Re: [PATCH v3 3/3] mm/vmalloc: add more comments to the adjust_va_to_fit_type()

[Michal Hocko]

On Wed 16-10-19 11:54:38, Uladzislau Rezki (Sony) wrote:
> When fit type is NE_FIT_TYPE there is a need in one extra object.
> Usually the "ne_fit_preload_node" per-CPU variable has it and
> there is no need in GFP_NOWAIT allocation, but there are exceptions.
> 
> This commit just adds more explanations, as a result giving
> answers on questions like when it can occur, how often, under
> which conditions and what happens if GFP_NOWAIT gets failed.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/vmalloc.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 593bf554518d..2290a0d270e4 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -969,6 +969,19 @@ adjust_va_to_fit_type(struct vmap_area *va,
>  			 * There are a few exceptions though, as an example it is
>  			 * a first allocation (early boot up) when we have "one"
>  			 * big free space that has to be split.
> +			 *
> +			 * Also we can hit this path in case of regular "vmap"
> +			 * allocations, if "this" current CPU was not preloaded.
> +			 * See the comment in alloc_vmap_area() why. If so, then
> +			 * GFP_NOWAIT is used instead to get an extra object for
> +			 * split purpose. That is rare and most time does not
> +			 * occur.
> +			 *
> +			 * What happens if an allocation gets failed. Basically,
> +			 * an "overflow" path is triggered to purge lazily freed
> +			 * areas to free some memory, then, the "retry" path is
> +			 * triggered to repeat one more time. See more details
> +			 * in alloc_vmap_area() function.
>  			 */
>  			lva = kmem_cache_alloc(vmap_area_cachep, GFP_NOWAIT);
>  			if (!lva)
> -- 
> 2.20.1
> 

-- 
Michal Hocko
SUSE Labs