Re: [PATCH v10 07/10] mm: Device exclusive memory access

[Alistair Popple]

On Friday, 11 June 2021 9:04:19 AM AEST Peter Xu wrote:
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> 
> On Fri, Jun 11, 2021 at 12:21:26AM +1000, Alistair Popple wrote:
> > > Hmm, the thing is.. to me FOLL_SPLIT_PMD should have similar effect to explicit
> > > call split_huge_pmd_address(), afaict.  Since both of them use __split_huge_pmd()
> > > internally which will generate that unwanted CLEAR notify.
> >
> > Agree that gup calls __split_huge_pmd() via split_huge_pmd_address()
> > which will always CLEAR. However gup only calls split_huge_pmd_address() if it
> > finds a thp pmd. In follow_pmd_mask() we have:
> >
> >       if (likely(!pmd_trans_huge(pmdval)))
> >               return follow_page_pte(vma, address, pmd, flags, &ctx->pgmap);
> >
> > So I don't think we have a problem here.
> 
> Sorry I didn't follow here..  We do FOLL_SPLIT_PMD after this check, right?  I
> mean, if it's a thp for the current mm, afaict pmd_trans_huge() should return
> true above, so we'll skip follow_page_pte(); then we'll check FOLL_SPLIT_PMD
> and do the split, then the CLEAR notify.  Hmm.. Did I miss something?

That seems correct - if the thp is not mapped with a pmd we won't split and we
won't CLEAR. If there is a thp pmd we will split and CLEAR, but in that case it
is fine - we will retry, but the retry will won't CLEAR because the pmd has
already been split.

The issue arises with doing it unconditionally in make device exclusive is that
you *always* CLEAR even if there is no thp pmd to split. Or at least that's my
understanding, please let me know if it doesn't make sense.

 - Alistair

> --
> Peter Xu
>