From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-185.mta0.migadu.com (out-185.mta0.migadu.com [91.218.175.185]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id E72B81925BB for ; Thu, 5 Sep 2024 07:18:46 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.185 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725520729; cv=none; b=AUWamckfrYCgmOQLCaBSilsjjnEGy/zdKaqk2I5T5kmiDqJ+cNeEm6oRc6a6M1beoB39mfpjZtEg9uOXNb44cPoc+a54Ad6lXdbb2a73ETQypgjb3sEDQx8hvqYtMwmyBCSsy1Okf1rfJfwM7AEp3td9V9+lEE4A5f+ha6WV4T0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1725520729; c=relaxed/simple; bh=m8yo0ygUNvRfurS6oe7vGjo/dY5uVKasvHnDoAUEHBw=; h=Content-Type:Mime-Version:Subject:From:In-Reply-To:Date:Cc: Message-Id:References:To; b=Gk194Q10VOkaJIVRmIrjhN/oAcv9M1NfMP3xTkPW15qPGBmJcd/29lppgbeZGoyPlOcHuLZFieuvhz4vaCwoIE0UqGgDbDV2UwYcjGqxYDtPShmOdyNiL+ckKq7jmaJ4Q5pj1emoN8OdO9poPPDXSV3ofouq/dcFWE9oR25BTsY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=dTTEQmPD; arc=none smtp.client-ip=91.218.175.185 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="dTTEQmPD" Content-Type: text/plain; charset=us-ascii DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1725520725; 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=T5FTfSc9tY5acYoQHjJ+ZBc2V1eSvg3XTLgGUoFubUE=; b=dTTEQmPDGj+7RglANjOO+JH0gBypKgwITBbfCfuZQGhtYghyVvL1Xr5PdWB/5k0gtIgSu3 kTBOUAi4gr1JjWOxr2Dg+EZKbh0VaXgUksSLWQ7gmyiuLZZ8w/FlUx1QBP9gFaTnqh0Y+6 s710lUGxLXjgUFPSKnt6EzoE/r88fBo= Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 (Mac OS X Mail 16.0 \(3776.700.51\)) Subject: Re: [PATCH v2 07/14] mm: khugepaged: collapse_pte_mapped_thp() use pte_offset_map_rw_nolock() X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Muchun Song In-Reply-To: <7f22c46c-2119-4de6-9d58-efcab05b5751@bytedance.com> Date: Thu, 5 Sep 2024 15:18:05 +0800 Cc: David Hildenbrand , Hugh Dickins , Matthew Wilcox , "Vlastimil Babka (SUSE)" , Andrew Morton , Mike Rapoport , Vishal Moola , Peter Xu , Ryan Roberts , christophe.leroy2@cs-soprasteria.com, LKML , Linux Memory Management List , linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org Content-Transfer-Encoding: quoted-printable Message-Id: <69F12D50-BCA9-4874-B558-71008EF82674@linux.dev> References: <24be821f-a95f-47f1-879a-c392a79072cc@linux.dev> <05955456-8743-448A-B7A4-BC45FABEA628@linux.dev> <7f22c46c-2119-4de6-9d58-efcab05b5751@bytedance.com> To: Qi Zheng X-Migadu-Flow: FLOW_OUT > On Sep 5, 2024, at 14:41, Qi Zheng wrote: >=20 >=20 >=20 > On 2024/9/5 14:32, Muchun Song wrote: >>> On Aug 30, 2024, at 14:54, Qi Zheng = wrote: >>>=20 >>>=20 >>>=20 >>> On 2024/8/29 16:10, Muchun Song wrote: >>>> On 2024/8/22 15:13, Qi Zheng wrote: >>>>> In collapse_pte_mapped_thp(), we may modify the pte and pmd entry = after >>>>> acquring the ptl, so convert it to using = pte_offset_map_rw_nolock(). At >>>>> this time, the write lock of mmap_lock is not held, and the = pte_same() >>>>> check is not performed after the PTL held. So we should get = pgt_pmd and do >>>>> pmd_same() check after the ptl held. >>>>>=20 >>>>> For the case where the ptl is released first and then the pml is = acquired, >>>>> the PTE page may have been freed, so we must do pmd_same() check = before >>>>> reacquiring the ptl. >>>>>=20 >>>>> Signed-off-by: Qi Zheng >>>>> --- >>>>> mm/khugepaged.c | 16 +++++++++++++++- >>>>> 1 file changed, 15 insertions(+), 1 deletion(-) >>>>>=20 >>>>> diff --git a/mm/khugepaged.c b/mm/khugepaged.c >>>>> index 53bfa7f4b7f82..15d3f7f3c65f2 100644 >>>>> --- a/mm/khugepaged.c >>>>> +++ b/mm/khugepaged.c >>>>> @@ -1604,7 +1604,7 @@ int collapse_pte_mapped_thp(struct mm_struct = *mm, unsigned long addr, >>>>> if (userfaultfd_armed(vma) && !(vma->vm_flags & VM_SHARED)) >>>>> pml =3D pmd_lock(mm, pmd); >>>>> - start_pte =3D pte_offset_map_nolock(mm, pmd, haddr, &ptl); >>>>> + start_pte =3D pte_offset_map_rw_nolock(mm, pmd, haddr, = &pgt_pmd, &ptl); >>>>> if (!start_pte) /* mmap_lock + page lock should = prevent this */ >>>>> goto abort; >>>>> if (!pml) >>>>> @@ -1612,6 +1612,9 @@ int collapse_pte_mapped_thp(struct mm_struct = *mm, unsigned long addr, >>>>> else if (ptl !=3D pml) >>>>> spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); >>>>> + if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) >>>>> + goto abort; >>>>> + >>>>> /* step 2: clear page table and adjust rmap */ >>>>> for (i =3D 0, addr =3D haddr, pte =3D start_pte; >>>>> i < HPAGE_PMD_NR; i++, addr +=3D PAGE_SIZE, pte++) { >>>>> @@ -1657,6 +1660,16 @@ int collapse_pte_mapped_thp(struct = mm_struct *mm, unsigned long addr, >>>>> /* step 4: remove empty page table */ >>>>> if (!pml) { >>>>> pml =3D pmd_lock(mm, pmd); >>>>> + /* >>>>> + * We called pte_unmap() and release the ptl before = acquiring >>>>> + * the pml, which means we left the RCU critical section, = so the >>>>> + * PTE page may have been freed, so we must do pmd_same() = check >>>>> + * before reacquiring the ptl. >>>>> + */ >>>>> + if (unlikely(!pmd_same(pgt_pmd, pmdp_get_lockless(pmd)))) = { >>>>> + spin_unlock(pml); >>>>> + goto pmd_change; >>>> Seems we forget to flush TLB since we've cleared some pte entry? >>>=20 >>> See comment above the ptep_clear(): >>>=20 >>> /* >>> * Must clear entry, or a racing truncate may re-remove it. >>> * TLB flush can be left until pmdp_collapse_flush() does it. >>> * PTE dirty? Shmem page is already dirty; file is read-only. >>> */ >>>=20 >>> The TLB flush was handed over to pmdp_collapse_flush(). If a >> But you skipped pmdp_collapse_flush(). >=20 > I skip it only in !pmd_same() case, at which time it must be cleared > by other thread, which will be responsible for flushing TLB: WOW! AMAZING! You are right. >=20 > CPU 0 CPU 1 > pmd_clear > spin_unlock > flushing tlb > spin_lock > if (!pmd_same)=20 > goto pmd_change; > pmdp_collapse_flush >=20 > Did I miss something? >=20 >>> concurrent thread free the PTE page at this time, the TLB will >>> also be flushed after pmd_clear(). >>>=20 >>>>> + } >>>>> if (ptl !=3D pml) >>>>> spin_lock_nested(ptl, SINGLE_DEPTH_NESTING); >>>>> } >>>>> @@ -1688,6 +1701,7 @@ int collapse_pte_mapped_thp(struct mm_struct = *mm, unsigned long addr, >>>>> pte_unmap_unlock(start_pte, ptl); >>>>> if (pml && pml !=3D ptl) >>>>> spin_unlock(pml); >>>>> +pmd_change: >>>>> if (notified) >>>>> mmu_notifier_invalidate_range_end(&range); >>>>> drop_folio: