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b=KNMSibocozxsOEboPUh9RzwdH6/04r2b9CNhhHRp61mEbqeBdpDKtdBsxnR29HH2p T8NW7atvdWqQcoP61SdwHnZT0VOORIr3HYAfhHJ4efdT1pTPfZBLgmncH4/5Dt9Yc/ GvrIS5FzN/06GoXdVwbt6N4WOjXb+JVDHPSD57HGdw1TxMAiMD8QmnSsBibY6ovo7p kpS7MkkiTOdGclxIQtQmQocIIUYsK/ytq/NF7+zx4AL5KE7hnaATQfZ6ZseC9F44kL RdHlmxG70wrFrqXrWUhBoJy4KPvb0XIYMt2tc2aHSRRjULaL6TVJ+gMPSpMaGGmm4n Pc1E1+kRsoF3g== Date: Thu, 21 May 2026 11:39:36 +0200 From: "Oscar Salvador (SUSE)" To: yizhang089@gmail.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, david@redhat.com, william.kucharski@linux.dev, karol.wachowski@linux.intel.com, yi.zhang@huawei.com, yi.zhang@huaweicloud.com, liuyongqiang13@huawei.com, wangkefeng.wang@huawei.com, yangerkun@huawei.com Subject: Re: [PATCH v3] mm: do not install PMD mappings when handling a COW fault Message-ID: References: <20260520151624.78370-1-yizhang089@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260520151624.78370-1-yizhang089@gmail.com> X-Rspam-User: X-Rspamd-Queue-Id: A7003140010 X-Rspamd-Server: rspam04 X-Stat-Signature: fmrkwsutji4titgmwunifrx4sjpxym9y X-HE-Tag: 1779356383-117429 X-HE-Meta: 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 egg8hS+a DdsPi6sV9ecIftzuER6eSJSo0t+7eye425jHAkDN/xwj9chQ5scqcoVgqZQ5ZGnr1PC1fl+fyvclqEqUrx3z8o88lIJArhrL4lknBhRy7srMyoxxzGL8SPW4Oj6/mP3ngVHH2+Z8adgiXMM0HCVJwYlIP+p6mOJI3npm2Nvj9q8oP0x23LD59rue6eEhA/b7253mckHBR/A2rp3LQ6+1dH54msExSQBIKMziwi3M6G9h1kdK7Yfa5P98OAX5HOUHdks5Fib7obUHNIQ7QW529xUWWwMDgyLqPoOTEfrMB507rWOGyOM7AC/QvdYTvwh7eaqrgvgydrmyP4YY= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Wed, May 20, 2026 at 11:16:24AM -0400, yizhang089@gmail.com wrote: > From: Zhang Yi > > When pinning a page with FOLL_LONGTERM in a CoW VMA and a PMD-aligned > (2MB on x86) large folio follow_page_mask() failed to obtain a valid > anonymous page, resulting in an infinite loop issue. The specific > triggering process is as follows: > > 1. User call mmap with a 2MB size in MAP_PRIVATE mode for a file that > has a 2MB large folio installed in the page cache. > > addr = mmap(NULL, 2*1024*1024, PROT_READ, MAP_PRIVATE, file_fd, 0); > > 2. The kernel driver pass this mapped address to pin_user_pages_fast() > in FOLL_LONGTERM mode. > > pin_user_pages_fast(addr, 512, FOLL_LONGTERM, pages); > > -> pin_user_pages_fast() > | gup_fast_fallback() > | __gup_longterm_locked() > | __get_user_pages_locked() > | __get_user_pages() > | follow_page_mask() > | follow_p4d_mask() > | follow_pud_mask() > | follow_pmd_mask() //pmd_leaf(pmdval) is true because the > | //huge PMD is installed. This is normal > | //in the first round, but it shouldn't > | //happen in the second round. > | follow_huge_pmd() //require an anonymous page > | return -EMLINK; > | faultin_page() > | handle_mm_fault() > | wp_huge_pmd() //remove PMD and fall back to PTE > | handle_pte_fault() > | do_pte_missing() > | do_fault() > | do_read_fault() //FAULT_FLAG_WRITE is not set > | finish_fault() > | do_set_pmd() //install a huge PMD again, this is wrong!!! > | do_wp_page() //create private anonymous pages > <- goto retry; > > Due to an incorrectly large PMD set in do_read_fault(), > follow_pmd_mask() always returns -EMLINK, causing an infinite loop. > > David pointed out that we can preallocate a page table and remap the PMD > to be mapped by a PTE table in wp_huge_pmd() in the future. But now we > can avoid this issue by not installing PMD mappings when handling a COW > and unshare fault in do_set_pmd(). > > Fixes: a7f226604170 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page") > Reported-by: Karol Wachowski > Closes: https://lore.kernel.org/linux-ext4/844e5cd4-462e-4b88-b3b5-816465a3b7e3@linux.intel.com/ > Suggested-by: David Hildenbrand > Signed-off-by: Zhang Yi > Acked-by: David Hildenbrand Reviewed-by: Oscar Salvador (SUSE) -- Oscar Salvador SUSE Labs