Re: [PATCH 6.1.y] mm/page_alloc: Separate THP PCP into movable and non-movable categories

[Greg KH <greg@kroah.com>]

On Tue, Jul 02, 2024 at 12:00:19PM +0800, yangge1116@126.com wrote:
> From: yangge <yangge1116@126.com>
> 
> 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.
> 
> Link: https://lkml.kernel.org/r/1718845190-4456-1-git-send-email-yangge1116@126.com
> Fixes: 5d0a661d808f ("mm/page_alloc: use only one PCP list for THP-sized allocations")
> Signed-off-by: yangge <yangge1116@126.com>
> Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
> Cc: Barry Song <21cnbao@gmail.com>
> Cc: Mel Gorman <mgorman@techsingularity.net>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> (cherry picked from commit bf14ed81f571f8dba31cd72ab2e50fbcc877cc31)
> Signed-off-by: yangge <yangge1116@126.com>
> ---
>  include/linux/mmzone.h | 9 ++++-----
>  mm/page_alloc.c        | 8 ++++++--
>  2 files changed, 10 insertions(+), 7 deletions(-)
> 

All now queued up, thanks.

greg k-h