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). > 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
