Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX_SIZE

From: Michal Hocko <mhocko@kernel.org>
To: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2] mm/kvmalloc: do not call kmalloc for size > KMALLOC_MAX_SIZE
Date: Mon, 5 Nov 2018 17:57:30 +0100	[thread overview]
Message-ID: <20181105165730.GN4361@dhcp22.suse.cz> (raw)
In-Reply-To: <d033db53-129d-c031-db78-ba7f9fed5bf4@yandex-team.ru>

On Mon 05-11-18 19:19:28, Konstantin Khlebnikov wrote:
> 
> 
> On 05.11.2018 16:03, Vlastimil Babka wrote:
> > On 11/1/18 11:09 AM, Konstantin Khlebnikov wrote:
> > > Allocations over KMALLOC_MAX_SIZE could be served only by vmalloc.
> > > 
> > > Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> > 
> > Makes sense regardless of warnings stuff.
> > 
> > Acked-by: Vlastimil Babka <vbabka@suse.cz>
> > 
> > But it must be moved below the GFP_KERNEL check!
> 
> But kmalloc cannot handle it regardless of GFP.
> 
> Ok maybe write something like this
> 
> if (size > KMALLOC_MAX_SIZE) {
> 	if (WARN_ON_ONCE((flags & GFP_KERNEL) != GFP_KERNEL)
> 		return NULL;
> 	goto do_vmalloc;
> }

Do we really have to be so defensive? I agree with Vlastimil that the
check should be done after GFP_KERNEL check (I should have noticed that).
kmalloc should already complain on the allocation size request.

> or fix that uncertainty right in vmalloc
> 
> For now comment in vmalloc declares
> 
>  *	Any use of gfp flags outside of GFP_KERNEL should be consulted
>  *	with mm people.

Which is what we want. There are some exceptional cases where using a
subset of GFP_KERNEL works fine (e.g. scope nofs/noio context).

-- 
Michal Hocko
SUSE Labs