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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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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> Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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