linux-kernel - Re: [PATCH 3/4] mm: cma: Ensure that reservations never cross the low/high mem boundary

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Message-ID: <1874192.vbdvJooA5D@avalon>
Date:	Fri, 24 Oct 2014 13:00:11 +0300
From:	Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Laurent Pinchart <laurent.pinchart+renesas@...asonboard.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linux-sh@...r.kernel.org,
	Marek Szyprowski <m.szyprowski@...sung.com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Michal Nazarewicz <mina86@...a86.com>
Subject: Re: [PATCH 3/4] mm: cma: Ensure that reservations never cross the low/high mem boundary

Hi Joonsoo,

Thank you for the review.

On Friday 24 October 2014 11:53:25 Joonsoo Kim wrote:
> On Thu, Oct 23, 2014 at 05:33:47PM +0300, Laurent Pinchart wrote:
> > Commit 95b0e655f914 ("ARM: mm: don't limit default CMA region only to
> > low memory") extended CMA memory reservation to allow usage of high
> > memory. It relied on commit f7426b983a6a ("mm: cma: adjust address limit
> > to avoid hitting low/high memory boundary") to ensure that the reserved
> > block never crossed the low/high memory boundary. While the
> > implementation correctly lowered the limit, it failed to consider the
> > case where the base..limit range crossed the low/high memory boundary
> > with enough space on each side to reserve the requested size on either
> > low or high memory.
> > 
> > Rework the base and limit adjustment to fix the problem. The function
> > now starts by rejecting the reservation altogether for fixed
> > reservations that cross the boundary, then adjust the limit if
> > reservation from high memory is impossible, and finally first try to
> > reserve from high memory first and then falls back to low memory.
> > 
> > Signed-off-by: Laurent Pinchart
> > <laurent.pinchart+renesas@...asonboard.com>
> > ---
> > 
> >  mm/cma.c | 58 ++++++++++++++++++++++++++++++++++++++++++++--------------
> >  1 file changed, 44 insertions(+), 14 deletions(-)
> > 
> > diff --git a/mm/cma.c b/mm/cma.c
> > index 6b14346..b83597b 100644
> > --- a/mm/cma.c
> > +++ b/mm/cma.c
> > @@ -247,23 +247,38 @@ int __init cma_declare_contiguous(phys_addr_t base,
> >  		return -EINVAL;
> >  	
> >  	/*
> > -	 * adjust limit to avoid crossing low/high memory boundary for
> > +	 * Adjust limit and base to avoid crossing low/high memory boundary
> > for
> >  	 * automatically allocated regions
> >  	 */
> > 
> > -	if (((limit == 0 || limit > memblock_end) &&
> > -	     (memblock_end - size < highmem_start &&
> > -	      memblock_end > highmem_start)) ||
> > -	    (!fixed && limit > highmem_start && limit - size <
> > highmem_start)) {
> > -		limit = highmem_start;
> > -	}
> > 
> > -	if (fixed && base < highmem_start && base+size > highmem_start) {
> > +	/*
> > +	 * If allocating at a fixed base the request region must not cross
> > the
> > +	 * low/high memory boundary.
> > +	 */
> > +	if (fixed && base < highmem_start && base + size > highmem_start) {
> >  		ret = -EINVAL;
> >  		pr_err("Region at %08lx defined on low/high memory boundary
> >  		(%08lx)\n",
> >  			(unsigned long)base, (unsigned long)highmem_start);
> >  		goto err;
> >  	}
> > 
> > +	/*
> > +	 * If the limit is unspecified or above the memblock end, its
> > effective
> > +	 * value will be the memblock end. Set it explicitly to simplify
> > further
> > +	 * checks.
> > +	 */
> > +	if (limit == 0 || limit > memblock_end)
> > +		limit = memblock_end;
> > +
> > +	/*
> > +	 * If the limit is above the highmem start by less than the reserved
> > +	 * size allocation in highmem won't be possible. Lower the limit to
> > the
> > +	 * lowmem end.
> > +	 */
> > +	if (limit > highmem_start && limit - size < highmem_start)
> > +		limit = highmem_start;
> > +
> 
> How about removing this check?
> Without this check, memblock_alloc_range would be failed and we can
> go fallback correctly. So, this is redundant, IMO.

Good point. I'll remove the check in v2.

-- 
Regards,

Laurent Pinchart

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