Re: [RFC 2/3] mm/vmalloc: add support for __GFP_NOFAIL

[Uladzislau Rezki <urezki@gmail.com>]

> From: Michal Hocko <mhocko@suse.com>
> 
> 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
> 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.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>
> ---
>  mm/vmalloc.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 7455c89598d3..3a5a178295d1 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -2941,8 +2941,10 @@ static void *__vmalloc_area_node(struct vm_struct *area, gfp_t gfp_mask,
>  	else if (!(gfp_mask & (__GFP_FS | __GFP_IO)))
>  		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);
> +	} while ((gfp_mask & __GFP_NOFAIL) && (ret < 0));
>  
>  	if ((gfp_mask & (__GFP_FS | __GFP_IO)) == __GFP_IO)
>  		memalloc_nofs_restore(flags);
> @@ -3032,6 +3034,8 @@ 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)
> +			goto again;
>  		goto fail;
>  	}
>  
> -- 
> 2.30.2
> 
I have checked the vmap code how it aligns with the __GFP_NOFAIL flag.
To me it looks correct from functional point of view.

There is one place though it is kasan_populate_vmalloc_pte(). It does
not use gfp_mask, instead it directly deals with GFP_KERNEL for its
internal purpose. If it fails the code will end up in loping in the
__vmalloc_node_range().

I am not sure how it is important to pass __GFP_NOFAIL into KASAN code.

Any thoughts about it?

--
Vlad Rezki