linux-kernel - Re: [RFC PATCH for 4.21 04/16] mm: Introduce vm_map_user_ram, vm_unmap_user

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Message-ID: <20181016143016.10da89bd@gandalf.local.home>
Date:   Tue, 16 Oct 2018 14:30:16 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>, Chris Lameter <cl@...ux.com>,
        Ben Maurer <bmaurer@...com>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Subject: Re: [RFC PATCH for 4.21 04/16] mm: Introduce vm_map_user_ram,
 vm_unmap_user_ram

On Wed, 10 Oct 2018 15:19:24 -0400
Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:

> + * vm_unmap_user_ram - unmap linear kernel address space set up by vm_map_user_ram
> + * @mem: the pointer returned by vm_map_user_ram
> + * @count: the count passed to that vm_map_user_ram call (cannot unmap partial)
> + */
> +void vm_unmap_user_ram(const void *mem, unsigned int count)
> +{
> +	unsigned long size = (unsigned long)count << PAGE_SHIFT;
> +	unsigned long addr = (unsigned long)mem;
> +	struct vmap_area *va;
> +
> +	might_sleep();
> +	BUG_ON(!addr);
> +	BUG_ON(addr < VMALLOC_START);
> +	BUG_ON(addr > VMALLOC_END);
> +	BUG_ON(!PAGE_ALIGNED(addr));
> +
> +	debug_check_no_locks_freed(mem, size);
> +	va = find_vmap_area(addr);
> +	BUG_ON(!va);
> +	free_unmap_vmap_area(va);
> +}
> +EXPORT_SYMBOL(vm_unmap_user_ram);
> +

Noticing this from Sergey's question in another patch, why are you
using BUG_ON()? That's rather extreme and something we are trying to
avoid adding more of (I still need to remove the BUG_ON()s I've added
over ten years ago). I don't see why all these BUG_ON's can't be turned
into:

	if (WARN_ON(x))
		return;

-- Steve