linux-kernel - Re: [PATCH v2 26/44] mm/damon: Stop using vma_mas

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Date:   Thu,  5 Jan 2023 20:16:56 +0000
From:   SeongJae Park <sj@...nel.org>
To:     Liam Howlett <liam.howlett@...cle.com>
Cc:     SeongJae Park <sj@...nel.org>,
        "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>,
        "damon@...ts.linux.dev" <damon@...ts.linux.dev>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v2 26/44] mm/damon: Stop using vma_mas_store() for maple tree store

Hi Liam,

On Thu, 5 Jan 2023 19:52:21 +0000 Liam Howlett <liam.howlett@...cle.com> wrote:

> * SeongJae Park <sj@...nel.org> [230105 14:33]:
> > Hi Liam,
> > 
> > On Thu, 5 Jan 2023 19:16:00 +0000 Liam Howlett <liam.howlett@...cle.com> wrote:
> > 
> > > From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
> > > 
> > > Prepare for the removal of the vma_mas_store() function by open coding
> > > the maple tree store in this test code.
> > 
> > But seems this series is not really removing 'vma_mas_store()'.  Wouldn't it
> > better to do the preparation and removal together in a same patch series?
> 
> It does from the all code but the nommu side.  The definition is dropped
> from the header and c file in "mmap: Convert __vma_adjust() to use vma
> iterator" [1].

Thank you for nice explanation.

> 
> > 
> > > Set the range of the maple
> > > state and call the store function directly.
> > > 
> > > Cc: SeongJae Park <sj@...nel.org>
> > > Cc: damon@...ts.linux.dev
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> > > ---
> > >  mm/damon/vaddr-test.h | 6 ++++--
> > >  1 file changed, 4 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/mm/damon/vaddr-test.h b/mm/damon/vaddr-test.h
> > > index bce37c487540..41532f7355d0 100644
> > > --- a/mm/damon/vaddr-test.h
> > > +++ b/mm/damon/vaddr-test.h
> > > @@ -24,8 +24,10 @@ static void __link_vmas(struct maple_tree *mt, struct vm_area_struct *vmas,
> > >  		return;
> > >  
> > >  	mas_lock(&mas);
> > > -	for (i = 0; i < nr_vmas; i++)
> > > -		vma_mas_store(&vmas[i], &mas);
> > > +	for (i = 0; i < nr_vmas; i++) {
> > > +		mas_set_range(&mas, vmas[i].vm_start, vmas[i].vm_end - 1);
> > > +		mas_store_gfp(&mas, &vmas[i], GFP_KERNEL);
> > > +	}
> > 
> > On the latest mm-unstable, vma_mas_store() uses mas_store_prealloc() instead of
> > mas_store_gfp().  Seems the difference would make no problem to this test code
> > in most cases, but could I ask the reason for this change?
> 
> mas_store_prealloc() expects the maple state to have the necessary
> memory to store the value.  Using this function is the right way of
> storing the range.  In fact, we would only need a single node since
> these values will be append operations anyways.

Again, thank you for nice explanation.

> 
> > 
> > Also, should we check the return value of mas_store_gfp()?
> 
> I can add this.  The only reason we would return an error is on ENOMEM
> which seems unlikely here.  Again, it is a single node that will be
> used.  The size is 256B, but it's safer to add the check.

You're right.  I'd prefer having the check, but I'd not block this for the
trivial nit.

Reviewed-by: SeongJae Park <sj@...nel.org>


Thanks,
SJ

> 
> [1] https://lore.kernel.org/linux-mm/20230105191517.3099082-28-Liam.Howlett@oracle.com/
> 
> 
> Thanks,
> Liam
>