Re: [PATCH 2/2] ipc: use kmalloc for msg_queue and shmid_kernel

From: Vasily Averin
Date: Wed Apr 28 2021 - 01:15:18 EST

On 4/26/21 1:25 PM, Michal Hocko wrote:
> Using kvmalloc for sub page size objects is suboptimal because kmalloc
> can easily fallback into vmalloc under memory pressure and smaller
> objects would fragment memory. Therefore replace kvmalloc by a simple
> kmalloc.

I think you're wrong here:
kvmalloc can failback to vmalloc for size > PAGE_SIZE only

Please take look at mm/util.c::kvmalloc_node()

if (size > PAGE_SIZE) {
kmalloc_flags |= __GFP_NOWARN;

if (!(kmalloc_flags & __GFP_RETRY_MAYFAIL))
kmalloc_flags |= __GFP_NORETRY;

ret = kmalloc_node(size, kmalloc_flags, node);

* It doesn't really make sense to fallback to vmalloc for sub page
* requests
if (ret || size <= PAGE_SIZE)
return ret;

return __vmalloc_node(size, 1, flags, node,

For small objects kvmalloc is not much different just from kmalloc,
so the patch is mostly cosmetic.