Messages in this thread | | | From | David Hildenbrand <> | Subject | Re: [PATCH v2 6/9] mm: free user PTE page table pages | Date | Wed, 1 Sep 2021 18:13:07 +0200 |
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On 01.09.21 17:32, Jason Gunthorpe wrote: > On Wed, Sep 01, 2021 at 03:57:09PM +0200, David Hildenbrand wrote: >> On 01.09.21 15:53, Jason Gunthorpe wrote: >>> On Thu, Aug 19, 2021 at 11:18:55AM +0800, Qi Zheng wrote: >>> >>>> diff --git a/mm/gup.c b/mm/gup.c >>>> index 2630ed1bb4f4..30757f3b176c 100644 >>>> +++ b/mm/gup.c >>>> @@ -500,6 +500,9 @@ static struct page *follow_page_pte(struct vm_area_struct *vma, >>>> if (unlikely(pmd_bad(*pmd))) >>>> return no_page_table(vma, flags); >>>> + if (!pte_try_get(mm, pmd)) >>>> + return no_page_table(vma, flags); >>>> + >>>> ptep = pte_offset_map_lock(mm, pmd, address, &ptl); >>> >>> This is not good on a performance path, the pte_try_get() is >>> locking/locking the same lock that pte_offset_map_lock() is getting. >> >> Yes, and we really need patch #8, anything else is just confusing reviewers. > > It is a bit better with patch 8, but it is still not optimal, we don't > need to do the atomic work at all if the entire ptep is accessed while > locked. So the above is stil not what I would expect here, even with > RCU. > > eg I would expect that this kind of change would work first with the > existing paired acessors, ie > > pte = pte_offset_map(pmd, address); > pte_unmap(pte); > > Should handle the refcount under the covers, and same kind of idea for > the _locked/_unlocked varient.
See my other mail.
> > Only places that don't already use that pairing should get modified. > > To do this we have to extend the API so that pte_offset_map() can > fail, or very cleverly return some kind of global non-present pte page > (I wonder if the zero page would work?)
I explored both ideas (returning NULL, return a specially prepared page) and it didn't work in some cases where we unmap+remap etc.
> >>> Also, I don't really understand how this scheme works with >>> get_user_pages_fast. >> >> With the RCU change it in #8 it should work just fine, because RCU >> synchronize has to wait either until all other CPUs have left the RCU read >> section, or re-enabled interrupts. > > So at this point in the series fast gup is broken, that does mean the > series presentation really needs to be reworked. The better > presentation is to add the API changes, with a > no-functional-difference implementation, push the new API in well > split patches to all the consumption sites, then change the API to > have the new semantics.
Exactly my thoughts.
> > RCU and refcount to free the page levels seems like a reasonable > approach, but I have to say I haven't thought it through fully - are > all the contexts that have the pte deref safe to do call_rcu?
Very good question. I'd assume so.
-- Thanks,
David / dhildenb
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