linux-kernel - Re: [PATCH 1/1] lib/ioremap.c: avoid endless loop under ioremapping page unaligned ranges

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Date:   Fri, 23 Sep 2016 15:33:31 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     zijun_hu <zijun_hu@...o.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org, zijun_hu@....com,
        Andrew Morton <akpm@...ux-foundation.org>, tj@...nel.org,
        mingo@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH 1/1] lib/ioremap.c: avoid endless loop under ioremapping
 page unaligned ranges

On Fri 23-09-16 21:00:18, zijun_hu wrote:
> On 09/23/2016 08:42 PM, Michal Hocko wrote:
> >>>> no, it don't work for many special case
> >>>> for example, provided  PMD_SIZE=2M
> >>>> mapping [0x1f8800, 0x208800) virtual range will be split to two ranges
> >>>> [0x1f8800, 0x200000) and [0x200000,0x208800) and map them separately
> >>>> the first range will cause dead loop
> >>>
> >>> I am not sure I see your point. How can we deadlock if _both_ addresses
> >>> get aligned to the page boundary and how does PMD_SIZE make any
> >>> difference.
> >>>
> >> i will take a example to illustrate my considerations
> >> provided PUD_SIZE == 1G, PMD_SIZE == 2M, PAGE_SIZE == 4K
> >> it is used by arm64 normally
> >>
> >> we want to map virtual range [0xffffffff_ffc08800, 0xffffffff_fffff800) by
> >> ioremap_page_range(),ioremap_pmd_range() is called to map the range
> >> finally, ioremap_pmd_range() will call
> >> ioremap_pte_range(pmd, 0xffffffff_ffc08800, 0xffffffff_fffe0000) and
> >> ioremap_pte_range(pmd, 0xffffffff_fffe0000, 0xffffffff fffff800) separately
> > 
> > but those ranges are not aligned and it ioremap_page_range fix them up
> > to _be_ aligned then there is no problem, right? So either I am missing
> > something or we are talking past each other.
> > 
> my complementary considerations are show below
> 
> why not to round up the range start boundary to page aligned?
> 1, it don't remain consistent with the original logic
>    take map [0x1800, 0x4800) as example
>    the original logic map range [0x1000, 0x2000), but rounding up start boundary
>    don't mapping the range [0x1000, 0x2000)

just look at how we do that for the mmap...

> 2, the rounding up start boundary maybe cause overflow, consider start boundary =
>    0xffffffff_fffff800  

this is just insane

-- 
Michal Hocko
SUSE Labs