Messages in this thread | | | Subject | Re: [PATCH v9 08/12] mm: zero reserved and unavailable struct pages | From | Pasha Tatashin <> | Date | Wed, 4 Oct 2017 11:08:59 -0400 |
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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
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