linux-kernel - Re: [PATCH v4 63/66] i915: Use the VMA iterator

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Date:   Thu, 20 Jan 2022 17:39:58 +0000
From:   Liam Howlett <liam.howlett@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Vlastimil Babka <vbabka@...e.cz>,
        "maple-tree@...ts.infradead.org" <maple-tree@...ts.infradead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Song Liu <songliubraving@...com>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        David Rientjes <rientjes@...gle.com>,
        Axel Rasmussen <axelrasmussen@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Rik van Riel <riel@...riel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michel Lespinasse <walken.cr@...il.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Minchan Kim <minchan@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>
Subject: Re: [PATCH v4 63/66] i915: Use the VMA iterator

* Matthew Wilcox <willy@...radead.org> [220120 10:50]:
> On Thu, Jan 20, 2022 at 03:59:11PM +0100, Vlastimil Babka wrote:
> > On 12/1/21 15:30, Liam Howlett wrote:
> > > From: "Matthew Wilcox (Oracle)" <willy@...radead.org>
> > > 
> > > Replace the O(n.log(n)) loop with an O(n) loop.
> > 
> > Not true?
> 
> Oh, right, that should have been just the linked-list walk.
> I misread it as calling find_vma() for each iteration instead
> of just the first one.  Liam, do you mind updating the changelog
> here?

I will update the changelog for v5.

> 
> I wonder whether we want a "for_each_contiguous_vma()" that
> will stop on a hole.  It seems like a relatively sensible thing
> to do -- walk across a contiguous range of memory and stop if
> there's no VMA mapping a page.  Like gup(), for example.

Hmm, so search for a non-zero gap across a range might be a reasonable
implementation.

> 
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> > > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > 
> > Acked-by: Vlastimil Babka <vbabka@...e.cz>
> > 
> > > ---
> > >  drivers/gpu/drm/i915/gem/i915_gem_userptr.c | 14 +++++---------
> > >  1 file changed, 5 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > > index 3173c9f9a040..39960973c130 100644
> > > --- a/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > > +++ b/drivers/gpu/drm/i915/gem/i915_gem_userptr.c
> > > @@ -425,12 +425,11 @@ static const struct drm_i915_gem_object_ops i915_gem_userptr_ops = {
> > >  static int
> > >  probe_range(struct mm_struct *mm, unsigned long addr, unsigned long len)
> > >  {
> > > -	const unsigned long end = addr + len;
> > > +	VMA_ITERATOR(vmi, mm, addr);
> > >  	struct vm_area_struct *vma;
> > > -	int ret = -EFAULT;
> > >  
> > >  	mmap_read_lock(mm);
> > > -	for (vma = find_vma(mm, addr); vma; vma = vma->vm_next) {
> > > +	for_each_vma_range(vmi, vma, addr + len) {
> > >  		/* Check for holes, note that we also update the addr below */
> > >  		if (vma->vm_start > addr)
> > >  			break;
> > > @@ -438,16 +437,13 @@ probe_range(struct mm_struct *mm, unsigned long addr, unsigned long len)
> > >  		if (vma->vm_flags & (VM_PFNMAP | VM_MIXEDMAP))
> > >  			break;
> > >  
> > > -		if (vma->vm_end >= end) {
> > > -			ret = 0;
> > > -			break;
> > > -		}
> > > -
> > >  		addr = vma->vm_end;
> > >  	}
> > >  	mmap_read_unlock(mm);
> > >  
> > > -	return ret;
> > > +	if (vma)
> > > +		return -EFAULT;
> > > +	return 0;
> > >  }
> > >  
> > >  /*
> > 
>