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Fri, 8 May 2026 08:34:35 +0000 (GMT) Date: Fri, 8 May 2026 10:34:34 +0200 From: Alexander Gordeev To: "David Hildenbrand (Arm)" Cc: Wei Yang , Andrew Morton , Lorenzo Stoakes , Anshuman Khandual , Oscar Salvador , linux-s390@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Gerald Schaefer , Heiko Carstens , Vasily Gorbik Subject: Re: [PATCH v2] mm/page_vma_mapped_walk: Use ptep_get_lockless() for lockless access Message-ID: References: <20260504130434.2285053-1-agordeev@linux.ibm.com> <20260507093433.q5xybwumryxclnqy@master> <43815634-29d3-4fa4-b967-0e251ff32f73-agordeev@linux.ibm.com> <20260508010040.o7bypjnfceeqloeq@master> <4da2d354-808d-463e-ac85-5b1f3fd51618-agordeev@linux.ibm.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-TM-AS-GCONF: 00 X-Proofpoint-Reinject: loops=2 maxloops=12 X-Proofpoint-ORIG-GUID: yvYtRSDiaxp9bcv13EZwXbYOjcFwU-1e X-Proofpoint-GUID: djMkVqanIwdlCx0yQ_e8h84ZNmHV7Z4I X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTA4MDA4MSBTYWx0ZWRfXwVnf+ft5fQnZ qMIFYtPCvRKpMBCzIDhRvat2S1+thRIDPV73WUWLInXoJRbmVuCpYYVkPkjIgESAoio21p6tCKz Wvn+ecyK/z/OYMRfMYZP+QFnjA6gG/qM/YF33r+EzHMvBLlSiWgDRHYQogQX6x5DkX6nI0ECpc9 xe8tGlW/b5uEVFQZYU26teaWxR7wpTIdmwP85+KrJhzhcIZo2pf2flOzXUvFlvb3C1nXzDkeQp6 j3sj7+EqQ4K85S4PZAMJQDaArnPxr1N0/uFvtfO9XSY0g8Gaptx5JmJkKOZo3ooeBAQbUCfdil5 xKphlii9onXYMwKDpWsNmSQLvCOdpUkxsLWfE36xLHZJxOBFEyxn/eGSMTJ+1eOPp4Blvc6kbXG L4nrfAmaGxj5w6rtahcRvcNOS/ky4rcXHaL/8Vlb6k0fg0GbH7KbwJtXtZrXjspXEMXKQIoyKMU SGsUhmHtJgMn8/0Kj+A== X-Authority-Analysis: v=2.4 cv=UbFhjqSN c=1 sm=1 tr=0 ts=69fda022 cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=kj9zAlcOel0A:10 a=NGcC8JguVDcA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=U7nrCbtTmkRpXpFmAIza:22 a=2aWb7pxy_uW5kUccelMA:9 a=CjuIK1q_8ugA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-05-07_02,2026-05-06_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 spamscore=0 lowpriorityscore=0 malwarescore=0 suspectscore=0 adultscore=0 priorityscore=1501 bulkscore=0 phishscore=0 impostorscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2604200000 definitions=main-2605080081 On Fri, May 08, 2026 at 10:17:16AM +0200, David Hildenbrand (Arm) wrote: > >> So the rule here is: > >> > >> * ptep_get_lockless() could be used for locked and not locked > >> * ptep_get() only used when locked > >> > >> Right? > > > > Yes, this is my assumption. > > I agree, ptep_get_lockless() simply makes sense to return something sensible if > there are concurrent modifications (which cannot happen when the PTL is held). > > That's why only 32bit with 64bit PTEs and arm64 even has to special-case it. > > > We should clarify in the patch description that in the do-while loop, we might > or might not hold the PTL, and that calling ptep_get_lockless() with the PTL > held is OK. > > I wonder if it's more efficient and clearer, to use the correct variant, though? > > > diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c > index a4d52fdb3056..36d97661a4e5 100644 > --- a/mm/page_vma_mapped.c > +++ b/mm/page_vma_mapped.c > @@ -187,6 +187,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > p4d_t *p4d; > pud_t *pud; > pmd_t pmde; > + pte_t pteval; > > /* The only possible pmd mapping has been handled on last iteration */ > if (pvmw->pmd && !pvmw->pte) > @@ -310,7 +311,11 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) > goto restart; > } > pvmw->pte++; > - } while (pte_none(ptep_get(pvmw->pte))); > + if (!pvmw->ptl) > + pteval = ptep_get_lockless(pvmw->pte); > + else > + pteval = ptep_get(pvmw->pte); > + } while (pte_none(pteval)); Looks fine to me. I will try and add it to the next version. > if (!pvmw->ptl) { > spin_lock(ptl); > > > -- > Cheers, > > David Thanks!