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.
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?
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.
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