Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages

[Pasha Tatashin]

On 10/04/2017 10:04 AM, Michal Hocko wrote:
> On Wed 04-10-17 09:28:55, Pasha Tatashin wrote:
> > > 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.

> > > 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
>
> > > 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