From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 BDA2E33B969 for ; Mon, 17 Aug 2026 17:45:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786988752; cv=none; b=KAXzmwro+VolYgXF7g/wpasgitETXx7SXvm9ia9TGs8e5lLAuvyO/cmwrcWwVW4ZSrVe379t2DQ2jDgCv9XrFzXhebsbAkvchmIcVPkAHtACGk4lbR+25wxXxQ/hT3/WvQC/nzgY4/z2nZNKRTJYwTKGWoWGtEEJdCQk9bZAqEI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786988752; c=relaxed/simple; bh=iCUxGH8M0s0Cmu5RCJcaDCf8fQXzyDltaat9z5M+uCw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=JJEHUyx66O2/vvzPfssGEJl+1Eq17Qo688enYgcLtNmo9HXZRXylsIPqe5pDS5SNTaxpkrl3rLD84GX0rCqZaW9VSOQLI0fBL8F49KlYvlTG3QdTX40vdvLRXgWPHFM1ejhaCOHykiAeYXB9jp8Yhl+n7HxLwIMpBGnZccrWJ5w= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Z6lCAadT; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="Z6lCAadT" Received: by smtp.kernel.org (Postfix) with ESMTPSA id B4A411F000E9; Mon, 17 Aug 2026 17:45:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786988750; bh=fAKo7TrZu9+ob9VDRDNL34HG8FM2DqWuvJo8ttA9XR0=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Z6lCAadT1fE7faFuJ2NmDgiosFfh6ejTBefrzHqr6GGcbbNigaLzYEBjOt40prDvR lyN7h+sbKHazVIoPNYRv3edYv48af0G+NYUC42N1LA4VEYONxZ91k3kCf6MBfiTZnV 1oIf69wm1kSUsd0x9lJANQQRxnhi88WgZwsTiM6dyXSHCRkPYkuZK7qnnFceA6BF83 AgyqyKmiL9OBCXKc694/LIaXxWHB8EmNPh2RaWkJfuCCvKaLCQshMu8iOYiRPRRYRH U8q6CKZXpVJe1MaNs5je5CpfV8nTJKJxTGIRiW/gAI/0SFWjn+0jk6XMeCSOHiL9km RbxVcocLbpAuQ== Message-ID: Date: Mon, 17 Aug 2026 19:45:44 +0200 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] mm: Use a folio in the softleaf_is_device_private path To: "Lorenzo Stoakes (ARM)" Cc: Hongfu Li , akpm@linux-foundation.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hongfu Li References: <20260803031554.30421-1-hongfu.li@linux.dev> <3ae2f1e1-c784-44ed-9b2c-5edf5699d627@kernel.org> 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 8/17/26 18:39, Lorenzo Stoakes (ARM) wrote: > On Mon, Aug 17, 2026 at 05:57:57PM +0200, David Hildenbrand (Arm) wrote: >> On 8/17/26 17:52, David Hildenbrand (Arm) wrote: >>> >>> Okay, so this breaks the HMM selftests: >>> >>> # ok 30 hmm.hmm_device_private.migrate_anon_huge_fault >>> # # RUN hmm.hmm_device_private.migrate_partial_unmap_fault ... >>> [ 14.589125] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP >>> [ 14.589178] Modules linked in: test_hmm >>> [ 14.589203] CPU: 25 UID: 0 PID: 653 Comm: hmm-tests Not tainted 7.2.0-rc5+ #14 PREEMPT >>> [ 14.589243] Hardware name: linux,dummy-virt (DT) >>> [ 14.589268] pstate: 63400005 (nZCv daif +PAN -UAO +TCO +DIT -SSBS BTYPE=--) >>> [ 14.589300] pc : folio_unlock+0x78/0x88 >>> [ 14.589326] lr : folio_unlock+0x78/0x88 >>> [ 14.589349] sp : ffff80008284bb90 >>> [ 14.589368] x29: ffff80008284bb90 x28: ffff0000cc8ce180 x27: 0000000000000000 >>> [ 14.589406] x26: 0000000000000000 x25: ffff0000ca395dc0 x24: 0000000000000000 >>> [ 14.589447] x23: 0000000020100073 x22: ffff0000c23d35c0 x21: ffffffffbfff8000 >>> [ 14.589485] x20: ffffffffbfff8000 x19: ffff80008284bca0 x18: 0000000000000010 >>> [ 14.589523] x17: 3030303030303030 x16: 2066666666666666 x15: 6630303030303030 >>> [ 14.589562] x14: 0000000000000000 x13: 29296f696c6f6628 x12: 64656b636f6c5f74 >>> [ 14.589603] x11: fffffffffffc7100 x10: fffffffffffc70b0 x9 : ffffb0504ff88f70 >>> [ 14.589640] x8 : 3fffffffffffdfff x7 : ffff000c79b24bc0 x6 : 00000000000001a7 >>> [ 14.589680] x5 : ffff000c79b24bc0 x4 : 0000000000000000 x3 : 0000000000000000 >>> [ 14.589718] x2 : 0000000000000000 x1 : ffff0000cc8ce180 x0 : 000000000000003f >>> [ 14.589756] Call trace: >>> [ 14.589769] folio_unlock+0x78/0x88 (P) >>> [ 14.589792] do_swap_page+0xf40/0x1580 >>> [ 14.589815] __handle_mm_fault+0x648/0xe68 >>> [ 14.589838] handle_mm_fault+0x9c/0x330 >>> [ 14.589858] do_page_fault+0x268/0x7e0 >>> [ 14.589882] do_translation_fault+0x5c/0x80 >>> [ 14.589904] do_mem_abort+0x48/0xa0 >>> [ 14.589927] el0_da+0x38/0xd0 >>> [ 14.589948] el0t_64_sync_handler+0xd0/0xe8 >>> [ 14.589970] el0t_64_sync+0x198/0x1a0 >>> [ 14.589990] Code: d65f03c0 d000eec1 913c4021 94015993 (d4210000) >>> >>> I can only speculate that we need another >>> >>> folio = page_folio(vmf->page); >>> >>> after the migrate_to_ram() call. >>> >>> Let me try that real quick. >>> >> >> Yeah, that fixes it: >> >> diff --git a/mm/memory.c b/mm/memory.c >> index 4134ac607ee0..83d1a2be957c 100644 >> --- a/mm/memory.c >> +++ b/mm/memory.c >> @@ -4937,6 +4937,7 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) >> pte_unmap_unlock(vmf->pte, vmf->ptl); >> pgmap = page_pgmap(vmf->page); >> ret = pgmap->ops->migrate_to_ram(vmf); >> + folio = page_folio(vmf->page); > > So after migrate_to_ram() the folio might have been split or otherwise somehow > the page doesn't belong to the same locked, refcount-incremented folio it did > before? I suspect a split. > > That's kinda a footgun... but this documents it at least. > > I think a comment explaining how this can happen would be helpful though as this > doesn't seem intuitive. I think, conceptually, calling into something that consumes a page (vmf->page) always needs care when operating on folios. Passing the vmf to some callback might be the odd thing here, because the vmf->page contract is not really clear. -- Cheers, David