linux-kernel - Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache

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Date:   Wed, 31 Aug 2016 14:35:11 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     "Huang\, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        tim.c.chen@...el.com, dave.hansen@...el.com, andi.kleen@...el.com,
        aaron.lu@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Shaohua Li <shli@...nel.org>, Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
        Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH -v2] mm: Don't use radix tree writeback tags for pages in swap cache

Mel Gorman <mgorman@...hsingularity.net> writes:
>
> __GFP_BITS_SHIFT + 5 (AS_NO_WRITEBACK_TAGS) = 31
>
> mapping->flags is a combination of AS and GFP flags so increasing
> __GFP_BITS_SHIFT overflows mapping->flags on 32-bit as gfp_t is an
> unsigned int.

Couldn't we just split mapping->flags into two fields?
I'm sure more GFP bits will be needed eventually.

-Andi