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Date:   Tue, 8 Aug 2023 04:26:48 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Miaohe Lin <linmiaohe@huawei.com>
Cc:     akpm@linux-foundation.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: remove unneeded __meminit annotation
Message-ID: <ZNG1+PV5nG8YZDev@casper.infradead.org>
References: <20230808015831.608454-1-linmiaohe@huawei.com>
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On Tue, Aug 08, 2023 at 04:16:57AM +0100, Matthew Wilcox wrote:
> On Tue, Aug 08, 2023 at 11:08:36AM +0800, Miaohe Lin wrote:
> > On 2023/8/8 10:31, Matthew Wilcox wrote:
> > > On Tue, Aug 08, 2023 at 09:58:31AM +0800, Miaohe Lin wrote:
> > >> kswapd_stop() and kcompactd_stop() are only called when MEMORY_HOTREMOVE
> > >> is enabled. So wrap them under CONFIG_MEMORY_HOTREMOVE and further remove
> > >> __meminit annotation. No functional change intended.
> > > 
> > > I don't understand why this is an improvement.  If CONFIG_MEMORY_HOTREMOVE
> > > is disabled, the linker drops this section (... right?)  If it's enabled,
> > 
> > When CONFIG_MEMORY_HOTREMOVE is disabled, without this patch:
> > 
> > size mm/compaction.o
> >    text	   data	    bss	    dec	    hex	filename
> >  103164	  30873	      0	 134037	  20b95	mm/compaction.o
> > 
> > size mm/vmscan.o
> >    text	   data	    bss	    dec	    hex	filename
> >  158775	  49612	     64	 208451	  32e43	mm/vmscan.o
> > 
> > while with this patch:
> > 
> > size mm/compaction.o
> >    text	   data	    bss	    dec	    hex	filename
> >  102915	  30865	      0	 133780	  20a94	mm/compaction.o
> > 
> > size mm/vmscan.o
> >    text	   data	    bss	    dec	    hex	filename
> >  158534	  49604	     64	 208202	  32d4a	mm/vmscan.o
> > 
> > We can reduce each .o by ~250 bytes.
> 
> But this is before the linker step!  That will be where the meminit
> sections get dropped.  Assuming they are; I haven't verified.  You need
> to compare before/after of the vmlinux, not the individual .o files.

Ah, found it:

#if defined(CONFIG_MEMORY_HOTPLUG)
#define MEM_KEEP(sec)    *(.mem##sec)
#define MEM_DISCARD(sec)
#else
#define MEM_KEEP(sec)
#define MEM_DISCARD(sec) *(.mem##sec)
#endif

in include/asm-generic/vmlinux.lds.h

So yeah, I think this patch is garbage.