Re: [PATCH v2 2/4] mm/vmalloc: add support for __GFP_NOFAIL

[Vlastimil Babka]

On 11/22/21 16:32, 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 large part of the vmalloc implementation already complies with the
> 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>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> @@ -2921,6 +2923,7 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  {
>  	const gfp_t nested_gfp = (gfp_mask & GFP_RECLAIM_MASK) | __GFP_ZERO;
>  	const gfp_t orig_gfp_mask = gfp_mask;
> +	bool nofail = gfp_mask & __GFP_NOFAIL;
>  	unsigned long addr = (unsigned long)area->addr;
>  	unsigned long size = get_vm_area_size(area);
>  	unsigned long array_size;
> @@ -2978,8 +2981,12 @@ 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 (nofail && (ret < 0))
> +			schedule_timeout_uninterruptible(1);
> +	} while (nofail && (ret < 0));

Kind of ugly to have the same condition twice here, but no hard feelings.

>  
>  	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
>  		memalloc_nofs_restore(flags);
> @@ -3074,9 +3081,14 @@ void *__vmalloc_node_range(unsigned long size, unsigned long align,
>  				  VM_UNINITIALIZED | vm_flags, start, end, node,
>  				  gfp_mask, caller);
>  	if (!area) {
> +		bool nofail = gfp_mask & __GFP_NOFAIL;
>  		warn_alloc(gfp_mask, NULL,
> -			"vmalloc error: size %lu, vm_struct allocation failed",
> -			real_size);
> +			"vmalloc error: size %lu, vm_struct allocation failed%s",
> +			real_size, (nofail) ? ". Retrying." : "");
> +		if (nofail) {
> +			schedule_timeout_uninterruptible(1);
> +			goto again;
> +		}
>  		goto fail;
>  	}
>  
>