On 10/04/2017 10:04 AM, Michal Hocko wrote:

On Wed 04-10-17 09:28:55, Pasha Tatashin wrote:

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I am not really familiar with the trim_low_memory_range code path. I am
not even sure we have to care about it because nobody should be walking
pfns outside of any zone.

According to commit comments first 4K belongs to BIOS, so I think the memory
exists but BIOS may or may not report it to Linux. So, reserve it to make
sure we never touch it.

Yes and that memory should be outside of any zones, no?

I am not totally sure, I think some x86 expert could help us here. But, 
in either case this issue can be fixed separately from the rest of the 
series.

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I am worried that this patch adds a code which
is not really used and it will just stay that way for ever because
nobody will dare to change it as it is too obscure and not explained
very well.

I could explain mine code better. Perhaps add more comments, and explain
when it can be removed?

More explanation would be definitely helpful

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trim_low_memory_range is a good example of this. Why do we
even reserve this range from the memory block allocator? The memory
shouldn't be backed by any real memory and thus not in the allocator in
the first place, no?

Since it is not enforced in memblock that everything in reserved list must
be part of memory list, we can have it, and we need to make sure kernel does
not panic. Otherwise, it is very hard to detect such bugs.

So, should we report such a memblock reservation API (ab)use to the log?
Are you actually sure that trim_low_memory_range is doing a sane and
really needed thing? In other words do we have a zone which contains
this no-memory backed pfns?

And, this patch reports it already:

+	pr_info("Reserved but unavailable: %lld pages", pgcnt);

I could add a comment above this print call, explain that such memory is 
probably bogus and must be studied/fixed. Also, add that this code can 
be removed once memblock is changed to allow reserve only memory that is 
backed by physical memory i.e. in "memory" list.

Pasha