linux-kernel - Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages

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Date:   Fri, 26 Mar 2021 10:08:09 +0100
From:   Thomas Hellström (Intel) 
        <thomas_os@...pmail.org>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
        "christian.koenig@....com" <christian.koenig@....com>,
        "airlied@...ux.ie" <airlied@...ux.ie>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH 1/2] mm,drm/ttm: Block fast GUP to TTM huge pages


On 3/25/21 7:24 PM, Jason Gunthorpe wrote:
> On Thu, Mar 25, 2021 at 07:13:33PM +0100, Thomas Hellström (Intel) wrote:
>> On 3/25/21 6:55 PM, Jason Gunthorpe wrote:
>>> On Thu, Mar 25, 2021 at 06:51:26PM +0100, Thomas Hellström (Intel) wrote:
>>>> On 3/24/21 9:25 PM, Dave Hansen wrote:
>>>>> On 3/24/21 1:22 PM, Thomas Hellström (Intel) wrote:
>>>>>>> We also have not been careful at *all* about how _PAGE_BIT_SOFTW* are
>>>>>>> used.  It's quite possible we can encode another use even in the
>>>>>>> existing bits.
>>>>>>>
>>>>>>> Personally, I'd just try:
>>>>>>>
>>>>>>> #define _PAGE_BIT_SOFTW5        57      /* available for programmer */
>>>>>>>
>>>>>> OK, I'll follow your advise here. FWIW I grepped for SW1 and it seems
>>>>>> used in a selftest, but only for PTEs AFAICT.
>>>>>>
>>>>>> Oh, and we don't care about 32-bit much anymore?
>>>>> On x86, we have 64-bit PTEs when running 32-bit kernels if PAE is
>>>>> enabled.  IOW, we can handle the majority of 32-bit CPUs out there.
>>>>>
>>>>> But, yeah, we don't care about 32-bit. :)
>>>> Hmm,
>>>>
>>>> Actually it makes some sense to use SW1, to make it end up in the same dword
>>>> as the PSE bit, as from what I can tell, reading of a 64-bit pmd_t on 32-bit
>>>> PAE is not atomic, so in theory a huge pmd could be modified while reading
>>>> the pmd_t making the dwords inconsistent.... How does that work with fast
>>>> gup anyway?
>>> It loops to get an atomic 64 bit value if the arch can't provide an
>>> atomic 64 bit load
>> Hmm, ok, I see a READ_ONCE() in gup_pmd_range(), and then the resulting pmd
>> is dereferenced either in try_grab_compound_head() or __gup_device_huge(),
>> before the pmd is compared to the value the pointer is currently pointing
>> to. Couldn't those dereferences be on invalid pointers?
> Uhhhhh.. That does look questionable, yes. Unless there is some tricky
> reason why a 64 bit pmd entry on a 32 bit arch either can't exist or
> has a stable upper 32 bits..
>
> The pte does it with ptep_get_lockless(), we probably need the same
> for the other levels too instead of open coding a READ_ONCE?
>
> Jason

TBH, ptep_get_lockless() also looks a bit fishy. it says
"it will not switch to a completely different present page without a TLB 
flush in between".

What if the following happens:

processor 1: Reads lower dword of PTE.
processor 2: Zaps PTE. Gets stuck waiting to do TLB flush
processor 1: Reads upper dword of PTE, which is now zero.
processor 3: Hits a TLB miss, reads an unpopulated PTE and faults in a 
new PTE value which happens to be the same as the original one before 
the zap.
processor 1: Reads the newly faulted in lower dword, compares to the old 
one, gives an OK and returns a bogus PTE.

/Thomas