linux-kernel - Re: [PATCH v1][cover-letter] mm/mempolicy.c: Fix get

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Date:   Fri, 06 Oct 2017 15:12:40 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Luis Felipe Sandoval Castro 
        <luis.felipe.sandoval.castro@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, vbabka@...e.cz, mhocko@...e.com,
        mingo@...nel.org, rientjes@...gle.com, n-horiguchi@...jp.nec.com,
        salls@...ucsb.edu
Subject: Re: [PATCH v1][cover-letter] mm/mempolicy.c: Fix get_nodes() off-by-one error.

Luis Felipe Sandoval Castro <luis.felipe.sandoval.castro@...el.com>
writes:

> According to mbind() and set_mempolicy()'s man pages the argument "maxnode"
> specifies the max number of bits in the "nodemask" (which is also to be passed
> to these functions) that should be considered for the memory policy. If maxnode
> = 2, only two bits are to be considered thus valid node masks are: 0b00, 0b01,
> 0b10 and 0b11.

We can't change this unfortunately, it would break old binaries (like
libnuma) which assume the old interface.

The only way to fix it would be to add a new system call and keep
the old one for compatibility, but that would seem like overkill just
for this.

You always have to add +1, sorry.

Perhaps it could be better documented.

-Andi