Re: [PATCH v2] mmap.2: MAP_FIXED updated documentation

On 12/04/2017 11:08 PM, Michal Hocko wrote:

On Mon 04-12-17 18:52:27, John Hubbard wrote:

quoted

On 12/04/2017 03:31 AM, Mike Rapoport wrote:

quoted

On Sun, Dec 03, 2017 at 06:14:11PM -0800, john.hubbard@gmail.com wrote:

quoted

From: John Hubbard <jhubbard@nvidia.com>

[...]

quoted

quoted

+.IP
+Given the above limitations, one of the very few ways to use this option
+safely is: mmap() a region, without specifying MAP_FIXED. Then, within that
+region, call mmap(MAP_FIXED) to suballocate regions. This avoids both the
+portability problem (because the first mmap call lets the kernel pick the
+address), and the address space corruption problem (because the region being
+overwritten is already owned by the calling thread).

Maybe "address space corruption problem caused by implicit calls to mmap"?
The region allocated with the first mmap is not exactly owned by the
thread and a multi-thread application can still corrupt its memory if
different threads use mmap(MAP_FIXED) for overlapping regions.

My 2 cents.

Hi Mike,

Yes, thanks for picking through this, and I agree that the above is misleading.
It should definitely not use the word "owned" at all. Re-doing the whole 
paragraph in order to make it all fit together nicely, I get this:

"Given the above limitations, one of the very few ways to use this option
safely is: mmap() an enclosing region, without specifying MAP_FIXED.
Then, within that region, call mmap(MAP_FIXED) to suballocate regions
within the enclosing region. This avoids both the portability problem 
(because the first mmap call lets the kernel pick the address), and the 
address space corruption problem (because implicit calls to mmap will 
not affect the already-mapped enclosing region)."

...how's that sound to you? I'll post a v3 soon with this.

It sounds to me you are trying to tell way to much while actually being
a bit misleading. Even sub-range MAP_FIXED is not multi-thread safe.

Really the more corner cases you will try to cover the worse the end
result will end up. I would just try to be simple here and mention the
address space corruption issues you've had earlier and be done with it.
Maybe add a note that some architectures might need a special alignement
and fail if it is not the case but nothing really specific.

Sure, I can drop the "how to use this safely" section.  It seemed like a good
idea at the time... :)

thanks,
John Hubbard
NVIDIA

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