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[2003:cb:c708:9d00:edd9:835b:4bfb:2ce3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4366112e780sm53169585e9.0.2024.12.20.11.06.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Dec 2024 11:06:51 -0800 (PST) Message-ID: <6254ce2c-4a47-4501-b518-dedaddcbf91a@redhat.com> Date: Fri, 20 Dec 2024 20:06:48 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 12/25] mm/memory: Enhance insert_page_into_pte_locked() to create writable mappings From: David Hildenbrand To: Alistair Popple , akpm@linux-foundation.org, dan.j.williams@intel.com, linux-mm@kvack.org Cc: lina@asahilina.net, zhang.lyra@gmail.com, gerald.schaefer@linux.ibm.com, vishal.l.verma@intel.com, dave.jiang@intel.com, logang@deltatee.com, bhelgaas@google.com, jack@suse.cz, jgg@ziepe.ca, catalin.marinas@arm.com, will@kernel.org, mpe@ellerman.id.au, npiggin@gmail.com, dave.hansen@linux.intel.com, ira.weiny@intel.com, willy@infradead.org, djwong@kernel.org, tytso@mit.edu, linmiaohe@huawei.com, peterx@redhat.com, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, nvdimm@lists.linux.dev, linux-cxl@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jhubbard@nvidia.com, hch@lst.de, david@fromorbit.com References: <25a23433cb70f0fe6af92042eb71e962fcbf092b.1734407924.git-series.apopple@nvidia.com> Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: CE3881C0015 X-Rspam-User: X-Rspamd-Server: rspam07 X-Stat-Signature: uodjseabai5e55c1bk6d39uiedznuynw X-HE-Tag: 1734721579-721160 X-HE-Meta: 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 F14AV7Om 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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 20.12.24 20:01, David Hildenbrand wrote: > On 17.12.24 06:12, Alistair Popple wrote: >> In preparation for using insert_page() for DAX, enhance >> insert_page_into_pte_locked() to handle establishing writable >> mappings. Recall that DAX returns VM_FAULT_NOPAGE after installing a >> PTE which bypasses the typical set_pte_range() in finish_fault. >> >> Signed-off-by: Alistair Popple >> Suggested-by: Dan Williams >> >> --- >> >> Changes since v2: >> >> - New patch split out from "mm/memory: Add dax_insert_pfn" >> --- >> mm/memory.c | 45 +++++++++++++++++++++++++++++++++++++-------- >> 1 file changed, 37 insertions(+), 8 deletions(-) >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 06bb29e..cd82952 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -2126,19 +2126,47 @@ static int validate_page_before_insert(struct vm_area_struct *vma, >> } >> >> static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte, >> - unsigned long addr, struct page *page, pgprot_t prot) >> + unsigned long addr, struct page *page, >> + pgprot_t prot, bool mkwrite) >> { >> struct folio *folio = page_folio(page); >> + pte_t entry = ptep_get(pte); >> pte_t pteval; >> >> - if (!pte_none(ptep_get(pte))) >> - return -EBUSY; >> + if (!pte_none(entry)) { >> + if (!mkwrite) >> + return -EBUSY; >> + >> + /* >> + * For read faults on private mappings the PFN passed in may not >> + * match the PFN we have mapped if the mapped PFN is a writeable >> + * COW page. In the mkwrite case we are creating a writable PTE >> + * for a shared mapping and we expect the PFNs to match. If they >> + * don't match, we are likely racing with block allocation and >> + * mapping invalidation so just skip the update. >> + */ > > Would it make sense to instead have here > > /* See insert_pfn(). */ > > But ... > >> + if (pte_pfn(entry) != page_to_pfn(page)) { >> + WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry))); >> + return -EFAULT; >> + } >> + entry = maybe_mkwrite(entry, vma); >> + entry = pte_mkyoung(entry); >> + if (ptep_set_access_flags(vma, addr, pte, entry, 1)) >> + update_mmu_cache(vma, addr, pte); > > ... I am not sure if we want the above at all. Someone inserted a page, > which is refcounted + mapcounted already. > > Now you ignore that and do like the second insertion "worked" ? > > No, that feels wrong, I suspect you will run into refcount+mapcount issues. > > If there is already something, inserting must fail IMHO. If you want to > change something to upgrade write permissions, then a different > interface should be used. Ah, now I realize that the early exit saves you because we won't adjust the refcount +mapcount. I still wonder if that really belongs in here, I would prefer to not play such tricks to upgrade write permissions if possible. -- Cheers, David / dhildenb