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imf27.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=oKmgI4wq; spf=pass (imf27.hostedemail.com: domain of david@kernel.org designates 172.234.252.31 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1778231711; a=rsa-sha256; cv=none; b=svpk+AKHtzwaLvvHMjbwg1P6Azdea4Kw499O0CTtjYSKNBflWf/yKCvft2MA/liqezPD4W bR1hOHWfEYz7u0qCyP9tHFBcSQ1b94pCBfBN38gPOCx60BckaHoHXYu7yzZWb62ddVCggn ZnJ0DK0X1aQTPUITCQdpcAdSanMPJeM= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by sea.source.kernel.org (Postfix) with ESMTP id 93371429C2; Fri, 8 May 2026 09:15:10 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6BE1DC2BCC7; Fri, 8 May 2026 09:15:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778231710; bh=NXZCU9O+VnXKW0Sj9R4wxcr+LevLi2lp2ovXFLv6vJk=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=oKmgI4wq7bbeVrOg4MRR6sfYHcNJ4J+g8kT2q+cfspcG0ggGGazKNsKJW9yHW8Yht UfKdlAf8CONiwguOIIXA8zzDMabe3eZZ9Wop21tmbFQk0s0+wricTFF+eBLpFZZii1 wsUO+jsMDHDypwLO8KVorS5qpt+ddDuwgG76To6GvFrYwOWYfEUxrQEHYC70YIyJA/ PiySftNL/Go2Daomk44GyADSVg1SXDY79SLFwr74xdcb0Otza2Sf1fpbp1T+pFWGyx KPXDafM+0hQ1IeWB5YV7nJirMYMI47Ly4IxIsc9ZgN7rOg3UzIMWwgdX+QsvNbiWYB Gv5SbUKY/Oplw== Message-ID: Date: Fri, 8 May 2026 11:15:05 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] fs/dax: check for empty/zero entries before calling pfn_to_page() To: Souvik Banerjee , dan.j.williams@intel.com Cc: willy@infradead.org, jack@suse.cz, apopple@nvidia.com, linux-fsdevel@vger.kernel.org, nvdimm@lists.linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, stable@vger.kernel.org References: <20260501233933.2614302-1-souvik@amlalabs.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzS5EYXZpZCBIaWxk ZW5icmFuZCAoQ3VycmVudCkgPGRhdmlkQGtlcm5lbC5vcmc+wsGQBBMBCAA6AhsDBQkmWAik AgsJBBUKCQgCFgICHgUCF4AWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaYJt/AIZAQAKCRBN 3hD3AP+DWriiD/9BLGEKG+N8L2AXhikJg6YmXom9ytRwPqDgpHpVg2xdhopoWdMRXjzOrIKD g4LSnFaKneQD0hZhoArEeamG5tyo32xoRsPwkbpIzL0OKSZ8G6mVbFGpjmyDLQCAxteXCLXz ZI0VbsuJKelYnKcXWOIndOrNRvE5eoOfTt2XfBnAapxMYY2IsV+qaUXlO63GgfIOg8RBaj7x 3NxkI3rV0SHhI4GU9K6jCvGghxeS1QX6L/XI9mfAYaIwGy5B68kF26piAVYv/QZDEVIpo3t7 /fjSpxKT8plJH6rhhR0epy8dWRHk3qT5tk2P85twasdloWtkMZ7FsCJRKWscm1BLpsDn6EQ4 jeMHECiY9kGKKi8dQpv3FRyo2QApZ49NNDbwcR0ZndK0XFo15iH708H5Qja/8TuXCwnPWAcJ DQoNIDFyaxe26Rx3ZwUkRALa3iPcVjE0//TrQ4KnFf+lMBSrS33xDDBfevW9+Dk6IISmDH1R HFq2jpkN+FX/PE8eVhV68B2DsAPZ5rUwyCKUXPTJ/irrCCmAAb5Jpv11S7hUSpqtM/6oVESC 3z/7CzrVtRODzLtNgV4r5EI+wAv/3PgJLlMwgJM90Fb3CB2IgbxhjvmB1WNdvXACVydx55V7 LPPKodSTF29rlnQAf9HLgCphuuSrrPn5VQDaYZl4N/7zc2wcWM7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20260501233933.2614302-1-souvik@amlalabs.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Queue-Id: 7A4DC40002 X-Rspamd-Server: rspam06 X-Stat-Signature: 455etfau6t8m6ibxngmcd4frbr8dmpb5 X-HE-Tag: 1778231711-752966 X-HE-Meta: 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 yxywrvdX zPZHWdqnDslQ2e5RR64gVU9arFrTWVAsLxI1c/gPBhrP04TlXejMBzYazZGsAknC+wQUJyMkRXWBHnwfIzL3m6ybzV1MiSqkm8eKYpwmgJdDxOlUUj37MAgbbX+bLyDhHdvix1d5AaOOdZvCc3FcN1YoxotudrCE9pWAEjEDatOZJQb5bP25BfpnOPZrKnL5Kh0rv+8vCp7SSEFpwb+JtEgI5E1MikhFrSvfxwEZwpY8dCWiTQt0/I7cVbA0Djv2f0KcbZM3OjCbgs5/hfFgmvwiVUE7MLvQYcs+a2Ty8TmelHoP3mPlLbCDlxXI+BSM04HU2lkGubUicEjTi0T5kmDd5MwIv2lTzvIT3NvmxGgnO80XK685i/kji94RIFgPC7ML11sPwT43Ikx3gzBdG6hey1Ib8Glefxq6G Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/2/26 01:39, Souvik Banerjee wrote: > Commit 98c183a4fccf ("fs/dax: don't disassociate zero page entries") > added zero/empty-entry early returns to dax_associate_entry() and > dax_disassociate_entry(), but placed them *after* the > `struct folio *folio = dax_to_folio(entry);` line. dax_to_folio() > expands to page_folio(pfn_to_page(dax_to_pfn(entry))), and page_folio() > performs READ_ONCE(page->compound_head) -- a real dereference of the > struct page pointer derived from a bogus PFN extracted from the > empty/zero XA value. > > On systems where vmemmap covers all of RAM that dereference reads > garbage and is harmless: the early return then discards the result. > On virtio-pmem with altmap (vmemmap stored inside the device), only > the real device PFN range is mapped, so the dereference triggers a > kernel paging fault from the truncate / invalidate path and from the > PMD-downgrade branch of dax_iomap_pte_fault when an entry is being > freed: > > Unable to handle kernel paging request at > virtual address ffff_fdff_bf00_0008 (vmemmap region) > Call trace: > dax_disassociate_entry.isra.0+0x20/0x50 > dax_iomap_pte_fault > dax_iomap_fault > erofs_dax_fault > > Close the residual gap by moving the dax_to_folio() call after the > zero/empty guard in dax_disassociate_entry(). Apply the same > treatment to dax_busy_page(), which has the identical pattern but > was not touched by the prior fix. > > Fixes: 98c183a4fccf ("fs/dax: don't disassociate zero page entries") > Fixes: 38607c62b34b ("fs/dax: properly refcount fs dax pages") > Cc: stable@vger.kernel.org # v6.15+ > Cc: Alistair Popple > Signed-off-by: Souvik Banerjee > --- > fs/dax.c | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/fs/dax.c b/fs/dax.c > index 6d175cd47a99..6878473265bb 100644 > --- a/fs/dax.c > +++ b/fs/dax.c > @@ -505,21 +505,23 @@ static void dax_associate_entry(void *entry, struct address_space *mapping, > static void dax_disassociate_entry(void *entry, struct address_space *mapping, > bool trunc) > { > - struct folio *folio = dax_to_folio(entry); > + struct folio *folio; > > if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) > return; > > + folio = dax_to_folio(entry); > dax_folio_put(folio); > } > > static struct page *dax_busy_page(void *entry) > { > - struct folio *folio = dax_to_folio(entry); > + struct folio *folio; > > if (dax_is_zero_entry(entry) || dax_is_empty_entry(entry)) > return NULL; > > + folio = dax_to_folio(entry); > if (folio_ref_count(folio) - folio_mapcount(folio)) > return &folio->page; > else Makes perfect sense to me. What about the usage in dax_associate_entry()? -- Cheers, David