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@xxxxxxx> 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
