged_enter_vma() can see MMF_VM_HUGEPAGE is set and return, then !slot clears it again. If there is no later khugepaged_enter_vma(), the mm still wouldn't get registered :) >Although one could argue the same about this patch, I will >still say it is important to fix "flag is set but not registered >with khugepaged" because that just feels wrong. +1 that is still better then keeping MMF_VM_HUGEPAGE set forever: any later attempt can retry and install the mm_slot :) Cheers, Lance