Re: [PATCH 03/12] percpu: introduce helper to determine if two regions overlap

[Dennis Zhou]

On Sat, Mar 02, 2019 at 01:37:37PM +0000, Peng Fan wrote:
> Hi Dennis,
> 
> > -----Original Message-----
> > From: owner-linux-mm@xxxxxxxxx [mailto:owner-linux-mm@xxxxxxxxx] On
> > Behalf Of Dennis Zhou
> > Sent: 2019å2æ28æ 10:19
> > To: Dennis Zhou <dennis@xxxxxxxxxx>; Tejun Heo <tj@xxxxxxxxxx>; Christoph
> > Lameter <cl@xxxxxxxxx>
> > Cc: Vlad Buslov <vladbu@xxxxxxxxxxxx>; kernel-team@xxxxxx;
> > linux-mm@xxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> > Subject: [PATCH 03/12] percpu: introduce helper to determine if two regions
> > overlap
> > 
> > While block hints were always accurate, it's possible when spanning across
> > blocks that we miss updating the chunk's contig_hint. Rather than rely on
> > correctness of the boundaries of hints, do a full overlap comparison.
> > 
> > Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
> > ---
> >  mm/percpu.c | 31 +++++++++++++++++++++++++++----
> >  1 file changed, 27 insertions(+), 4 deletions(-)
> > 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 69ca51d238b5..b40112b2fc59 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -546,6 +546,24 @@ static inline int pcpu_cnt_pop_pages(struct
> > pcpu_chunk *chunk, int bit_off,
> >  	       bitmap_weight(chunk->populated, page_start);  }
> > 
> > +/*
> > + * pcpu_region_overlap - determines if two regions overlap
> > + * @a: start of first region, inclusive
> > + * @b: end of first region, exclusive
> > + * @x: start of second region, inclusive
> > + * @y: end of second region, exclusive
> > + *
> > + * This is used to determine if the hint region [a, b) overlaps with
> > +the
> > + * allocated region [x, y).
> > + */
> > +static inline bool pcpu_region_overlap(int a, int b, int x, int y) {
> > +	if ((x >= a && x < b) || (y > a && y <= b) ||
> > +	    (x <= a && y >= b))
> 
> I think this could be simplified:
>  (a < y) && (x < b) could be used to do overlap check.
> 

I'll change it to be the negative.

Thanks,
Dennis

> 
> > +		return true;
> > +	return false;
> > +}
> > +
> >  /**
> >   * pcpu_chunk_update - updates the chunk metadata given a free area
> >   * @chunk: chunk of interest
> > @@ -710,8 +728,11 @@ static void pcpu_block_update_hint_alloc(struct
> > pcpu_chunk *chunk, int bit_off,
> >  					PCPU_BITMAP_BLOCK_BITS,
> >  					s_off + bits);
> > 
> > -	if (s_off >= s_block->contig_hint_start &&
> > -	    s_off < s_block->contig_hint_start + s_block->contig_hint) {
> > +	if (pcpu_region_overlap(s_block->contig_hint_start,
> > +				s_block->contig_hint_start +
> > +				s_block->contig_hint,
> > +				s_off,
> > +				s_off + bits)) {
> >  		/* block contig hint is broken - scan to fix it */
> >  		pcpu_block_refresh_hint(chunk, s_index);
> >  	} else {
> > @@ -764,8 +785,10 @@ static void pcpu_block_update_hint_alloc(struct
> > pcpu_chunk *chunk, int bit_off,
> >  	 * contig hint is broken.  Otherwise, it means a smaller space
> >  	 * was used and therefore the chunk contig hint is still correct.
> >  	 */
> > -	if (bit_off >= chunk->contig_bits_start  &&
> > -	    bit_off < chunk->contig_bits_start + chunk->contig_bits)
> > +	if (pcpu_region_overlap(chunk->contig_bits_start,
> > +				chunk->contig_bits_start + chunk->contig_bits,
> > +				bit_off,
> > +				bit_off + bits))
> >  		pcpu_chunk_refresh_hint(chunk);
> >  }
> > 
> > --
> > 2.17.1
>