On Fri 23-09-16 20:29:20, zijun_hu wrote:

On 2016/9/23 16:45, Michal Hocko wrote:

quoted

On Thu 22-09-16 23:13:17, zijun_hu wrote:

quoted

On 2016/9/22 20:47, Michal Hocko wrote:

quoted

On Wed 21-09-16 12:19:53, zijun_hu wrote:

quoted

From: zijun_hu <redacted>

endless loop maybe happen if either of parameter addr and end is not
page aligned for kernel API function ioremap_page_range()

Does this happen in practise or this you found it by reading the code?

i found it by reading the code, this is a kernel API function and there
are no enough hint for parameter requirements, so any parameters
combination maybe be used by user, moreover, it seems appropriate for
many bad parameter combination, for example, provided  PMD_SIZE=2M and
PAGE_SIZE=4K, 0x00 is used for aligned very well address
a user maybe want to map virtual range[0x1ff800, 0x200800) to physical address
0x300800, it will cause endless loop

Well, we are relying on the kernel to do the sane thing otherwise we
would be screwed anyway. If this can be triggered by a userspace then it
would be a different story. Just look at how we are doing mmap, we
sanitize the page alignment at the high level and the lower level
functions just assume sane values.

ioremap_page_range() is exported by EXPORT_SYMBOL_GPL() as a kernel interface
so perhaps it is called by not only any kernel module authors but also other
kernel parts

if the bad range is used by a careless kernel user really, it seems a better
choice to alert the warning message or panic the kernel than hanging the system
due to endless loop, it can help them locate problem usefully

I absolutely do not want to panic my system just because a crapy module
or whatnot doesn't provide an aligned address. Warning and a fixup
sounds much more sane to me.

[...]

quoted

quoted

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.

-- 
Michal Hocko
SUSE Labs

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