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[2003:cb:c711:c600:c1d6:7158:f946:f083]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3749efb37efsm1695666f8f.109.2024.08.29.08.31.21 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Aug 2024 08:31:22 -0700 (PDT) Message-ID: <8cbd44d9-f39b-4ee8-b1c1-ba89c12c0e23@redhat.com> Date: Thu, 29 Aug 2024 17:31:21 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2 01/14] mm: pgtable: introduce pte_offset_map_{ro|rw}_nolock() To: Qi Zheng , muchun.song@linux.dev Cc: hughd@google.com, willy@infradead.org, vbabka@kernel.org, akpm@linux-foundation.org, rppt@kernel.org, vishal.moola@gmail.com, peterx@redhat.com, ryan.roberts@arm.com, christophe.leroy2@cs-soprasteria.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org References: <4481a0e4-a7a5-4223-a8ab-d1215d7c6352@bytedance.com> <42aba316-2b01-4fdb-9aff-9e670aac4c6e@redhat.com> <63ef0611-50c2-49b5-ba3f-c6ea81f9fbce@bytedance.com> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 2AD6940007 X-Rspamd-Server: rspam01 X-Stat-Signature: 11g9dsptk9efm51gz3yfzmyq5ysxorgd X-HE-Tag: 1724945486-44234 X-HE-Meta: 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 BLcL1xuO 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 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: List-Subscribe: List-Unsubscribe: On 29.08.24 12:59, Qi Zheng wrote: > > > On 2024/8/28 18:48, David Hildenbrand wrote: >> On 27.08.24 06:33, Qi Zheng wrote: > > [...] > >>> sufficient AFAIUK. >> >> Drop the "AFAIUK" :) >> >> "For R/O access this is sufficient." >> >>> >>> pte_offset_map_rw_nolock(mm, pmd, addr, pmdvalp, ptlp), above, is like >>> pte_offset_map_ro_nolock(); but when successful, it also outputs the >>> pdmval. For R/W access, the callers can not accept that the page table >>> it sees has been unmapped and is about to get freed. The pmdval can help >>> callers to recheck pmd_same() to identify this case once the spinlock is >>> taken. For some cases where exclusivity is already guaranteed, such as >>> holding the write lock of mmap_lock, or in cases where checking is >>> sufficient, such as a !pte_none() pte will be rechecked after the >>> spinlock is taken, there is no need to recheck pdmval. >> >> Right, using pte_same() one can achieve a similar result, assuming that >> the freed page table gets all ptes set to pte_none(). >> >> page_table_check_pte_clear_range() before pte_free_defer() in >> retract_page_tables/collapse_pte_mapped_thp() sanity checks that I think. > > Since commit 1d65b771bc08, retract_page_tables() only holds the > i_mmap_lock_read(mapping) but not mmap_lock, so it seems that > holding the write lock of mmap_lock cannot guarantee the stability > of the PTE page. Guess it depends. khugepaged on anonymous memory will block any page table walkers (like anon THP collapse does) -- per-VMA lock, mmap lock, mapping lock/RMAP lock ... so it *should* be sufficient to hold any of these, right? So at least for now, these (anonymous memory) cases would be ok. Likely that will change when reclaiming empty page tables. > > IIUC, I will also perform a pmd_same() check on the case where the > write lock of mmap_lock is held in v3. Or do I miss something? Can you spell out the instances where you think it might be required. -- Cheers, David / dhildenb