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bh=0sV2KOgy6g6NiXOUj3SUT7h86UfP0gGqBaWtBYQWHwM=; b=WZTXQRJTBDnUq536dPYTZHHnanujLV1aChdQZGg9io5q3rrhHCJfG+jgHnvDb3r9M54ivu o41cPQkZy7b/HKtAUn32EjYH5eoV0x5LbCcsh4ZZDS4DhBbNdvhgR/iaD+/o71perLeV4q cUnPnpNGch5NpIZk65rZqrZYt1mx/Oc= X-Stat-Signature: fnyx6k1ycp3xxis3uayromy7qfipmic5 X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 618571A0004 X-Rspam-User: Authentication-Results: imf19.hostedemail.com; dkim=pass header.d=oracle.com header.s=corp-2022-7-12 header.b=eckHQrtp; dkim=pass header.d=oracle.onmicrosoft.com header.s=selector2-oracle-onmicrosoft-com header.b=tiVAP4c+; dmarc=pass (policy=none) header.from=oracle.com; arc=pass ("microsoft.com:s=arcselector9901:i=1"); spf=pass (imf19.hostedemail.com: domain of mike.kravetz@oracle.com designates 205.220.177.32 as permitted sender) smtp.mailfrom=mike.kravetz@oracle.com X-HE-Tag: 1667500723-531927 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 11/03/22 14:11, Peter Xu wrote: > On Thu, Nov 03, 2022 at 08:42:01AM -0700, Mike Kravetz wrote: > > On 10/30/22 17:29, Peter Xu wrote: > > Not sure if it is worth calling out that we are safe if the process owning the > > page table being walked is single threaded? Although, a pmd can be 'unshared' > > due to an operation in another process, the primary is when the pmd is cleared > > which only happens when the unshare is initiated by a thread of the process > > owning the page tables being walked. > > Even if the process is single threaded, the pmd unshare can still trigger > from other threads too, am I right? > > Looking at huge_pmd_unshare() callers, the major ones that doesn't need > current mm context are: > > - __unmap_hugepage_range() (e.g. hole punch from other process on file?) > - try_to_unmap_one() > - try_to_migrate_one() > > So for example, even for a single thread process, if its pmd shared with > another process, the other process can do (1) punch hole on pmd shared > region, then (2) munmap() the pmd shared region, then it seems the single > thread process can be still on risk of accessing freed pgtable. Yes, you are correct. I was not thinking about an unmap initiated by another process doing something like hole punch or truncation. -- Mike Kravetz