On Sat, Mar 02, 2019 at 01:32:04PM +0000, Peng Fan wrote: > Hi Dennis, > > > -----Original Message----- > > From: owner-linux...@kvack.org [mailto:owner-linux...@kvack.org] On > > Behalf Of Dennis Zhou > > Sent: 2019年2月28日 10:18 > > To: Dennis Zhou <den...@kernel.org>; Tejun Heo <t...@kernel.org>; Christoph > > Lameter <c...@linux.com> > > Cc: Vlad Buslov <vla...@mellanox.com>; kernel-t...@fb.com; > > linux...@kvack.org; linux-kernel@vger.kernel.org > > 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 <den...@kernel.org> > > --- > > 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....@nxp.com> > > 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
