linux-kernel - Re: [PATCH v2 1/2] mm: cma: allocate cma areas bottom-up

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Message-ID: <20201223163537.GA4011967@carbon.DHCP.thefacebook.com>
Date:   Wed, 23 Dec 2020 08:35:37 -0800
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Mike Rapoport <rppt@...nel.org>, <linux-mm@...ck.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Rik van Riel <riel@...riel.com>,
        Michal Hocko <mhocko@...nel.org>,
        <linux-kernel@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH v2 1/2] mm: cma: allocate cma areas bottom-up

On Tue, Dec 22, 2020 at 08:06:06PM -0800, Andrew Morton wrote:
> On Mon, 21 Dec 2020 09:05:51 -0800 Roman Gushchin <guro@...com> wrote:
> 
> > Subject: [PATCH v3 1/2] mm: cma: allocate cma areas bottom-up
> 
> i386 allmodconfig:
> 
> In file included from ./include/vdso/const.h:5,
>                  from ./include/linux/const.h:4,
>                  from ./include/linux/bits.h:5,
>                  from ./include/linux/bitops.h:6,
>                  from ./include/linux/kernel.h:11,
>                  from ./include/asm-generic/bug.h:20,
>                  from ./arch/x86/include/asm/bug.h:93,
>                  from ./include/linux/bug.h:5,
>                  from ./include/linux/mmdebug.h:5,
>                  from ./include/linux/mm.h:9,
>                  from ./include/linux/memblock.h:13,
>                  from mm/cma.c:24:
> mm/cma.c: In function ‘cma_declare_contiguous_nid’:
> ./include/uapi/linux/const.h:20:19: warning: conversion from ‘long long unsigned int’ to ‘phys_addr_t’ {aka ‘unsigned int’} changes value from ‘4294967296’ to ‘0’ [-Woverflow]
>  #define __AC(X,Y) (X##Y)
>                    ^~~~~~
> ./include/uapi/linux/const.h:21:18: note: in expansion of macro ‘__AC’
>  #define _AC(X,Y) __AC(X,Y)
>                   ^~~~
> ./include/linux/sizes.h:46:18: note: in expansion of macro ‘_AC’
>  #define SZ_4G    _AC(0x100000000, ULL)
>                   ^~~
> mm/cma.c:349:53: note: in expansion of macro ‘SZ_4G’
>     addr = memblock_alloc_range_nid(size, alignment, SZ_4G,
>                                                      ^~~~~
> 

I thought that (!memblock_bottom_up() && memblock_end >= SZ_4G + size)
can't be true on a 32-bit platform, so the whole if clause can be compiled out.
Maybe it's because memblock_end can be equal to SZ_4G and if the size == 0...

I have no better idea than wrapping everything into
#if BITS_PER_LONG > 32
#endif.

Thanks!

--

diff --git a/mm/cma.c b/mm/cma.c
index 4fe74c9d83b0..5d69b498603a 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -344,12 +344,14 @@ int __init cma_declare_contiguous_nid(phys_addr_t base,
                 * Avoid using first 4GB to not interfere with constrained zones
                 * like DMA/DMA32.
                 */
+#if BITS_PER_LONG > 32
                if (!memblock_bottom_up() && memblock_end >= SZ_4G + size) {
                        memblock_set_bottom_up(true);
                        addr = memblock_alloc_range_nid(size, alignment, SZ_4G,
                                                        limit, nid, true);
                        memblock_set_bottom_up(false);
                }
+#endif
 
                if (!addr) {
                        addr = memblock_alloc_range_nid(size, alignment, base,