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[2003:cb:c708:9d00:edd9:835b:4bfb:2ce3]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-38a1c8b874asm4653683f8f.109.2024.12.20.11.01.03 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 20 Dec 2024 11:01:05 -0800 (PST) Message-ID: Date: Fri, 20 Dec 2024 20:01:02 +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 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> From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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. > + return 0; > + } > + > /* Ok, finally just insert the thing.. */ > pteval = mk_pte(page, prot); > if (unlikely(is_zero_folio(folio))) { > pteval = pte_mkspecial(pteval); > } else { > folio_get(folio); > + entry = mk_pte(page, prot); > + if (mkwrite) { > + entry = pte_mkyoung(entry); > + entry = maybe_mkwrite(pte_mkdirty(entry), vma);> + } > inc_mm_counter(vma->vm_mm, mm_counter_file(folio)); > folio_add_file_rmap_pte(folio, page, vma); > } > @@ -2147,7 +2175,7 @@ static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte, > } > > static int insert_page(struct vm_area_struct *vma, unsigned long addr, > - struct page *page, pgprot_t prot) > + struct page *page, pgprot_t prot, bool mkwrite) > { > int retval; > pte_t *pte; > @@ -2160,7 +2188,8 @@ static int insert_page(struct vm_area_struct *vma, unsigned long addr, > pte = get_locked_pte(vma->vm_mm, addr, &ptl); > if (!pte) > goto out; > - retval = insert_page_into_pte_locked(vma, pte, addr, page, prot); > + retval = insert_page_into_pte_locked(vma, pte, addr, page, prot, > + mkwrite); Alignment looks odd. Likely you can also just put it into a single line. -- Cheers, David / dhildenb