linux-kernel - Re: [PATCH 2/2] mm/hmm: make full use of walk_page

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Date:   Wed, 24 Jul 2019 11:53:42 +0000
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Christoph Hellwig <hch@....de>
CC:     Ralph Campbell <rcampbell@...dia.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jérôme Glisse <jglisse@...hat.com>
Subject: Re: [PATCH 2/2] mm/hmm: make full use of walk_page_range()

On Wed, Jul 24, 2019 at 08:51:46AM +0200, Christoph Hellwig wrote:
> On Tue, Jul 23, 2019 at 04:30:16PM -0700, Ralph Campbell wrote:
> > hmm_range_snapshot() and hmm_range_fault() both call find_vma() and
> > walk_page_range() in a loop. This is unnecessary duplication since
> > walk_page_range() calls find_vma() in a loop already.
> > Simplify hmm_range_snapshot() and hmm_range_fault() by defining a
> > walk_test() callback function to filter unhandled vmas.
> 
> I like the approach a lot!
> 
> But we really need to sort out the duplication between hmm_range_fault
> and hmm_range_snapshot first, as they are basically the same code.  I
> have patches here:
> 
> http://git.infradead.org/users/hch/misc.git/commitdiff/a34ccd30ee8a8a3111d9e91711c12901ed7dea74
> 
> http://git.infradead.org/users/hch/misc.git/commitdiff/81f442ebac7170815af7770a1efa9c4ab662137e

Yeah, that is a straightforward improvement, maybe Ralph should grab
these two as part of his series?

> That being said we don't really have any users for the snapshot mode
> or non-blocking faults, and I don't see any in the immediate pipeline
> either.

If this code was production ready I'd use it in ODP right away.

When we first create a ODP MR we'd want to snapshot to pre-load the
NIC tables with something other than page fault, but not fault
anything.

This would be a big performance improvement for ODP.

Jason