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Message-ID: <2123f339-2487-4b1c-abb1-313e9a012242@redhat.com>
Date: Thu, 29 Aug 2024 08:35:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>, Peter Xu <peterx@...hat.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org,
Gavin Shan <gshan@...hat.com>, Catalin Marinas <catalin.marinas@....com>,
x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Paolo Bonzini <pbonzini@...hat.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Thomas Gleixner <tglx@...utronix.de>, Alistair Popple <apopple@...dia.com>,
kvm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
Sean Christopherson <seanjc@...gle.com>, Oscar Salvador <osalvador@...e.de>,
Borislav Petkov <bp@...en8.de>, Zi Yan <ziy@...dia.com>,
Axel Rasmussen <axelrasmussen@...gle.com>, Yan Zhao <yan.y.zhao@...el.com>,
Will Deacon <will@...nel.org>, Kefeng Wang <wangkefeng.wang@...wei.com>,
Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v2 06/19] mm/pagewalk: Check pfnmap for folio_walk_start()
On 29.08.24 01:46, Jason Gunthorpe wrote:
> On Wed, Aug 28, 2024 at 03:45:49PM -0400, Peter Xu wrote:
>
>> Meanwhile I'm actually not 100% sure pte_special is only needed in
>> gup-fast. See vm_normal_page() and for VM_PFNMAP when pte_special bit is
>> not defined:
>>
>> } else {
>> unsigned long off;
>> off = (addr - vma->vm_start) >> PAGE_SHIFT;
>> if (pfn == vma->vm_pgoff + off) <------------------ [1]
>> return NULL;
>> if (!is_cow_mapping(vma->vm_flags))
>> return NULL;
>> }
>>
>> I suspect things can go wrong when there's assumption on vm_pgoff [1]. At
>> least vfio-pci isn't storing vm_pgoff for the base PFN, so this check will
>> go wrong when pte_special is not supported on any arch but when vfio-pci is
>> present. I suspect more drivers can break it.
Fortunately, we did an excellent job at documenting vm_normal_page():
* There are 2 broad cases. Firstly, an architecture may define a pte_special()
* pte bit, in which case this function is trivial. Secondly, an architecture
* may not have a spare pte bit, which requires a more complicated scheme,
* described below.
*
* A raw VM_PFNMAP mapping (ie. one that is not COWed) is always considered a
* special mapping (even if there are underlying and valid "struct pages").
* COWed pages of a VM_PFNMAP are always normal.
*
* The way we recognize COWed pages within VM_PFNMAP mappings is through the
* rules set up by "remap_pfn_range()": the vma will have the VM_PFNMAP bit
* set, and the vm_pgoff will point to the first PFN mapped: thus every special
* mapping will always honor the rule
*
* pfn_of_page == vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT)
*
* And for normal mappings this is false.
*
remap_pfn_range_notrack() will currently handle that for us:
if (is_cow_mapping(vma->vm_flags)) {
if (addr != vma->vm_start || end != vma->vm_end)
return -EINVAL;
}
Even if [1] would succeed, the is_cow_mapping() check will return NULL and it will
all work as expected, even without pte_special().
Because VM_PFNMAP is easy: in a !COW mapping, everything is special.
>
> I think that is a very important point.
>
> IIRC this was done magically in one of the ioremap pfns type calls,
> and if VFIO is using fault instead it won't do it.
>
> This probably needs more hand holding for the driver somehow..
As long as these drivers don't support COW-mappings. It's all good.
And IIUC, we cannot support COW mappings if we don't use remap_pfn_range().
For this reason, remap_pfn_range() also bails out if not the whole VMA is covered
in a COW mapping.
It would be great if we could detect and fail that. Likely when trying to insert
PFNs (*not* using remap_pfn_range) manually we would have to WARN if we stumble over
a COW mapping.
In the meantime, we should really avoid any new VM_PFNMAP COW users ...
>
>> So I wonder if it's really the case in real life that only gup-fast would
>> need the special bit. It could be that we thought it like that, but nobody
>> really seriously tried run it without special bit yet to see things broke.
>
> Indeed.
VM_PFNMAP for sure works.
VM_MIXEDMAP, I am not so sure. The s390x introduction of pte_special() [again,
I posted the commit] raised why they need it: because pfn_valid() could have
returned non-refcounted pages. One would have to dig if that is even still the
case as of today, and if other architectures have similar constraints.
>
> What arches even use the whole 'special but not special' system?
>
> Can we start banning some of this stuff on non-special arches?
Again, VM_PFNMAP is not a problem. Only VM_MIXEDMAP, and I would love to
see that go. There are some, but not that many users ... but I'm afraid it's
not that easy :)
--
Cheers,
David / dhildenb
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