Re: [PATCH 26/31] sched, numa, mm: Add fault driven placement andmigration policy

[Ingo Molnar]

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> [
>   task_numa_work() performance side note:
> 
>   We are also *very* close to be able to use down_read() instead
>   of down_write() in the sampling-unmap code in 
>   task_numa_work(), as it should be safe in theory to call 
>   change_protection(PROT_NONE) in parallel - but there's one 
>   regression that disagrees with this theory so we use 
>   down_write() at the moment.
> 
>   Maybe you could help us there: can you see a reason why the
>   change_prot_none()->change_protection() call in
>   task_numa_work() can not occur in parallel to a page fault in
>   another thread on another CPU? It should be safe - yet if we 
>   change it I can see occasional corruption of user-space state: 
>   segfaults and register corruption.
> ]

Oh, just found the reason:

the ptep_modify_prot_start()/modify()/commit() sequence is 
SMP-unsafe - it has to be done under the mmap_sem write-locked.

It is safe against *hardware* updates to the PTE, but not safe 
against itself.

This is apparently a hidden cost of paravirt, it is forcing that 
weird sequence and thus the down_write() ...

Thanks,

	Ingo
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