Re: [PATCH 2/4] mm/vmalloc: add support for __GFP_NOFAIL

[NeilBrown]

On Tue, 26 Oct 2021, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
> 
> Dave Chinner has mentioned that some of the xfs code would benefit from
> kvmalloc support for __GFP_NOFAIL because they have allocations that
> cannot fail and they do not fit into a single page.
> 
> The larg part of the vmalloc implementation already complies with the

*large*

> given gfp flags so there is no work for those to be done. The area
> and page table allocations are an exception to that. Implement a retry
> loop for those.
> 
> Add a short sleep before retrying. 1 jiffy is a completely random
> timeout. Ideally the retry would wait for an explicit event - e.g.
> a change to the vmalloc space change if the failure was caused by
> the space fragmentation or depletion. But there are multiple different
> reasons to retry and this could become much more complex. Keep the retry
> simple for now and just sleep to prevent from hogging CPUs.
> 
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
>  mm/vmalloc.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index c6cc77d2f366..602649919a9d 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2941,8 +2941,12 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  	else if ((gfp_mask & (__GFP_FS | __GFP_IO)) == 0)
>  		flags = memalloc_noio_save();
>  
> -	ret = vmap_pages_range(addr, addr + size, prot, area->pages,
> +	do {
> +		ret = vmap_pages_range(addr, addr + size, prot, area->pages,
>  			page_shift);
> +		if (ret < 0)
> +			schedule_timeout_uninterruptible(1);
> +	} while ((gfp_mask & __GFP_NOFAIL) && (ret < 0));
>  
>  	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
>  		memalloc_nofs_restore(flags);
> @@ -3032,6 +3036,10 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
>  		warn_alloc(gfp_mask, NULL,
>  			"vmalloc error: size %lu, vm_struct allocation failed",
>  			real_size);
> +		if (gfp_mask & __GFP_NOFAIL) {
> +			schedule_timeout_uninterruptible(1);
> +			goto again;
> +		}

Shouldn't the retry happen *before* the warning?

NeilBrown


>  		goto fail;
>  	}
>  
> -- 
> 2.30.2
> 
>