Re: [patch 3/3] mm: fault handler to replace nopage and populate

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Benjamin Herrenschmidt wrote:


Could it be an option to make it safe for the fault handler to temporarily drop the mmap_sem read lock given that some conditions TBD are met? In that case it can retake the mmap_sem write lock, do the VMA flags modifications, downgrade and do the pte modifications using a helper, or even use remap_pfn_range() during the time the write lock is held?
When you drop and retake the mmap_sem, you need to start again from
find_vma. At which point you technically probably want to start again
from the architecture specfic fault code. It sounds difficult but I
won't say it can't be done.

I can be done with returning NOPAGE_REFAULT but as you said, I don't
think it's necessary.
Still, even with NOPAGE_REFAULT or the equivalent with the new fault() code,
in the case we need to take this route, (and it looks like we won't have to), I guess we still need to restart from find_vma() in the fault()/nopage() handler to make sure the VMA is still present. The object mutex need to be dropped as well to avoid deadlocks. Sounds complicated.

Cheers,
Ben.


/Thomas

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