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    Subject[PATCH -mm -V3 07/21] mm, THP, swap: Support PMD swap mapping in split_swap_cluster()
    Date
    From: Huang Ying <ying.huang@intel.com>

    When splitting a THP in swap cache or failing to allocate a THP when
    swapin a huge swap cluster, the huge swap cluster will be split. In
    addition to clear the huge flag of the swap cluster, the PMD swap
    mapping count recorded in cluster_count() will be set to 0. But we
    will not touch PMD swap mappings themselves, because it is hard to
    find them all sometimes. When the PMD swap mappings are operated
    later, it will be found that the huge swap cluster has been split and
    the PMD swap mappings will be split at that time.

    Unless splitting a THP in swap cache (specified via "force"
    parameter), split_swap_cluster() will return -EEXIST if there is
    SWAP_HAS_CACHE flag in swap_map[offset]. Because this indicates there
    is a THP corresponds to this huge swap cluster, and it isn't desired
    to split the THP.

    When splitting a THP in swap cache, the position to call
    split_swap_cluster() is changed to before unlocking sub-pages. So
    that all sub-pages will be kept locked from the THP has been split to
    the huge swap cluster is split. This makes the code much easier to be
    reasoned.

    Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
    Cc: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Shaohua Li <shli@kernel.org>
    Cc: Hugh Dickins <hughd@google.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Rik van Riel <riel@redhat.com>
    Cc: Dave Hansen <dave.hansen@linux.intel.com>
    Cc: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com>
    Cc: Zi Yan <zi.yan@cs.rutgers.edu>
    ---
    include/linux/swap.h | 4 ++--
    mm/huge_memory.c | 18 ++++++++++++------
    mm/swapfile.c | 45 ++++++++++++++++++++++++++++++---------------
    3 files changed, 44 insertions(+), 23 deletions(-)

    diff --git a/include/linux/swap.h b/include/linux/swap.h
    index bb9de2cb952a..878f132dabc0 100644
    --- a/include/linux/swap.h
    +++ b/include/linux/swap.h
    @@ -617,10 +617,10 @@ static inline swp_entry_t get_swap_page(struct page *page)
    #endif /* CONFIG_SWAP */

    #ifdef CONFIG_THP_SWAP
    -extern int split_swap_cluster(swp_entry_t entry);
    +extern int split_swap_cluster(swp_entry_t entry, bool force);
    extern int split_swap_cluster_map(swp_entry_t entry);
    #else
    -static inline int split_swap_cluster(swp_entry_t entry)
    +static inline int split_swap_cluster(swp_entry_t entry, bool force)
    {
    return 0;
    }
    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index 84d5d8ff869e..e363e13f6751 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -2502,6 +2502,17 @@ static void __split_huge_page(struct page *page, struct list_head *list,

    unfreeze_page(head);

    + /*
    + * Split swap cluster before unlocking sub-pages. So all
    + * sub-pages will be kept locked from THP has been split to
    + * swap cluster is split.
    + */
    + if (PageSwapCache(head)) {
    + swp_entry_t entry = { .val = page_private(head) };
    +
    + split_swap_cluster(entry, true);
    + }
    +
    for (i = 0; i < HPAGE_PMD_NR; i++) {
    struct page *subpage = head + i;
    if (subpage == page)
    @@ -2728,12 +2739,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
    __dec_node_page_state(page, NR_SHMEM_THPS);
    spin_unlock(&pgdata->split_queue_lock);
    __split_huge_page(page, list, flags);
    - if (PageSwapCache(head)) {
    - swp_entry_t entry = { .val = page_private(head) };
    -
    - ret = split_swap_cluster(entry);
    - } else
    - ret = 0;
    + ret = 0;
    } else {
    if (IS_ENABLED(CONFIG_DEBUG_VM) && mapcount) {
    pr_alert("total_mapcount: %u, page_count(): %u\n",
    diff --git a/mm/swapfile.c b/mm/swapfile.c
    index 05f53c4c0cfe..1e723d3a9a6f 100644
    --- a/mm/swapfile.c
    +++ b/mm/swapfile.c
    @@ -1414,21 +1414,6 @@ static void swapcache_free_cluster(swp_entry_t entry)
    }
    }
    }
    -
    -int split_swap_cluster(swp_entry_t entry)
    -{
    - struct swap_info_struct *si;
    - struct swap_cluster_info *ci;
    - unsigned long offset = swp_offset(entry);
    -
    - si = _swap_info_get(entry);
    - if (!si)
    - return -EBUSY;
    - ci = lock_cluster(si, offset);
    - cluster_clear_huge(ci);
    - unlock_cluster(ci);
    - return 0;
    -}
    #else
    static inline void swapcache_free_cluster(swp_entry_t entry)
    {
    @@ -4072,6 +4057,36 @@ int split_swap_cluster_map(swp_entry_t entry)
    unlock_cluster(ci);
    return 0;
    }
    +
    +int split_swap_cluster(swp_entry_t entry, bool force)
    +{
    + struct swap_info_struct *si;
    + struct swap_cluster_info *ci;
    + unsigned long offset = swp_offset(entry);
    + int ret = 0;
    +
    + si = get_swap_device(entry);
    + if (!si)
    + return -EINVAL;
    + ci = lock_cluster(si, offset);
    + /* The swap cluster has been split by someone else */
    + if (!cluster_is_huge(ci))
    + goto out;
    + VM_BUG_ON(!is_cluster_offset(offset));
    + VM_BUG_ON(cluster_count(ci) < SWAPFILE_CLUSTER);
    + /* If not forced, don't split swap cluster has swap cache */
    + if (!force && si->swap_map[offset] & SWAP_HAS_CACHE) {
    + ret = -EEXIST;
    + goto out;
    + }
    + cluster_set_count(ci, SWAPFILE_CLUSTER);
    + cluster_clear_huge(ci);
    +
    +out:
    + unlock_cluster(ci);
    + put_swap_device(si);
    + return ret;
    +}
    #endif

    static int __init swapfile_init(void)
    --
    2.16.1
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