On Mon, Apr 06, 2020 at 12:53:00PM -0700, Ralph Campbell wrote:
>
> On 4/3/20 4:29 AM, Kirill A. Shutemov wrote:
> > __collapse_huge_page_swapin() check number of referenced PTE to decide
> > if the memory range is hot enough to justify swapin.
> >
> > The problem is that it stops collapse altogether if there's not enough
> > referenced pages, not only swappingin.
> >
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> > Fixes: 0db501f7a34c ("mm, thp: convert from optimistic swapin collapsing to conservative")
> > Reviewed-by: Zi Yan <ziy@xxxxxxxxxx> ---
> > mm/khugepaged.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 99bab7e4d05b..14d7afc90786 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -905,7 +905,8 @@ static bool __collapse_huge_page_swapin(struct mm_struct *mm,
> > /* we only decide to swapin, if there is enough young ptes */
> > if (referenced < HPAGE_PMD_NR/2) {
> > trace_mm_collapse_huge_page_swapin(mm, swapped_in, referenced, 0);
>
> The trace point is recording the return value. Shouldn't you s/0/1 to match?
Good catch.
>
> > - return false;
> > + /* Do not block collapse, only skip swapping in */
> > + return true;
> > }
> > vmf.pte = pte_offset_map(pmd, address);
> > for (; vmf.address < address + HPAGE_PMD_NR*PAGE_SIZE;
> >
>
> So "referenced < HPAGE_PMD_NR/2" means swapped out pages aren't faulted back in
> but there could still be no swapped out pages, just "old" pages so collapse could
> succeed. Seems like this check could have been made in khugepaged_scan_pmd() when
> "referenced" is being computed and "unmapped" is available. Just skip setting
> "ret = 1" if unmapped > 0 && referenced < HPAGE_PMD_NR/2.
Good point. I'll change the code to address it.
--
Kirill A. Shutemov
