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 08:40:11 -0400 |
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>>> Could you be more specific where is such a memory reserved? >>> >> >> I know of one example: trim_low_memory_range() unconditionally reserves from >> pfn 0, but e820__memblock_setup() might provide the exiting memory from pfn >> 1 (i.e. KVM). > > Then just initialize struct pages for that mapping rigth there where a > special API is used. > >> But, there could be more based on this comment from linux/page-flags.h: >> >> 19 * PG_reserved is set for special pages, which can never be swapped out. >> Some >> 20 * of them might not even exist (eg empty_bad_page)... > > I have no idea wht empty_bad_page is but a quick grep shows that this is > never used. I might be wrong here but if somebody is reserving a memory > in a special way then we should handle the initialization right there. > E.g. create an API for special memblock reservations. >
Hi Michal,
The reservations happen before struct pages are allocated and mapped. So, it is not always possible to do it at call sites.
Previously, I have solved this problem like this:
https://patchwork.kernel.org/patch/9886163
But, I was not too happy with that approach, so I replaced it with the current approach as it is more generic, and solves similar issues if they happen in other places. Also, the comment in page-flags got me scared that there are probably other places perhaps on other architectures that can have the similar issue.
In addition, I did not like my solution, I was simply shrinking the low reservation from: [0 - reserve_low) to [min_pfn - reserve_low), but if min_pfn > reserve_low can we skip low reservation entirely? I was not sure.
The current approach notifies us if there are such pages, and we can fix/remove them in the future without crashing kernel in the meantime.
Pasha
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