Re: [PATCH -mm -v4 03/21] mm, THP, swap: Support PMD swap mapping in swap_duplicate()

[Dan Williams]

On Thu, Jun 21, 2018 at 8:55 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> From: Huang Ying <ying.huang@xxxxxxxxx>
>
> To support to swapin the THP as a whole, we need to create PMD swap
> mapping during swapout, and maintain PMD swap mapping count.  This
> patch implements the support to increase the PMD swap mapping
> count (for swapout, fork, etc.)  and set SWAP_HAS_CACHE flag (for
> swapin, etc.) for a huge swap cluster in swap_duplicate() function
> family.  Although it only implements a part of the design of the swap
> reference count with PMD swap mapping, the whole design is described
> as follow to make it easy to understand the patch and the whole
> picture.
>
> A huge swap cluster is used to hold the contents of a swapouted THP.
> After swapout, a PMD page mapping to the THP will become a PMD
> swap mapping to the huge swap cluster via a swap entry in PMD.  While
> a PTE page mapping to a subpage of the THP will become the PTE swap
> mapping to a swap slot in the huge swap cluster via a swap entry in
> PTE.
>
> If there is no PMD swap mapping and the corresponding THP is removed
> from the page cache (reclaimed), the huge swap cluster will be split
> and become a normal swap cluster.
>
> The count (cluster_count()) of the huge swap cluster is
> SWAPFILE_CLUSTER (= HPAGE_PMD_NR) + PMD swap mapping count.  Because
> all swap slots in the huge swap cluster are mapped by PTE or PMD, or
> has SWAP_HAS_CACHE bit set, the usage count of the swap cluster is
> HPAGE_PMD_NR.  And the PMD swap mapping count is recorded too to make
> it easy to determine whether there are remaining PMD swap mappings.
>
> The count in swap_map[offset] is the sum of PTE and PMD swap mapping
> count.  This means when we increase the PMD swap mapping count, we
> need to increase swap_map[offset] for all swap slots inside the swap
> cluster.  An alternative choice is to make swap_map[offset] to record
> PTE swap map count only, given we have recorded PMD swap mapping count
> in the count of the huge swap cluster.  But this need to increase
> swap_map[offset] when splitting the PMD swap mapping, that may fail
> because of memory allocation for swap count continuation.  That is
> hard to dealt with.  So we choose current solution.
>
> The PMD swap mapping to a huge swap cluster may be split when unmap a
> part of PMD mapping etc.  That is easy because only the count of the
> huge swap cluster need to be changed.  When the last PMD swap mapping
> is gone and SWAP_HAS_CACHE is unset, we will split the huge swap
> cluster (clear the huge flag).  This makes it easy to reason the
> cluster state.
>
> A huge swap cluster will be split when splitting the THP in swap
> cache, or failing to allocate THP during swapin, etc.  But when
> splitting the huge swap cluster, we will not try to split all PMD swap
> mappings, because we haven't enough information available for that
> sometimes.  Later, when the PMD swap mapping is duplicated or swapin,
> etc, the PMD swap mapping will be split and fallback to the PTE
> operation.
>
> When a THP is added into swap cache, the SWAP_HAS_CACHE flag will be
> set in the swap_map[offset] of all swap slots inside the huge swap
> cluster backing the THP.  This huge swap cluster will not be split
> unless the THP is split even if its PMD swap mapping count dropped to
> 0.  Later, when the THP is removed from swap cache, the SWAP_HAS_CACHE
> flag will be cleared in the swap_map[offset] of all swap slots inside
> the huge swap cluster.  And this huge swap cluster will be split if
> its PMD swap mapping count is 0.
>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Zi Yan <zi.yan@xxxxxxxxxxxxxx>
> Cc: Daniel Jordan <daniel.m.jordan@xxxxxxxxxx>
> ---
>  include/linux/huge_mm.h |   5 +
>  include/linux/swap.h    |   9 +-
>  mm/memory.c             |   2 +-
>  mm/rmap.c               |   2 +-
>  mm/swap_state.c         |   2 +-
>  mm/swapfile.c           | 287 +++++++++++++++++++++++++++++++++---------------
>  6 files changed, 214 insertions(+), 93 deletions(-)

I'm probably missing some background, but I find the patch hard to
read. Can you disseminate some of this patch changelog into kernel-doc
commentary so it's easier to follow which helpers do what relative to
THP swap.

>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index d3bbf6bea9e9..213d32e57c39 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -80,6 +80,11 @@ extern struct kobj_attribute shmem_enabled_attr;
>  #define HPAGE_PMD_ORDER (HPAGE_PMD_SHIFT-PAGE_SHIFT)
>  #define HPAGE_PMD_NR (1<<HPAGE_PMD_ORDER)
>
> +static inline bool thp_swap_supported(void)
> +{
> +       return IS_ENABLED(CONFIG_THP_SWAP);
> +}
> +
>  #ifdef CONFIG_TRANSPARENT_HUGEPAGE
>  #define HPAGE_PMD_SHIFT PMD_SHIFT
>  #define HPAGE_PMD_SIZE ((1UL) << HPAGE_PMD_SHIFT)
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index f73eafcaf4e9..57aa655ab27d 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -451,8 +451,8 @@ extern swp_entry_t get_swap_page_of_type(int);
>  extern int get_swap_pages(int n, bool cluster, swp_entry_t swp_entries[]);
>  extern int add_swap_count_continuation(swp_entry_t, gfp_t);
>  extern void swap_shmem_alloc(swp_entry_t);
> -extern int swap_duplicate(swp_entry_t);
> -extern int swapcache_prepare(swp_entry_t);
> +extern int swap_duplicate(swp_entry_t *entry, bool cluster);

This patch introduces a new flag to swap_duplicate(), but then all all
usages still pass 'false' so why does this patch change the argument.
Seems this change belongs to another patch?

> +extern int swapcache_prepare(swp_entry_t entry, bool cluster);

Rather than add a cluster flag to these helpers can the swp_entry_t
carry the cluster flag directly?