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[2003:cb:c719:1700:56dc:6a88:b509:d3f3]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-43656b3b214sm635741635e9.28.2025.01.07.03.29.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 07 Jan 2025 03:29:42 -0800 (PST) Message-ID: <35b64e41-b9b1-4fdb-af4f-1296602592ff@redhat.com> Date: Tue, 7 Jan 2025 12:29:40 +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 Cc: akpm@linux-foundation.org, dan.j.williams@intel.com, linux-mm@kvack.org, 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> <6254ce2c-4a47-4501-b518-dedaddcbf91a@redhat.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 06.01.25 03:07, Alistair Popple wrote: > On Fri, Dec 20, 2024 at 08:06:48PM +0100, David Hildenbrand wrote: >> 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. > > Right. > >> I still wonder if that really belongs in here, I would prefer to not play >> such tricks to upgrade write permissions if possible. > > As you have pointed out this was all inspired (ie. mostly copied) > from the existing insert_pfn() implementation which is used from > vmf_insert_mixed{_mkwrite}(). > > I agree a different interface to upgrade permissions would be nice. However > it's tricky because in general callers of these functions (eg. FS DAX) aren't > aware if the page is already mapped by a PTE/PMD. They only know a fault has > occured and the faulting permissions. > > This wouldn't be impossible to fix - the mm does provide vm_ops->page_mkwrite() > for permission upgrades. The difficulty is that most filesystems that support > FS DAX (ie. ext4, XFS) don't treat a vm_ops->page_mkwrite() call any differently > from a vm_ops->fault() call due to write fault. Therefore the FS DAX code is > unaware of whether or not this is a permission upgrade or initial writeable > mapping of the page in the VMA. > > A further issue in there is currently no vm_ops->huge_mkwrite() callback. > > Obviously this could all be plumbed through the MM/FS layers, but that would > require a separate patch series. Given the current implementation has no issues > beyond the cosmetic I'd rather not delay this series any longer, especially as > the cosmetic defect is largely pre-existing (vmf_insert_mixed{_mkwrite}() could > have equally had a separate upgrade interface). Fine with me, just stumbled over it an thought "that looks odd". -- Cheers, David / dhildenb