From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 94D14384CEC; Tue, 12 May 2026 06:48:53 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778568533; cv=none; b=cDZxdnxwBQPWgQHzWcsiQpPardl8RHuQhBpIZJKvE5RspqGSoFLtzbn7yiZ68QK+UzyIwH+4QDrAWwPopjJwoNlaw014bDj/byxG0Zw+6VRl1SHepU7Tm0N1rKrbfcI7R1FX1/aA0yxlLnF+GMlVjOErr2B+LJ8OptbakGyUKkQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778568533; c=relaxed/simple; bh=znGDfq/bJ1wkLHdJvnjCKiqpvwQQkaqjjb16iDxb1y0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=AWD1z+NEpygVJw2ZuogNmzDMWFeAWdYZ7Mj9Tx+SyxGlfRFiC3ZwLaba5rE/tFJ4X9pesM3renKEK5iY5KlWXx67CGqft9Jvs1L30i2Mm5NHoOXSlSdlFQREPeIBhl7WcJ0GlOBS4P2tBq1q3fTsbuKIsP9LVbS3Fpf6OeOBuaY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=PXIV3apF; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="PXIV3apF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B61E5C2BCB8; Tue, 12 May 2026 06:48:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778568530; bh=znGDfq/bJ1wkLHdJvnjCKiqpvwQQkaqjjb16iDxb1y0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=PXIV3apFaDjZyXED5iKly1omTUB+j7+uTG8N0AEXXmIG4szMxGvHOGJvhXKMfajMq wxD6uvJcSMQ1ylrb4bjecX0VUZLBZ0W2/oTL8lbeJiKkENf6XoAUEOyvJVw1IHituK OrdJGdhe/tUf8fNAnDd6VLaumzn45ZZ70B/aE+mI7Tunu4X2aJKL4whsroD6qa/0FL bIKBFjf6IroQSs2hPJyuEiSaqJIqXBE33PYQFuu2DDMHmrl3Yui4pjzlHSgJfzi7gx 5kJzv8jAqSLXx2HFuMbGWF0/i6XdtqLk1VZVX439ViJAEpCUYUmUJWgj+tE/rsyBZ8 HXmm1ZulcjYfQ== Message-ID: <7a223c95-fcec-4ae4-9c22-f31c75e3fa93@kernel.org> Date: Tue, 12 May 2026 08:48:47 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] fs/dax: check for empty/zero entries before calling pfn_to_page() To: Souvik Banerjee , djbw@kernel.org 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: <20260511214020.208939-1-souvik@amlalabs.com> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit On 5/11/26 23:40, 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))), which calls > _compound_head() and performs READ_ONCE(page->compound_info) -- 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 both dax_associate_entry() and > dax_disassociate_entry(). Apply the same treatment to dax_busy_page(), > which has the identical pattern but was not touched by the prior fix. > dax_associate_entry() is reachable with a zero entry via > dax_insert_entry() -> dax_associate_entry(new_entry, ...), where > new_entry can carry DAX_ZERO_PAGE (built by dax_make_entry() in > dax_load_hole() / dax_pmd_load_hole()). dax_disassociate_entry() and > dax_busy_page() additionally see DAX_EMPTY entries created by > grab_mapping_entry(). > > The remaining users of dax_to_folio() / dax_to_pfn() in fs/dax.c are > either guarded or only reachable on real-PFN entries, so this exhausts > the anti-pattern. > > 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 > Suggested-by: David Hildenbrand > Signed-off-by: Souvik Banerjee Reviewed-by: David Hildenbrand (Arm) -- Cheers, David