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

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Message-ID: <20190328234357.GL13560@redhat.com>
Date:   Thu, 28 Mar 2019 19:43:58 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH v2 07/11] mm/hmm: add default fault flags to avoid the
 need to pre-fill pfns arrays.

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@...hat.com wrote:
> >>>>>>> From: Jérôme Glisse <jglisse@...hat.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.

The code you were pointing at is temporary ie once this get merge that code
will get remove in release N+2 ie merge code in N, update nouveau in N+1 and
remove this temporary code in N+2

When updating HMM API it is easier to stage API update over release like that
so there is no need to synchronize accross multiple tree (mm, drm, rdma, ...)

> Either way, you can add:
> 
> 	Reviewed-by: John Hubbard <jhubbard@...dia.com>
> 
> thanks,
> -- 
> John Hubbard
> NVIDIA