linux-kernel - Re: [PATCH 1/4] mm/zsmalloc: always set movable/highmem flag to the zspage

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Date:   Thu, 23 Mar 2017 11:10:23 +0900
From:   Joonsoo Kim <iamjoonsoo.kim@....com>
To:     Minchan Kim <minchan@...nel.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, kernel-team@....com
Subject: Re: [PATCH 1/4] mm/zsmalloc: always set movable/highmem flag to the
 zspage

On Tue, Mar 21, 2017 at 08:10:05PM +0900, Minchan Kim wrote:
> Hi Joonsoo,
> 
> On Thu, Mar 16, 2017 at 11:46:35AM +0900, js1304@...il.com wrote:
> > From: Joonsoo Kim <iamjoonsoo.kim@....com>
> > 
> > Zspage is always movable and is used through zs_map_object() function
> > which returns directly accessible pointer that contains content of
> > zspage. It is independent on the user's allocation flag.
> > Therefore, it's better to always set movable/highmem flag to the zspage.
> > After that, we don't need __GFP_MOVABLE/__GFP_HIGHMEM clearing in
> > cache_alloc_handle()/cache_alloc_zspage() since there is no zs_malloc
> > caller who specifies __GFP_MOVABLE/__GFP_HIGHMEM.
> 
> Hmm, I wanted this when you pointed out to me firstly but when I think
> again, I don't see it's improvement. Sorry for that.
> The zs_malloc is exported symbol and it has gfp_t argument so user can
> do whatever he want with any zone modifiers flags. IOW, if someuser want
> to allocate pages from {normal|dma} zone by whatever reason, he can
> omit __GFP_HIGHMEM from the gfp flag to fullfill the goal.

Hello,

I don't think that such flexibility makes things better. User cannot
fully understand what flags are the best since it highly depends on
implementation detail. For example, __GFP_MOVABLE is needed to
optimize memory fragmentation and user cannot know it and there is no
reason that user need to know it. __GFP_HIGHMEM is the similar case.
He cannot know that he can pass __GFP_HIGHMEM without knowing the
implementation detail and he cannot know the impact of __GFP_HIGHMEM
here. So, I think that adding these flags in zsmalloc can be justified.

Anyway, this patch isn't so important for this series so if you don't
like it, I will drop it.

Thanks.