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David Alan Gilbert" , "Matthew Wilcox (Oracle)" , Vlastimil Babka , Baolin Wang , Miaohe Lin , Yang Shi , Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH v2 10/47] hugetlb: add hugetlb_pte to track HugeTLB page table entries Message-ID: References: <20221021163703.3218176-1-jthoughton@google.com> <20221021163703.3218176-11-jthoughton@google.com> Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-ClientProxiedBy: MW4PR04CA0314.namprd04.prod.outlook.com (2603:10b6:303:82::19) To BY5PR10MB4196.namprd10.prod.outlook.com (2603:10b6:a03:20d::23) MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: BY5PR10MB4196:EE_|SA2PR10MB4491:EE_ X-MS-Office365-Filtering-Correlation-Id: 5611d505-8163-4529-9aa3-08dadd3a20b2 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0; X-Microsoft-Antispam-Message-Info: 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 xgCFSF7h Tk8KAysztYsK6KqFr0rEQ4fEYkC/J+DAs9MB9rwEtfHGVSlo0e1uWTQE2AtazaSoKSR7aK6HhvAF7ZTWfBNFwE+5yrXT3bSoaVRBB6vNqYrayfCY= 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 12/09/22 11:02, James Houghton wrote: > On Wed, Dec 7, 2022 at 7:46 PM Mina Almasry wrote: > > On Fri, Oct 21, 2022 at 9:37 AM James Houghton wrote: > > > > > > +bool hugetlb_pte_present_leaf(const struct hugetlb_pte *hpte, pte_t pte) > > > > I also don't know if this is obvious to other readers, but I'm quite > > confused that we pass both hugetlb_pte and pte_t here, especially when > > hpte has a pte_t inside of it. Maybe a comment would help. > > It's possible for the value of the pte to change if we haven't locked > the PTL; we only store a pte_t* in hugetlb_pte, not the value itself. I had comments similar to Mina and Peter on other parts of this patch. Calling this without some type of locking is 'interesting'. I have not yet looked at callers (without locking), but I assume such callers can handle stale results. > Thinking about this... we *do* store `shift` which technically depends > on the value of the PTE. If the PTE is pte_none, the true `shift` of > the PTE is ambiguous, and so we just provide what the user asked for. > That could lead to a scenario where UFFDIO_CONTINUE(some 4K page) then > UFFDIO_CONTINUE(CONT_PTE_SIZE range around that page) can both succeed > because we merely check if the *first* PTE in the contiguous bunch is > none/has changed. Right, Yuck! > > So, in the case of a contiguous PTE where we *think* we're overwriting > a bunch of none PTEs, we need to check that each PTE we're overwriting > is still none while holding the PTL. That means that the PTL we use > for cont PTEs and non-cont PTEs of the same level must be the same. > > So for the next version, I'll: > - add some requirement that contiguous and non-contiguous PTEs on the > same level must use the same PTL > - think up some kind of API like all_contig_ptes_none(), but it only > really applies for arm64, so I think actually putting it in can wait. > I'll at least put a comment in hugetlb_mcopy_atomic_pte and > hugetlb_no_page (near the final huge_pte_none() and pte_same() > checks). > -- Mike Kravetz