RE: [PATCH] arm: mm: lockless get_user_pages_fast

[Chanho Park <chanho61.park@samsusng.com>]

> Apologies for the tardy response, this patch slipped past me.

Never mind.

> I've tested this patch out, unfortunately it treats huge pmds as regular
> pmds and attempts to traverse them rather than fall back to a slow path.
> The fix for this is very minor, please see my suggestion below.
OK. I'll fix it.

> 
> As an aside, I would like to extend this fast_gup to include full huge
> page support and include a __get_user_pages_fast implementation. This will
> hopefully fix a problem that was brought to my attention by Grazvydas
> Ignotas whereby a FUTEX_WAIT on a THP tail page will cause an infinite
> loop due to the stock implementation of __get_user_pages_fast always
> returning 0.

I'll add the __get_user_pages_fast implementation. BTW, HugeTLB on ARM
wasn't
supported yet. There is no problem to add gup_huge_pmd. But I think it need
a test
for hugepages.

> I would suggest:
> 		if (pmd_none(*pmdp) || pmd_bad(*pmdp))
> 			return 0;
> as this will pick up pmds that can't be traversed, and fall back to the
> slow path.

Thanks for your suggestion.
I'll prepare the v2 patch.

Best regards,
Chanho Park

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