Question

Since the pages are stored in address_space within each inode, how does the background page cache flush thread know all dirty pages?

Answer 1

They’re all in one place:

struct bdi_writeback {
        struct backing_dev_info *bdi;   /* our parent bdi */
        unsigned int nr;

        unsigned long last_old_flush;   /* last old data flush */
        unsigned long last_active;      /* last time bdi thread was active */

        struct task_struct *task;       /* writeback thread */
        struct timer_list wakeup_timer; /* used for delayed bdi thread wakeup */
        struct list_head b_dirty;       /* dirty inodes */
        struct list_head b_io;          /* parked for writeback */
        struct list_head b_more_io;     /* parked for more writeback */
        spinlock_t list_lock;           /* protects the b_* lists */
};

b_dirty is the list you’re looking for.

For some information on how the flushing happens, take a look in here. The code is kind of complex though. To summarize (alot), consider that in the default configuration data written to disk will sit in memory until either a) they’re more than 30 seconds old, or b) the dirty pages have consumed more than 10% of the active, working memory.