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: <20190329015919.GF16680@redhat.com>
Date:   Thu, 28 Mar 2019 21:59:19 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Ira Weiny <ira.weiny@...el.com>, 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 09:42:59PM -0400, Jerome Glisse wrote:
> On Thu, Mar 28, 2019 at 06:30:26PM -0700, John Hubbard wrote:
> > On 3/28/19 6:17 PM, Jerome Glisse wrote:
> > > 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@...hat.com wrote:
> > >>>>>>>>>> From: Jérôme Glisse <jglisse@...hat.com>
> > >>> [...]
> > >> 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.
> > > 
> > 
> > Once the dust settles, I would expect that a name change for this could go
> > via Andrew's tree, right? It seems incredible to claim that we've built something
> > that effectively does not allow any minor changes!
> > 
> > I do think it's worth some *minor* trouble to improve the name, assuming that we
> > can do it in a simple patch, rather than some huge maintainer-level effort.
> 
> Change to nouveau have to go through nouveau tree so changing name means:
>  -  release N add function with new name, maybe make the old function just
>     a wrapper to the new function
>  -  release N+1 update user to use the new name
>  -  release N+2 remove the old name
> 
> So it is do-able but it is painful so i rather do that one latter that now
> as i am sure people will then complain again about some little thing and it
> will post pone this whole patchset on that new bit. To avoid post-poning
> RDMA and bunch of other patchset that build on top of that i rather get
> this patchset in and then do more changes in the next cycle.
> 
> This is just a capacity thing.

Also for clarity changes to API i am doing in this patchset is to make
the ODP convertion easier and thus they bring a real hard value. Renaming
those function is esthetic, i am not saying it is useless, i am saying it
does not have the same value as those other changes and i would rather not
miss another merge window just for esthetic changes.

Cheers,
Jérôme