removing flush_tlb_mm as a generic hook ?

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

Hi folks !

While toying around with various MM callbacks, I found out that
flush_tlb_mm() as a generic hook provided by the archs has been mostly
obsoleted by the mmu_gather stuff.

(I'm not talking about archs internally wanting to implement it and use
it as a tlb_flush(), I'm talking about possibly making that optional :-)

I see two remaining users:

 - fs/proc/task_mmu.c, which I easily converted to use the mmu_gather
(I'll send a patch if people agree it's worth doing)

 - kernel/fork.c uses it to flush the "old" mm. That's the "meat".

I wonder if it's worth pursuing, that is converting copy_page_range to
use an mmu_gather on the source instead of using flush_tlb_mm. It might
allow some archs that can't just "flush all" easily but have to go
through every PTE individually to improve things a bit on fork, and it
allow them to remove the flush_tlb_mm() logic.

There is one reason why it's not a trivial conversion though, is that
copy_page_range() calls copy_hugetlb_page_range() for huge pages, and
I'm not sure about mixing up the hugetlb stuff with the mmu_gather
stuff, I need to do a bit more code auditing to figure out whether
that's an ok thing to do.

Nothing very urgent or important, it's just that one less hook seems
like a good idea ;-)

Cheers,
Ben.


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