linux-kernel - Re: [RFC PATCH 1/8] mm: Switch mm->get_unmapped

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Date: Thu, 15 Feb 2024 16:30:46 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Rick Edgecombe <rick.p.edgecombe@...el.com>, Liam.Howlett@...cle.com,
 akpm@...ux-foundation.org, debug@...osinc.com, broonie@...nel.org,
 kirill.shutemov@...ux.intel.com, keescook@...omium.org, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
 luto@...nel.org, peterz@...radead.org, hpa@...or.com, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/8] mm: Switch mm->get_unmapped_area() to a flag

On 2/15/24 15:13, Rick Edgecombe wrote:
> The mm_struct contains a function pointer *get_unmapped_area(), which
> is set to either arch_get_unmapped_area() or
> arch_get_unmapped_area_topdown() during the initialization of the mm.
> 
> Since the function pointer only ever points to two functions that are named
> the same across all arch's, a function pointer is not really required. In
> addition future changes will want to add versions of the functions that
> take additional arguments. So to save a pointers worth of bytes in
> mm_struct, and prevent adding additional function pointers to mm_struct in
> future changes, remove it and keep the information about which
> get_unmapped_area() to use in a flag.

Indirect calls are just all kinds of evil, especially when Spectre-v2 is
in play.  This is a really good idea.

Acked-by: Dave Hansen <dave.hansen@...ux.intel.com>