linux-kernel - Re: [PATCH 4/8] mm/percpu: add comment to state the empty populated pages accounting

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Date:   Tue, 25 Oct 2022 11:47:51 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Dennis Zhou <dennis@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org
Subject: Re: [PATCH 4/8] mm/percpu: add comment to state the empty populated
 pages accounting

On 10/24/22 at 09:56am, Dennis Zhou wrote:
> On Mon, Oct 24, 2022 at 04:14:31PM +0800, Baoquan He wrote:
> > When allocating an area from a chunk, pcpu_block_update_hint_alloc()
> > is called to update chunk metadata, including chunk's and global
> > nr_empty_pop_pages. However, if the allocation is not atomic, some
> > blocks may not be populated with pages yet, while we still account it
> > here. The number of pages will be subtracted with pcpu_chunk_populated()
> > when populating pages.
> > 
> > Adding code comment to make that more understandable.
> > 
> > Signed-off-by: Baoquan He <bhe@...hat.com>
> > ---
> >  mm/percpu.c | 14 +++++++++++---
> >  1 file changed, 11 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index a8121302a79c..09e407338573 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -831,13 +831,15 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
> >  
> >  	/*
> >  	 * Update s_block.
> > -	 * block->first_free must be updated if the allocation takes its place.
> > -	 * If the allocation breaks the contig_hint, a scan is required to
> > -	 * restore this hint.
> >  	 */
> >  	if (s_block->contig_hint == PCPU_BITMAP_BLOCK_BITS)
> >  		nr_empty_pages++;
> >  
> > +	/*
> > +	 * block->first_free must be updated if the allocation takes its place.
> > +	 * If the allocation breaks the contig_hint, a scan is required to
> > +	 * restore this hint.
> > +	 */
> >  	if (s_off == s_block->first_free)
> >  		s_block->first_free = find_next_zero_bit(
> >  					pcpu_index_alloc_map(chunk, s_index),
> > @@ -912,6 +914,12 @@ static void pcpu_block_update_hint_alloc(struct pcpu_chunk *chunk, int bit_off,
> >  		}
> >  	}
> >  
> > +	/*
> > +	 * If the allocation is not atomic, some blocks may not
> > +	 * be populated with pages, while we account it here.
> > +	 * The number of pages will be subtracted with
> > +	 * pcpu_chunk_populated() when populating pages.
> > +	 */
> >  	if (nr_empty_pages)
> >  		pcpu_update_empty_pages(chunk, -nr_empty_pages);
> >  
> > -- 
> > 2.34.1
> > 
> 
> Heh, that's a little more subtle than I remember it being.
> 
> Acked-by: Dennis Zhou <dennis@...nel.org>

Thanks for reviewing, Dennis.

When I rechecked the code comment, I realized I had said the opposite in
v1 about the newly added code comment. It subtracts the nr_empty_pop_pages
in pcpu_block_update_hint_alloc() whether it's atomic allocation or not.
If non atomic case, the number will be added back in pcpu_chunk_populated()
when populating pages.

I have updated and resent v2, sorry for the mistake.

Thanks
Baoquan