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    Subject[PATCH] splitlru: BDI_CAP_SWAP_BACKED
    The split-lru patches put file and swap-backed pages on different lrus.
    shmem/tmpfs pages are awkward because they are swap-backed file pages.
    Since it's difficult to change lru midstream, they are treated as swap-
    backed throughout, with SetPageSwapBacked on allocation in shmem_getpage.

    However, splice read (used by loop and sendfile) and readahead* allocate
    pages first, add_to_page_cache_lru, and then call into the filesystem
    through ->readpage. Under memory pressure, the shmem pages arrive at
    add_to_swap_cache and hit its BUG_ON(!PageSwapBacked(page)).

    I've not yet found a better way to handle this than a "capability"
    flag in shmem_backing_dev_info, tested by add_to_page_cache_lru.
    And solely because it would look suspicious without it, set that
    BDI_CAP_SWAP_BACKED in swap_backing_dev_info also.

    * readahead on shmem/tmpfs? I'd always thought ra_pages 0 prevented
    that; but in fact readahead(2), fadvise(POSIX_FADV_WILLNEED) and
    madvise(MADV_WILLNEED) all force_page_cache_readahead and get there.

    Signed-off-by: Hugh Dickins <hugh@veritas.com>
    ---
    Should follow mmotm's vmscan-split-lru-lists-into-anon-file-sets.patch

    include/linux/backing-dev.h | 13 +++++++++++++
    mm/filemap.c | 13 ++++++++++++-
    mm/shmem.c | 2 +-
    mm/swap_state.c | 2 +-
    4 files changed, 27 insertions(+), 3 deletions(-)

    --- mmotm/include/linux/backing-dev.h 2008-05-03 21:55:10.000000000 +0100
    +++ linux/include/linux/backing-dev.h 2008-06-27 17:02:45.000000000 +0100
    @@ -175,6 +175,8 @@ int bdi_set_max_ratio(struct backing_dev
    * BDI_CAP_READ_MAP: Can be mapped for reading
    * BDI_CAP_WRITE_MAP: Can be mapped for writing
    * BDI_CAP_EXEC_MAP: Can be mapped for execution
    + *
    + * BDI_CAP_SWAP_BACKED: Count shmem/tmpfs objects as swap-backed.
    */
    #define BDI_CAP_NO_ACCT_DIRTY 0x00000001
    #define BDI_CAP_NO_WRITEBACK 0x00000002
    @@ -184,6 +186,7 @@ int bdi_set_max_ratio(struct backing_dev
    #define BDI_CAP_WRITE_MAP 0x00000020
    #define BDI_CAP_EXEC_MAP 0x00000040
    #define BDI_CAP_NO_ACCT_WB 0x00000080
    +#define BDI_CAP_SWAP_BACKED 0x00000100

    #define BDI_CAP_VMFLAGS \
    (BDI_CAP_READ_MAP | BDI_CAP_WRITE_MAP | BDI_CAP_EXEC_MAP)
    @@ -248,6 +251,11 @@ static inline bool bdi_cap_account_write
    BDI_CAP_NO_WRITEBACK));
    }

    +static inline bool bdi_cap_swap_backed(struct backing_dev_info *bdi)
    +{
    + return bdi->capabilities & BDI_CAP_SWAP_BACKED;
    +}
    +
    static inline bool mapping_cap_writeback_dirty(struct address_space *mapping)
    {
    return bdi_cap_writeback_dirty(mapping->backing_dev_info);
    @@ -258,4 +266,9 @@ static inline bool mapping_cap_account_d
    return bdi_cap_account_dirty(mapping->backing_dev_info);
    }

    +static inline bool mapping_cap_swap_backed(struct address_space *mapping)
    +{
    + return bdi_cap_swap_backed(mapping->backing_dev_info);
    +}
    +
    #endif /* _LINUX_BACKING_DEV_H */
    --- mmotm/mm/filemap.c 2008-06-27 13:39:20.000000000 +0100
    +++ linux/mm/filemap.c 2008-06-27 18:16:07.000000000 +0100
    @@ -493,7 +493,18 @@ EXPORT_SYMBOL(add_to_page_cache);
    int add_to_page_cache_lru(struct page *page, struct address_space *mapping,
    pgoff_t offset, gfp_t gfp_mask)
    {
    - int ret = add_to_page_cache(page, mapping, offset, gfp_mask);
    + int ret;
    +
    + /*
    + * Splice_read and readahead add shmem/tmpfs pages into the page cache
    + * before shmem_readpage has a chance to mark them as SwapBacked: they
    + * need to go on the active_anon lru below, and mem_cgroup_cache_charge
    + * (called in add_to_page_cache) needs to know where they're going too.
    + */
    + if (mapping_cap_swap_backed(mapping))
    + SetPageSwapBacked(page);
    +
    + ret = add_to_page_cache(page, mapping, offset, gfp_mask);
    if (ret == 0) {
    if (page_is_file_cache(page))
    lru_cache_add_file(page);
    --- mmotm/mm/shmem.c 2008-06-27 13:39:20.000000000 +0100
    +++ linux/mm/shmem.c 2008-06-27 17:25:41.000000000 +0100
    @@ -201,7 +201,7 @@ static struct vm_operations_struct shmem

    static struct backing_dev_info shmem_backing_dev_info __read_mostly = {
    .ra_pages = 0, /* No readahead */
    - .capabilities = BDI_CAP_NO_ACCT_AND_WRITEBACK,
    + .capabilities = BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
    .unplug_io_fn = default_unplug_io_fn,
    };

    --- mmotm/mm/swap_state.c 2008-06-27 13:39:20.000000000 +0100
    +++ linux/mm/swap_state.c 2008-06-27 17:26:49.000000000 +0100
    @@ -33,7 +33,7 @@ static const struct address_space_operat
    };

    static struct backing_dev_info swap_backing_dev_info = {
    - .capabilities = BDI_CAP_NO_ACCT_AND_WRITEBACK,
    + .capabilities = BDI_CAP_NO_ACCT_AND_WRITEBACK | BDI_CAP_SWAP_BACKED,
    .unplug_io_fn = swap_unplug_io_fn,
    };


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