Re: [PATCHv9 26/36] mm: rework mapcount accounting to enable 4k mapping of THPs

[Jerome Marchand <jmarchan@redhat.com>]

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On 07/20/2015 04:20 PM, Kirill A. Shutemov wrote:
> We're going to allow mapping of individual 4k pages of THP compound.
> It means we need to track mapcount on per small page basis.
> 
> Straight-forward approach is to use ->_mapcount in all subpages to track
> how many time this subpage is mapped with PMDs or PTEs combined. But
> this is rather expensive: mapping or unmapping of a THP page with PMD
> would require HPAGE_PMD_NR atomic operations instead of single we have
> now.
> 
> The idea is to store separately how many times the page was mapped as
> whole -- compound_mapcount. This frees up ->_mapcount in subpages to
> track PTE mapcount.
> 
> We use the same approach as with compound page destructor and compound
> order to store compound_mapcount: use space in first tail page,
> ->mapping this time.
> 
> Any time we map/unmap whole compound page (THP or hugetlb) -- we
> increment/decrement compound_mapcount. When we map part of compound page
> with PTE we operate on ->_mapcount of the subpage.
> 
> page_mapcount() counts both: PTE and PMD mappings of the page.
> 
> Basically, we have mapcount for a subpage spread over two counters.
> It makes tricky to detect when last mapcount for a page goes away.
> 
> We introduced PageDoubleMap() for this. When we split THP PMD for the
> first time and there's other PMD mapping left we offset up ->_mapcount
> in all subpages by one and set PG_double_map on the compound page.
> These additional references go away with last compound_mapcount.

So this stays even if all PTE mappings goes and the page is again mapped
only with PMD. I'm not sure how often that happen and if it's an issue
worth caring about.

Acked-by: Jerome Marchand <jmarchan@redhat.com>

> 
> This approach provides a way to detect when last mapcount goes away on
> per small page basis without introducing new overhead for most common
> cases.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> ---
>  include/linux/mm.h         | 26 +++++++++++-
>  include/linux/mm_types.h   |  1 +
>  include/linux/page-flags.h | 37 +++++++++++++++++
>  include/linux/rmap.h       |  4 +-
>  mm/debug.c                 |  5 ++-
>  mm/huge_memory.c           |  2 +-
>  mm/hugetlb.c               |  4 +-
>  mm/memory.c                |  2 +-
>  mm/migrate.c               |  2 +-
>  mm/page_alloc.c            | 14 +++++--
>  mm/rmap.c                  | 99 +++++++++++++++++++++++++++++++++++-----------
>  11 files changed, 161 insertions(+), 35 deletions(-)
> 



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