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a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1725520762; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=NdpqiZxj8ma7ioX7XyPbBng0l15ju36eQ8GWCQEKGXU=; b=GpLSHLumwSE9X1dOCuP0uNTzYHT0jjbaboRmJWeFgVijYBeFCQ/grQZRdlWFmPXbdnwE+5 qQOHirVXp+IXolqliwYmmH09kFwvZtNakvtP4lemivnlNmvCUDUEoemYTFkQhVwzMtUN/1 xD0WXgrUwz1JrroWkBny+vhGqwiawak= Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 5914EFEC; Thu, 5 Sep 2024 00:21:06 -0700 (PDT) Received: from [10.57.87.65] (unknown [10.57.87.65]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 5716C3F66E; Thu, 5 Sep 2024 00:20:38 -0700 (PDT) Message-ID: Date: Thu, 5 Sep 2024 08:20:36 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH RFC] mm: arm64: advance pte for contpte_ptep_set_access_flags Content-Language: en-GB To: Barry Song <21cnbao@gmail.com> Cc: Will Deacon , David Hildenbrand , akpm@linux-foundation.org, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, Barry Song , Ard Biesheuvel , John Hubbard , Mark Rutland , Catalin Marinas References: <20240831083537.62111-1-21cnbao@gmail.com> <20240904151304.GB13919@willie-the-truck> <629ee1a6-c606-4a8d-bfd6-a2be31feddcf@arm.com> From: Ryan Roberts In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: CD1C2120007 X-Stat-Signature: ktnthc4b6up48heh8qgbhrxffqrjatir X-Rspam-User: X-HE-Tag: 1725520840-957126 X-HE-Meta: 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 /127K2+N WyOn5qcI0z/Vxkemxdb5ypQoq9t4mUWSpZf8NNWlUiBphkzGKofMXe8Js+ILKsavbb+bUoYZEo0UHmzMLfyLUMtQpuMh7udBu7VSm7gFFfOsG3ck9Myh8GoH4Av5i9n14IBJ1BkbkT8bAryN3U122Ko/pYvHD3IhVv4rXmrMVRk8iKHsJC3NgMIfCKLXF16tAMm1BPPm/KBz01yUliIeRjxXB3RKC7poSkJne8SgqaZlY6nUAyCxev9tAz7RZJ6mOjAltEVHZw1AZq3aVDuRPXP0atM4Efv+HMKwKRj8nf44gYGY0XawdpXixfFtk9Ej8O1Gdc1UB/7iBJRrZ/WeaFkZgnuY3Wh2i73qb9swE0249uLIXu+WT8rgiow== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 05/09/2024 04:27, Barry Song wrote: > On Thu, Sep 5, 2024 at 3:50 AM Ryan Roberts wrote: >> >> On 04/09/2024 16:13, Will Deacon wrote: >>> (Adding Ryan, since you're asking him a question!) >> >> Thanks, Will! >> >> Afraid I don't do a good job of monitoring the list; I'm guessing there are >> automated ways to filter for mentions of my name so I catch this sort of thing >> in future? > > It's not your fault. I just realized that, for some unknown reason, I forgot to > CC you. No worries. I was just asking if there is a general approach that people take to monitor mail that they are not explicitly cc'ed on, but I guess that's a bit off topic. > >> >>> >>> On Sat, Aug 31, 2024 at 10:06:40PM +1200, Barry Song wrote: >>>> On Sat, Aug 31, 2024 at 9:54 PM David Hildenbrand wrote: >>>>> >>>>> On 31.08.24 10:35, Barry Song wrote: >>>>>> From: Barry Song >>>>>> >>>>>> Hi Ryan, David, >>>>>> it seems contpte_ptep_set_access_flags() has never advanced >>>>>> pte pfn, and it is setting all entries' pfn to the first >>>>>> subpage. But I feel quite strange we never have a bug reported. >>>>>> Am I missing something? >>>>>> >>>>>> Fixes: 4602e5757bcc ("arm64/mm: wire up PTE_CONT for user mappings") >>>>>> Cc: Ard Biesheuvel >>>>>> Cc: John Hubbard >>>>>> Cc: Mark Rutland >>>>>> Cc: Catalin Marinas >>>>>> Cc: David Hildenbrand >>>>>> Cc: Will Deacon >>>>>> Signed-off-by: Barry Song >>>>>> --- >>>>>> arch/arm64/mm/contpte.c | 4 +++- >>>>>> 1 file changed, 3 insertions(+), 1 deletion(-) >>>>>> >>>>>> diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c >>>>>> index a3edced29ac1..10dcd2641184 100644 >>>>>> --- a/arch/arm64/mm/contpte.c >>>>>> +++ b/arch/arm64/mm/contpte.c >>>>>> @@ -421,8 +421,10 @@ int contpte_ptep_set_access_flags(struct vm_area_struct *vma, >>>>>> ptep = contpte_align_down(ptep); >>>>>> start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE); >>>>>> >>>>>> - for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) >>>>>> + for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) { >>>>>> __ptep_set_access_flags(vma, addr, ptep, entry, 0); >>>>>> + entry = pte_advance_pfn(entry, 1); >>>>>> + } >>>>>> >>>>>> if (dirty) >>>>>> __flush_tlb_range(vma, start_addr, addr, >>>>> >>>>> Taking a closer look at __ptep_set_access_flags(), there is: >>>>> >>>>> /* only preserve the access flags and write permission * >>>>> pte_val(entry) &= PTE_RDONLY | PTE_AF | PTE_WRITE | PTE_DIRTY; >>>>> >>>>> So it looks like it doesn't need the PFN? >> >> Correct, I don't believe there is a bug here; __ptep_set_access_flags() only >> consumes the access flags from entry. >> >>>> >>>> right. >>>> >>>>> >>>>> >>>>> OTOH, there is the initial: >>>>> >>>>> >>>>> if (pte_same(pte, entry)) >>>>> return 0; >>>>> >>>>> check that might accelerate things. >> >> There is an equivalent check in contpte_ptep_set_access_flags() which is >> checking for the whole contpte block and returning early if so. So I don't think >> there is a problem here either. >> >>>>> >>>>> So unless I am missing something, this works as expected? (and if the >>>>> pte_same() would frequently be taken with your change would be worthwile >>>>> to optimize) >>>> >>>> >>>> Right. From page 1 to page (nr_pages - 1), we consistently get FALSE >>>> for pte_same(). >>>> This seems quite strange. I think we might need to "fix" it, at least >>>> for the sake of code >>>> semantics. on the other hand, if pte_same() is not important, it >>>> should be dropped. >>>> >>>> Hi Ryan, >>>> what is your take on this? >> >> The code is correct and working as intended, AFAICT. But I accept that this is >> not exactly obvious. I'd be happy to Rb your proposed change if you feel it >> clarifies things. > > If this is the case, I'd rather add some comments instead in v2? > > diff --git a/arch/arm64/mm/contpte.c b/arch/arm64/mm/contpte.c > index a3edced29ac1..55107d27d3f8 100644 > --- a/arch/arm64/mm/contpte.c > +++ b/arch/arm64/mm/contpte.c > @@ -421,6 +421,12 @@ int contpte_ptep_set_access_flags(struct > vm_area_struct *vma, > ptep = contpte_align_down(ptep); > start_addr = addr = ALIGN_DOWN(addr, CONT_PTE_SIZE); > > + /* > + * We are not advancing entry because __ptep_set_access_flags() > + * only consumes access flags from entry. And since we > have checked > + * for the whole contpte block and returned early, pte_same() > + * within __ptep_set_access_flags() is likely false. > + */ > for (i = 0; i < CONT_PTES; i++, ptep++, addr += PAGE_SIZE) > __ptep_set_access_flags(vma, addr, ptep, entry, 0); LGTM: Reviewed-by: Ryan Roberts