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

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Message-ID: <20190724065146.GA2061@lst.de>
Date:   Wed, 24 Jul 2019 08:51:46 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ralph Campbell <rcampbell@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Jérôme Glisse <jglisse@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 2/2] mm/hmm: make full use of walk_page_range()

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

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.  It might actually be a better idea to just kill that stuff off
for now until we have a user, as code without users is per definition
untested and will just bitrot and break.

> +	const unsigned long device_vma = VM_IO | VM_PFNMAP | VM_MIXEDMAP;
> +	struct hmm_vma_walk *hmm_vma_walk = walk->private;
> +	struct hmm_range *range = hmm_vma_walk->range;
> +	struct vm_area_struct *vma = walk->vma;
> +
> +	/* If range is no longer valid, force retry. */
> +	if (!range->valid)
> +		return -EBUSY;
> +
> +	if (vma->vm_flags & device_vma)

Can we just kill off this odd device_vma variable?

	if (vma->vm_flags & (VM_IO | VM_PFNMAP | VM_MIXEDMAP))

and maybe add a comment on why we are skipping them (because they
don't have struct page backing I guess..)