Re: [PATCH -next] mm: delete oversized WARN_ON() in kvmalloc() calls

[Matthew Wilcox]

On Thu, Dec 02, 2021 at 05:23:42PM +0200, Leon Romanovsky wrote:
> The problem is that this WARN_ON() is triggered by the users.

... or the problem is that you don't do a sanity check between the user
and the MM system.  I mean, that's what this conversation is about --
is it a bug to be asking for this much memory in the first place?

> At least in the RDMA world, users can provide huge sizes and they expect
> to get plain -ENOMEM and not dump stack, because it happens indirectly
> to them.
> 
> In our case, these two kvcalloc() generates WARN_ON().
> 
> 		umem_odp->pfn_list = kvcalloc(
> 			npfns, sizeof(*umem_odp->pfn_list), GFP_KERNEL);

Does it really make sense for the user to specify 2^31 PFNs in a single
call?  I mean, that's 8TB of memory.  Should RDMA put its own limit
in here, or should it rely on kvmalloc returning -ENOMEM?