Re: [PATCH v4 5/6] mm: vmscan: Avoid split during shrink_folio_list()

[Barry Song]

On Tue, Mar 12, 2024 at 9:12 PM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
>
> On 11/03/2024 22:30, Barry Song wrote:
> > On Mon, Mar 11, 2024 at 11:01 PM Ryan Roberts <ryan.roberts@armcom> wrote:
> >>
> >> Now that swap supports storing all mTHP sizes, avoid splitting large
> >> folios before swap-out. This benefits performance of the swap-out path
> >> by eliding split_folio_to_list(), which is expensive, and also sets us
> >> up for swapping in large folios in a future series.
> >>
> >> If the folio is partially mapped, we continue to split it since we want
> >> to avoid the extra IO overhead and storage of writing out pages
> >> uneccessarily.
> >>
> >> Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
> >> ---
> >>  mm/vmscan.c | 9 +++++----
> >>  1 file changed, 5 insertions(+), 4 deletions(-)
> >>
> >> diff --git a/mm/vmscan.c b/mm/vmscan.c
> >> index cf7d4cf47f1a..0ebec99e04c6 100644
> >> --- a/mm/vmscan.c
> >> +++ b/mm/vmscan.c
> >> @@ -1222,11 +1222,12 @@ static unsigned int shrink_folio_list(struct list_head *folio_list,
> >>                                         if (!can_split_folio(folio, NULL))
> >>                                                 goto activate_locked;
> >>                                         /*
> >> -                                        * Split folios without a PMD map right
> >> -                                        * away. Chances are some or all of the
> >> -                                        * tail pages can be freed without IO.
> >> +                                        * Split partially mapped folios map
> >> +                                        * right away. Chances are some or all
> >> +                                        * of the tail pages can be freed
> >> +                                        * without IO.
> >>                                          */
> >> -                                       if (!folio_entire_mapcount(folio) &&
> >> +                                       if (!list_empty(&folio->_deferred_list) &&
> >
> > Hi Ryan,
> > After reconsidering our previous discussion about PMD-mapped large
> > folios, I've pondered
> > the possibility of PMD-mapped Transparent Huge Pages (THPs) being
> > mapped by multiple
> > processes. In such a scenario, if one process decides to unmap a
> > portion of the folio while
> > others retain the entire mapping, it raises questions about how the
> > system should handle
> > this situation. Would the large folio be placed in a deferred list?
>
> No - if the large folio is entirely mapped (via PMD), then the folio will not be
> put on the deferred split list in the first place. See __folio_remove_rmap():
>
>         last = (last < ENTIRELY_MAPPED);
>
> means that nr will never be incremented above 0. (_nr_pages_mapped is
> incremented by ENTIRELY_MAPPED for every PMD map).

you are right, I missed this part, we are breaking early in RMAP_LEVEL_PTE.
so we won't get to if (nr). Thanks for your clarification. now we get
unified code
for both pmd-mapped and pte-mapped large folios. feel free to add,

Reviewed-by: Barry Song <v-songbaohua@xxxxxxxx>

>
> > If
> > so, splitting it might not
> > yield benefits, as neither I/O nor swap slots would increase in this
> > case by not splitting it.
> >
> > Regarding PTE-mapped large folios, the absence of an indicator like
> > "entire_map" makes it
> > challenging to identify cases where the entire folio is mapped. Thus,
> > splitting seems to be
> > the only viable solution in such circumstances.
> >
> >>                                             split_folio_to_list(folio,
> >>                                                                 folio_list))
> >>                                                 goto activate_locked;
> >> --
> >> 2.25.1
>

Thanks
Barry