On Wed, Jun 19, 2024 at 08:54:32PM +0800, yangge1116@xxxxxxx wrote: > From: yangge <yangge1116@xxxxxxx> > > Since commit 5d0a661d808f ("mm/page_alloc: use only one PCP list for > THP-sized allocations") no longer differentiates the migration type > of pages in THP-sized PCP list, it's possible that non-movable > allocation requests may get a CMA page from the list, in some cases, > it's not acceptable. > > If a large number of CMA memory are configured in system (for > example, the CMA memory accounts for 50% of the system memory), > starting a virtual machine with device passthrough will get stuck. > During starting the virtual machine, it will call > pin_user_pages_remote(..., FOLL_LONGTERM, ...) to pin memory. Normally > if a page is present and in CMA area, pin_user_pages_remote() will > migrate the page from CMA area to non-CMA area because of > FOLL_LONGTERM flag. But if non-movable allocation requests return > CMA memory, migrate_longterm_unpinnable_pages() will migrate a CMA > page to another CMA page, which will fail to pass the check in > check_and_migrate_movable_pages() and cause migration endless. > Call trace: > pin_user_pages_remote > --__gup_longterm_locked // endless loops in this function > ----_get_user_pages_locked > ----check_and_migrate_movable_pages > ------migrate_longterm_unpinnable_pages > --------alloc_migration_target > > This problem will also have a negative impact on CMA itself. For > example, when CMA is borrowed by THP, and we need to reclaim it > through cma_alloc() or dma_alloc_coherent(), we must move those > pages out to ensure CMA's users can retrieve that contigous memory. > Currently, CMA's memory is occupied by non-movable pages, meaning > we can't relocate them. As a result, cma_alloc() is more likely to > fail. > > To fix the problem above, we add one PCP list for THP, which will > not introduce a new cacheline for struct per_cpu_pages. THP will > have 2 PCP lists, one PCP list is used by MOVABLE allocation, and > the other PCP list is used by UNMOVABLE allocation. MOVABLE > allocation contains GPF_MOVABLE, and UNMOVABLE allocation contains > GFP_UNMOVABLE and GFP_RECLAIMABLE. > > Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations") > Signed-off-by: yangge <yangge1116@xxxxxxx> Too late to be relevant but Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> It would have been preferred if the comment stated why GFP_UNMOVABLE is needed because the original assumption that THP would mostly be MOVABLE allocations did not age well but git blame is enough. Maybe one day GFP_RECLAIMABLE will also be added to the list. However, I suspect the bigger problem later will be multiple THP sizes becoming more common that are not necessarily fitting within PAGE_ALLOC_COSTLY_ORDER and per-cpu not being enough to mitigate zone->lock contention in general. That's beyond the scope of this patch. Thanks. -- Mel Gorman SUSE Labs