Re: [PATCH v2 07/11] mm/hmm: add default fault flags to avoid the need to pre-fill pfns arrays.

[Jerome Glisse <jglisse@redhat.com>]

On Thu, Mar 28, 2019 at 09:42:31AM -0700, Ira Weiny wrote:
> On Thu, Mar 28, 2019 at 04:28:47PM -0700, John Hubbard wrote:
> > On 3/28/19 4:21 PM, Jerome Glisse wrote:
> > > On Thu, Mar 28, 2019 at 03:40:42PM -0700, John Hubbard wrote:
> > >> On 3/28/19 3:31 PM, Jerome Glisse wrote:
> > >>> On Thu, Mar 28, 2019 at 03:19:06PM -0700, John Hubbard wrote:
> > >>>> On 3/28/19 3:12 PM, Jerome Glisse wrote:
> > >>>>> On Thu, Mar 28, 2019 at 02:59:50PM -0700, John Hubbard wrote:
> > >>>>>> On 3/25/19 7:40 AM, jglisse@redhat.com wrote:
> > >>>>>>> From: Jérôme Glisse <jglisse@redhat.com>
> > [...]
> > >> Hi Jerome,
> > >>
> > >> I think you're talking about flags, but I'm talking about the mask. The 
> > >> above link doesn't appear to use the pfn_flags_mask, and the default_flags 
> > >> that it uses are still in the same lower 3 bits:
> > >>
> > >> +static uint64_t odp_hmm_flags[HMM_PFN_FLAG_MAX] = {
> > >> +	ODP_READ_BIT,	/* HMM_PFN_VALID */
> > >> +	ODP_WRITE_BIT,	/* HMM_PFN_WRITE */
> > >> +	ODP_DEVICE_BIT,	/* HMM_PFN_DEVICE_PRIVATE */
> > >> +};
> > >>
> > >> So I still don't see why we need the flexibility of a full 0xFFFFFFFFFFFFFFFF
> > >> mask, that is *also* runtime changeable. 
> > > 
> > > So the pfn array is using a device driver specific format and we have
> > > no idea nor do we need to know where the valid, write, ... bit are in
> > > that format. Those bits can be in the top 60 bits like 63, 62, 61, ...
> > > we do not care. They are device with bit at the top and for those you
> > > need a mask that allows you to mask out those bits or not depending on
> > > what the user want to do.
> > > 
> > > The mask here is against an _unknown_ (from HMM POV) format. So we can
> > > not presume where the bits will be and thus we can not presume what a
> > > proper mask is.
> > > 
> > > So that's why a full unsigned long mask is use here.
> > > 
> > > Maybe an example will help let say the device flag are:
> > >     VALID (1 << 63)
> > >     WRITE (1 << 62)
> > > 
> > > Now let say that device wants to fault with at least read a range
> > > it does set:
> > >     range->default_flags = (1 << 63)
> > >     range->pfn_flags_mask = 0;
> > > 
> > > This will fill fault all page in the range with at least read
> > > permission.
> > > 
> > > Now let say it wants to do the same except for one page in the range
> > > for which its want to have write. Now driver set:
> > >     range->default_flags = (1 << 63);
> > >     range->pfn_flags_mask = (1 << 62);
> > >     range->pfns[index_of_write] = (1 << 62);
> > > 
> > > With this HMM will fault in all page with at least read (ie valid)
> > > and for the address: range->start + index_of_write << PAGE_SHIFT it
> > > will fault with write permission ie if the CPU pte does not have
> > > write permission set then handle_mm_fault() will be call asking for
> > > write permission.
> > > 
> > > 
> > > Note that in the above HMM will populate the pfns array with write
> > > permission for any entry that have write permission within the CPU
> > > pte ie the default_flags and pfn_flags_mask is only the minimun
> > > requirement but HMM always returns all the flag that are set in the
> > > CPU pte.
> > > 
> > > 
> > > Now let say you are an "old" driver like nouveau upstream, then it
> > > means that you are setting each individual entry within range->pfns
> > > with the exact flags you want for each address hence here what you
> > > want is:
> > >     range->default_flags = 0;
> > >     range->pfn_flags_mask = -1UL;
> > > 
> > > So that what we do is (for each entry):
> > >     (range->pfns[index] & range->pfn_flags_mask) | range->default_flags
> > > and we end up with the flags that were set by the driver for each of
> > > the individual range->pfns entries.
> > > 
> > > 
> > > Does this help ?
> > > 
> > 
> > Yes, the key point for me was that this is an entirely device driver specific
> > format. OK. But then we have HMM setting it. So a comment to the effect that
> > this is device-specific might be nice, but I'll leave that up to you whether
> > it is useful.
> 
> Indeed I did not realize there is an hmm "pfn" until I saw this function:
> 
> /*
>  * hmm_pfn_from_pfn() - create a valid HMM pfn value from pfn
>  * @range: range use to encode HMM pfn value
>  * @pfn: pfn value for which to create the HMM pfn
>  * Returns: valid HMM pfn for the pfn
>  */
> static inline uint64_t hmm_pfn_from_pfn(const struct hmm_range *range,
>                                         unsigned long pfn)
> 
> So should this patch contain some sort of helper like this... maybe?
> 
> I'm assuming the "hmm_pfn" being returned above is the device pfn being
> discussed here?
> 
> I'm also thinking calling it pfn is confusing.  I'm not advocating a new type
> but calling the "device pfn's" "hmm_pfn" or "device_pfn" seems like it would
> have shortened the discussion here.
> 

That helper is also use today by nouveau so changing that name is not that
easy it does require the multi-release dance. So i am not sure how much
value there is in a name change.

Cheers,
Jérôme