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Date:   Wed, 16 Dec 2020 10:04:53 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Rik van Riel <riel@...riel.com>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH] mm: cma: allocate cma areas bottom-up

On Tue, Dec 15, 2020 at 01:43:41PM -0800, Roman Gushchin wrote:
> On Tue, Dec 15, 2020 at 11:36:23PM +0200, Mike Rapoport wrote:
> > Hi Roman,
> > 
> > On Tue, Dec 15, 2020 at 11:36:15AM -0800, Roman Gushchin wrote:
> > > Currently cma areas without a fixed base address are allocated
> > > close to the end of the node. This placement is sub-optimal because
> > > of how the compaction works: it effectively moves pages into
> > > the cma area. In particular, it often brings in hot executable pages,
> > > even if there is a plenty of free memory on the machine.
> > > This results in more cma allocation failures.
> > > 
> > > Instead let's place cma areas close to the beginning of a node.
> > > Cma first tries to start with highmem_start, so we shouldn't mess
> > > up with DMA32. In this case the compaction will help to free cma
> > > areas, resulting in better cma allocation success rates.
> > > 
> > > Signed-off-by: Roman Gushchin <guro@...com>
> > > ---
> > >  include/linux/memblock.h |  5 +++--
> > >  mm/cma.c                 |  4 ++--
> > >  mm/memblock.c            | 26 +++++++++++++++-----------
> > >  3 files changed, 20 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/include/linux/memblock.h b/include/linux/memblock.h
> > > index 9c5cc95c7cee..698188066450 100644
> > > --- a/include/linux/memblock.h
> > > +++ b/include/linux/memblock.h
> > > @@ -384,8 +384,9 @@ static inline int memblock_get_region_node(const struct memblock_region *r)
> > >  phys_addr_t memblock_phys_alloc_range(phys_addr_t size, phys_addr_t align,
> > >  				      phys_addr_t start, phys_addr_t end);
> > >  phys_addr_t memblock_alloc_range_nid(phys_addr_t size,
> > > -				      phys_addr_t align, phys_addr_t start,
> > > -				      phys_addr_t end, int nid, bool exact_nid);
> > > +				     phys_addr_t align, phys_addr_t start,
> > > +				     phys_addr_t end, int nid, bool exact_nid,
> > > +				     bool bottom_up);
> > >  phys_addr_t memblock_phys_alloc_try_nid(phys_addr_t size, phys_addr_t align, int nid);
> > >  
> > >  static inline phys_addr_t memblock_phys_alloc(phys_addr_t size,
> > > diff --git a/mm/cma.c b/mm/cma.c
> > > index 20c4f6f40037..1b42be6d059b 100644
> > > --- a/mm/cma.c
> > > +++ b/mm/cma.c
> > > @@ -332,13 +332,13 @@ int __init cma_declare_contiguous_nid(phys_addr_t base,
> > >  		 */
> > >  		if (base < highmem_start && limit > highmem_start) {
> > >  			addr = memblock_alloc_range_nid(size, alignment,
> > > -					highmem_start, limit, nid, true);
> > > +					highmem_start, limit, nid, true, true);
> > >  			limit = highmem_start;
> > >  		}
> > >  
> > >  		if (!addr) {
> > >  			addr = memblock_alloc_range_nid(size, alignment, base,
> > > -					limit, nid, true);
> > > +					limit, nid, true, true);
> > >  			if (!addr) {
> > >  				ret = -ENOMEM;
> > >  				goto err;
> > > diff --git a/mm/memblock.c b/mm/memblock.c
> > > index b8b7be0561c4..c334b401fe16 100644
> > > --- a/mm/memblock.c
> > > +++ b/mm/memblock.c
> > > @@ -272,6 +272,7 @@ __memblock_find_range_top_down(phys_addr_t start, phys_addr_t end,
> > >   *       %MEMBLOCK_ALLOC_ACCESSIBLE
> > >   * @nid: nid of the free area to find, %NUMA_NO_NODE for any node
> > >   * @flags: pick from blocks based on memory attributes
> > > + * @bottom_up: force bottom-up allocation
> > 
> > Why wouldn't you use memblock_set_bottom_up() around the allocations in
> > CMA, e.g.
> > 
> > 	bool bottom_up = memblock_bottom_up();
> > 
> > 	if (!bottom_up)
> > 		memblock_set_bottom_up(true);
> > 
> > 	/* allocate memory */
> > 
> > 	memblock_set_bottom_up(bottom_up);
> 
> Hi Mike!
> 
> Wouldn't it open a possibility for a race? If somebody else is doing an allocation
> in parallel, their allocation could become affected.

This happens a lot earlier than we can have concurrency, so there is no
such possibility.

> Thanks!
> 

-- 
Sincerely yours,
Mike.

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