linux-kernel - Re: [PATCH] mm: Remove VM_BUG_ON in __alloc_pages

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Message-ID: <20190605142246.GH15685@dhcp22.suse.cz>
Date:   Wed, 5 Jun 2019 16:22:46 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Bharath Vedartham <linux.bhar@...il.com>
Cc:     akpm@...ux-foundation.org, vbabka@...e.cz, rientjes@...gle.com,
        khalid.aziz@...cle.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Remove VM_BUG_ON in __alloc_pages_node

On Wed 05-06-19 18:37:28, Bharath Vedartham wrote:
> [Not replying inline as my mail is bouncing back]
> 
> This patch is based on reading the code rather than a kernel crash. My
> thought process was that if an invalid node id was passed to
> __alloc_pages_node, it would be better to add a VM_WARN_ON and fail the
> allocation rather than crashing the kernel. 

This makes some sense to me because BUG_ONs are usually a wrong way to
handle wrong usage of the API. On the other hand VM_BUG_ON is special in
the way that production although some distributions enable it by default
IIRC.

> I feel it would be better to fail the allocation early in the hot path
> if an invalid node id is passed. This is irrespective of whether the
> VM_[BUG|WARN]_*s are enabled or not. I do not see any checks in the hot
> path for the node id, which in turn may cause NODE_DATA(nid) to fail to
> get the pglist_data pointer for the node id. 
> We can optimise the branch by wrapping it around in unlikely(), if
> performance is the issue?

unlikely will just move the return NULL ouside of the main code flow.
The check will still be done.

> What are your thoughts on this? 

I don't know. I would leave the code as it is now or remove the
VM_BUG_ON. I do not remember this would be catching any real issues in
the past.
-- 
Michal Hocko
SUSE Labs