Re: [PATCH 02/12] percpu: do not search past bitmap when allocating an area

[Dennis Zhou]

On Sat, Mar 02, 2019 at 01:32:04PM +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 10:18
> > 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 02/12] percpu: do not search past bitmap when allocating an
> > area
> > 
> > pcpu_find_block_fit() guarantees that a fit is found within
> > PCPU_BITMAP_BLOCK_BITS. Iteration is used to determine the first fit as it
> > compares against the block's contig_hint. This can lead to incorrectly scanning
> > past the end of the bitmap. The behavior was okay given the check after for
> > bit_off >= end and the correctness of the hints from pcpu_find_block_fit().
> > 
> > This patch fixes this by bounding the end offset by the number of bits in a
> > chunk.
> > 
> > Signed-off-by: Dennis Zhou <dennis@xxxxxxxxxx>
> > ---
> >  mm/percpu.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/mm/percpu.c b/mm/percpu.c
> > index 53bd79a617b1..69ca51d238b5 100644
> > --- a/mm/percpu.c
> > +++ b/mm/percpu.c
> > @@ -988,7 +988,8 @@ static int pcpu_alloc_area(struct pcpu_chunk *chunk,
> > int alloc_bits,
> >  	/*
> >  	 * Search to find a fit.
> >  	 */
> > -	end = start + alloc_bits + PCPU_BITMAP_BLOCK_BITS;
> > +	end = min_t(int, start + alloc_bits + PCPU_BITMAP_BLOCK_BITS,
> > +		    pcpu_chunk_map_bits(chunk));
> >  	bit_off = bitmap_find_next_zero_area(chunk->alloc_map, end, start,
> >  					     alloc_bits, align_mask);
> >  	if (bit_off >= end)
> > --
> 
> From pcpu_alloc_area itself, I think this is correct to avoid bitmap_find_next_zero_area
> scan past the boundaries of alloc_map, so
> 
> Reviewed-by: Peng Fan <peng.fan@xxxxxxx>
> 
> There are a few points I did not understand well,
> Per understanding pcpu_find_block_fit is to find the first bit off in a chunk which could satisfy
> the bits allocation, so bits might be larger than PCPU_BITMAP_BLOCK_BITS. And if
> pcpu_find_block_fit returns a good off, it means there is a area in the chunk could satisfy
> the bits allocation, then the following pcpu_alloc_area will not scan past the boundaries of
> alloc_map, right?
> 

pcpu_find_block_fit() finds the chunk offset corresponding to the block
that will be able to fit the chunk. Allocations are done by first fit,
so scanning begins from the first_free of a block. Because the hints are
always accurate, you never fail to find a fit in pcpu_alloc_area() if
pcpu_find_block_fit() gives you an offset. This means you never scan
past the end anyway.

Thanks,
Dennis