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Message-ID: <20180119142133.379d5145@ezekiel.suse.cz>
Date: Fri, 19 Jan 2018 14:21:33 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: Michal Hocko <mhocko@...nel.org>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
Mel Gorman <mgorman@...hsingularity.net>,
Johannes Weiner <hannes@...xchg.org>,
Kemi Wang <kemi.wang@...el.com>,
YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
Andrey Ryabinin <aryabinin@...tuozzo.com>,
Nikolay Borisov <nborisov@...e.com>
Subject: Re: [PATCH] Fix explanation of lower bits in the SPARSEMEM mem_map
pointer
On Fri, 19 Jan 2018 13:39:56 +0100
Michal Hocko <mhocko@...nel.org> wrote:
> On Fri 19-01-18 08:09:08, Petr Tesarik wrote:
> [...]
> > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> > index 67f2e3c38939..7522a6987595 100644
> > --- a/include/linux/mmzone.h
> > +++ b/include/linux/mmzone.h
> > @@ -1166,8 +1166,16 @@ extern unsigned long usemap_size(void);
> >
> > /*
> > * We use the lower bits of the mem_map pointer to store
> > - * a little bit of information. There should be at least
> > - * 3 bits here due to 32-bit alignment.
> > + * a little bit of information. The pointer is calculated
> > + * as mem_map - section_nr_to_pfn(pnum). The result is
> > + * aligned to the minimum alignment of the two values:
> > + * 1. All mem_map arrays are page-aligned.
> > + * 2. section_nr_to_pfn() always clears PFN_SECTION_SHIFT
> > + * lowest bits. PFN_SECTION_SHIFT is arch-specific
> > + * (equal SECTION_SIZE_BITS - PAGE_SHIFT), and the
> > + * worst combination is powerpc with 256k pages,
> > + * which results in PFN_SECTION_SHIFT equal 6.
> > + * To sum it up, at least 6 bits are available.
> > */
>
> This is _much_ better indeed. Do you think we can go one step further
> and add BUG_ON into the sparse code to guarantee that every mmemap
> is indeed aligned properly so that SECTION_MAP_LAST_BIT-1 bits are never
> used?
This is easy for the section_nr_to_pfn() part. I'd just add:
BUILD_BUG_ON(PFN_SECTION_SHIFT < SECTION_MAP_LAST_BIT);
But for the mem_map arrays... Do you mean adding a run-time BUG_ON into
all allocation paths?
Note that mem_map arrays can be allocated by:
a) __earlyonly_bootmem_alloc
b) memblock_virt_alloc_try_nid
c) memblock_virt_alloc_try_nid_raw
d) alloc_remap (only arch/tile still has it)
Some allocation paths are in mm/sparse.c, others are
mm/sparse-vmemmap.c, so it becomes a bit messy, but since it's
a single line in each, it may work.
Petr T
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