Re: Memory management bug

[schwidefsky@de.ibm.com]

>>
>> If they absolutely needs 4 pages for pmd pagetables due hardware
constraints
>> I'd recommend to use _four_ hardware pages for each softpage, not two.
>
>Yes.
>
>However, it definitely is an issue of making trade-offs. Most 64-bit MMU
>models tend to have some flexibility in how you set up the page tables,
>and it may be possible to just move bits around too (ie making both the
>pmd and the pgd twice as large, and getting the expansion of 4 by doing
>two expand-by-two's, for example, if the hardware has support for doing
>things like that).

Unluckly we don't have any flexibility. The segment index (pmd) has 11
bits,
pointers are 8 byte. That makes 16K segment table. I have understood that
this is a problem if the system is really low on memory. But low on memory
does mean low on real memory + swap space, doesn't it ? The system has
enough swap space but it isn't using any of it when the BUG hits. I think
the "if (!order)" statements before the "goto try_again" in __alloc_pages
have something to do with it. To test this assumption I removed the ifs and

I didn't see any "__alloc_pages: %lu-order allocation failed." message
before I hit yet another BUG in swap_state.c:60.
Whats the reasoning behind these ifs ?

blue skies,
   Martin

Linux/390 Design & Development, IBM Deutschland Entwicklung GmbH
Schönaicherstr. 220, D-71032 Böblingen, Telefon: 49 - (0)7031 - 16-2247
E-Mail: schwidefsky@de.ibm.com


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