On 20.10.20 12:22, Lecopzer Chen wrote:
> The cma_mutex which protects alloc_contig_range() was first appeared in
> commit 7ee793a62fa8c ("cma: Remove potential deadlock situation"),
> at that time, there is no guarantee the behavior of concurrency inside
> alloc_contig_range().
>
> After the commit 2c7452a075d4db2dc
> ("mm/page_isolation.c: make start_isolate_page_range() fail if already isolated")
> > However, two subsystems (CMA and gigantic
> > huge pages for example) could attempt operations on the same range. If
> > this happens, one thread may 'undo' the work another thread is doing.
> > This can result in pageblocks being incorrectly left marked as
> > MIGRATE_ISOLATE and therefore not available for page allocation.
> The concurrency inside alloc_contig_range() was clarified.
>
> Now we can find that hugepage and virtio call alloc_contig_range() without
> any lock, thus cma_mutex is "redundant" in cma_alloc() now.
>
> Signed-off-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
> ---
> mm/cma.c | 4 +---
> 1 file changed, 1 insertion(+), 3 deletions(-)
>
> diff --git a/mm/cma.c b/mm/cma.c
> index 7f415d7cda9f..3692a34e2353 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -38,7 +38,6 @@
>
> struct cma cma_areas[MAX_CMA_AREAS];
> unsigned cma_area_count;
> -static DEFINE_MUTEX(cma_mutex);
>
> phys_addr_t cma_get_base(const struct cma *cma)
> {
> @@ -454,10 +453,9 @@ struct page *cma_alloc(struct cma *cma, size_t count, unsigned int align,
> mutex_unlock(&cma->lock);
>
> pfn = cma->base_pfn + (bitmap_no << cma->order_per_bit);
> - mutex_lock(&cma_mutex);
> ret = alloc_contig_range(pfn, pfn + count, MIGRATE_CMA,
> GFP_KERNEL | (no_warn ? __GFP_NOWARN : 0));
> - mutex_unlock(&cma_mutex);
> +
> if (ret == 0) {
> page = pfn_to_page(pfn);
> break;
>
I guess this is fine. In case there is a race we return with -EBUSY -
which is suboptimal (as it could just be a temporary issue if the other
user backs off), but should be good enough for now.
Acked-by: David Hildenbrand <david@xxxxxxxxxx>
--
Thanks,
David / dhildenb
