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

From: Peter Xu
Date: Thu Jun 10 2021 - 19:04:26 EST

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?

Peter Xu