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 16:27:00 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     zijun_hu <zijun_hu@...o.com>
Cc:     zijun_hu@....com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        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 22:14:40, zijun_hu wrote:
> On 2016/9/23 21:33, Michal Hocko wrote:
> > 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...
>
> okay
> i don't familiar with mmap code very well now

mmap basically does addr &= PAGE_MASK (modulo mmap_min_addr) and
len = PAGE_ALIGN(len).

this is [star, end) raher than [start, start+len) but you should get the
point I guess.
-- 
Michal Hocko
SUSE Labs