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