/*
 *  linux/mm/vmscan.c
 *
 *  Copyright (C) 1991, 1992, 1993, 1994  Linus Torvalds
 *
 *  Swap reorganised 29.12.95, Stephen Tweedie.
 *  kswapd added: 7.1.96  sct
 *  Removed kswapd_ctl limits, and swap out as many pages as needed
 *  to bring the system back to freepages.high: 2.4.97, Rik van Riel.
 *  Version: $Id: vmscan.c,v 1.5 1998/02/23 22:14:28 sct Exp $
 */

#include <linux/slab.h>
#include <linux/kernel_stat.h>
#include <linux/swap.h>
#include <linux/swapctl.h>
#include <linux/smp_lock.h>
#include <linux/pagemap.h>
#include <linux/init.h>

#include <asm/pgtable.h>

/*
 * The swap-out functions return 1 if they successfully
 * threw something out, and we got a free page. It returns
 * zero if it couldn't do anything, and any other value
 * indicates it decreased rss, but the page was shared.
 *
 * NOTE! If it sleeps, it *must* return 1 to make sure we
 * don't continue with the swap-out. Otherwise we may be
 * using a process that no longer actually exists (it might
 * have died while we slept).
 */
static int try_to_swap_out(struct task_struct * tsk, struct vm_area_struct* vma,
        unsigned long address, pte_t * page_table, int gfp_mask)
{
        pte_t pte;
        unsigned long entry;
        unsigned long page;
        struct page * page_map;

        pte = *page_table;
        if (!pte_present(pte))
                return 0;
        page = pte_page(pte);
        if (MAP_NR(page) >= max_mapnr)
                return 0;
        page_map = mem_map + MAP_NR(page);

        if (pte_young(pte)) {
                /*
                 * Transfer the "accessed" bit from the page
                 * tables to the global page map.
                 */
                set_pte(page_table, pte_mkold(pte));
                flush_tlb_page(vma, address);
                set_bit(PG_referenced, &page_map->flags);
                return 0;
        }

        if (PageReserved(page_map)
            || PageLocked(page_map)
            || ((gfp_mask & __GFP_DMA) && !PageDMA(page_map)))
                return 0;

        /*
         * Is the page already in the swap cache? If so, then
         * we can just drop our reference to it without doing
         * any IO - it's already up-to-date on disk.
         *
         * Return 0, as we didn't actually free any real
         * memory, and we should just continue our scan.
         */
        if (PageSwapCache(page_map)) {
                entry = page_map->offset;
                swap_duplicate(entry);
                set_pte(page_table, __pte(entry));
drop_pte:
                vma->vm_mm->rss--;
                flush_tlb_page(vma, address);
                __free_page(page_map);
                return 0;
        }

        /*
         * Is it a clean page? Then it must be recoverable
         * by just paging it in again, and we can just drop
         * it..
         *
         * However, this won't actually free any real
         * memory, as the page will just be in the page cache
         * somewhere, and as such we should just continue
         * our scan.
         *
         * Basically, this just makes it possible for us to do
         * some real work in the future in "shrink_mmap()".
         */
        if (!pte_dirty(pte)) {
                if (page_map->inode && pgcache_under_min())
                        /* unmapping this page would be useless */
                        return 0;
                flush_cache_page(vma, address);
                pte_clear(page_table);
                goto drop_pte;
        }

        /*
         * Don't go down into the swap-out stuff if
         * we cannot do I/O! Avoid recursing on FS
         * locks etc.
         */
        if (!(gfp_mask & __GFP_IO) || current->fs_locks)
                return 0;

        /*
         * Ok, it's really dirty. That means that
         * we should either create a new swap cache
         * entry for it, or we should write it back
         * to its own backing store.
         *
         * Note that in neither case do we actually
         * know that we make a page available, but
         * as we potentially sleep we can no longer
         * continue scanning, so we migth as well
         * assume we free'd something.
         *
         * NOTE NOTE NOTE! This should just set a
         * dirty bit in page_map, and just drop the
         * pte. All the hard work would be done by
         * shrink_mmap().
         *
         * That would get rid of a lot of problems.
         */
        flush_cache_page(vma, address);
        if (vma->vm_ops && vma->vm_ops->swapout) {
                pid_t pid = tsk->pid;
                pte_clear(page_table);
                flush_tlb_page(vma, address);
                vma->vm_mm->rss--;
                
                if (vma->vm_ops->swapout(vma, page_map))
                        kill_proc(pid, SIGBUS, 1);
                __free_page(page_map);
                return 1;
        }

        /*
         * This is a dirty, swappable page.  First of all,
         * get a suitable swap entry for it, and make sure
         * we have the swap cache set up to associate the
         * page with that swap entry.
         */
        entry = get_swap_page();
        if (!entry)
                return 0; /* No swap space left */
                
        vma->vm_mm->rss--;
        tsk->nswap++;
        set_pte(page_table, __pte(entry));
        flush_tlb_page(vma, address);
        swap_duplicate(entry);  /* One for the process, one for the swap cache */
        add_to_swap_cache(page_map, entry);
        /* We checked we were unlocked way up above, and we
           have been careful not to stall until here */
        set_bit(PG_locked, &page_map->flags);

        /* OK, do a physical asynchronous write to swap.  */
        rw_swap_page(WRITE, entry, (char *) page, 0);

        __free_page(page_map);
        return 1;
}

/*
 * A new implementation of swap_out().  We do not swap complete processes,
 * but only a small number of blocks, before we continue with the next
 * process.  The number of blocks actually swapped is determined on the
 * number of page faults, that this process actually had in the last time,
 * so we won't swap heavily used processes all the time ...
 *
 * Note: the priority argument is a hint on much CPU to waste with the
 *       swap block search, not a hint, of how much blocks to swap with
 *       each process.
 *
 * (C) 1993 Kai Petzke, wpp@marie.physik.tu-berlin.de
 */

static inline int swap_out_pmd(struct task_struct * tsk, struct vm_area_struct * vma,
        pmd_t *dir, unsigned long address, unsigned long end, int gfp_mask)
{
        pte_t * pte;
        unsigned long pmd_end;

        if (pmd_none(*dir))
                return 0;
        if (pmd_bad(*dir)) {
                printk("swap_out_pmd: bad pmd (%08lx)\n", pmd_val(*dir));
                pmd_clear(dir);
                return 0;
        }
        
        pte = pte_offset(dir, address);
        
        pmd_end = (address + PMD_SIZE) & PMD_MASK;
        if (end > pmd_end)
                end = pmd_end;

        do {
                int result;
                tsk->mm->swap_address = address + PAGE_SIZE;
                result = try_to_swap_out(tsk, vma, address, pte, gfp_mask);
                if (result)
                        return result;
                if (current->need_resched)
                        return 2;
                address += PAGE_SIZE;
                pte++;
        } while (address < end);
        return 0;
}

static inline int swap_out_pgd(struct task_struct * tsk, struct vm_area_struct * vma,
        pgd_t *dir, unsigned long address, unsigned long end, int gfp_mask)
{
        pmd_t * pmd;
        unsigned long pgd_end;

        if (pgd_none(*dir))
                return 0;
        if (pgd_bad(*dir)) {
                printk("swap_out_pgd: bad pgd (%08lx)\n", pgd_val(*dir));
                pgd_clear(dir);
                return 0;
        }

        pmd = pmd_offset(dir, address);

        pgd_end = (address + PGDIR_SIZE) & PGDIR_MASK;  
        if (end > pgd_end)
                end = pgd_end;
        
        do {
                int result = swap_out_pmd(tsk, vma, pmd, address, end, gfp_mask);
                if (result)
                        return result;
                address = (address + PMD_SIZE) & PMD_MASK;
                pmd++;
        } while (address < end);
        return 0;
}

static int swap_out_vma(struct task_struct * tsk, struct vm_area_struct * vma,
        unsigned long address, int gfp_mask)
{
        pgd_t *pgdir;
        unsigned long end;

        /* Don't swap out areas which are locked down */
        if (vma->vm_flags & VM_LOCKED)
                return 0;

        pgdir = pgd_offset(tsk->mm, address);

        end = vma->vm_end;
        while (address < end) {
                int result = swap_out_pgd(tsk, vma, pgdir, address, end, gfp_mask);
                if (result)
                        return result;
                address = (address + PGDIR_SIZE) & PGDIR_MASK;
                pgdir++;
        }
        return 0;
}

static int swap_out_process(struct task_struct * p, int gfp_mask)
{
        unsigned long address;
        struct vm_area_struct* vma;

        /*
         * Go through process' page directory.
         */
        address = p->mm->swap_address;

        /*
         * Find the proper vm-area
         */
        vma = find_vma(p->mm, address);
        if (vma) {
                if (address < vma->vm_start)
                        address = vma->vm_start;

                for (;;) {
                        int result = swap_out_vma(p, vma, address, gfp_mask);
                        if (result)
                                return result;
                        vma = vma->vm_next;
                        if (!vma)
                                break;
                        address = vma->vm_start;
                }
        }

        /* We didn't find anything for the process */
        p->mm->swap_cnt = 0;
        p->mm->swap_address = 0;
        return 0;
}

/*
 * Select the task with maximal swap_cnt and try to swap out a page.
 * N.B. This function returns only 0 or 1.  Return values != 1 from
 * the lower level routines result in continued processing.
 */
static int swap_out(unsigned int priority, int gfp_mask)
{
        struct task_struct * p, * pbest;
        int assign = 0, counter;
        unsigned long max_cnt;

        /* 
         * We make one or two passes through the task list, indexed by 
         * assign = {0, 1}:
         *   Pass 1: select the swappable task with maximal RSS that has
         *         not yet been swapped out. 
         *   Pass 2: re-assign rss swap_cnt values, then select as above.
         *
         * With this approach, there's no need to remember the last task
         * swapped out.  If the swap-out fails, we clear swap_cnt so the 
         * task won't be selected again until all others have been tried.
         *
         * Think of swap_cnt as a "shadow rss" - it tells us which process
         * we want to page out (always try largest first).
         */
        counter = nr_tasks / priority;
        if (counter < 1)
                counter = 1;

        for (; counter >= 0; counter--) {
                max_cnt = 0;
                pbest = NULL;
        select:
                read_lock(&tasklist_lock);
                p = init_task.next_task;
                for (; p != &init_task; p = p->next_task) {
                        if (!p->swappable)
                                continue;
                        if (p->mm->rss <= 0)
                                continue;
                        /* Refresh swap_cnt? */
                        if (assign == 1)
                                p->mm->swap_cnt = p->mm->rss;
                        if (p->mm->swap_cnt > max_cnt) {
                                max_cnt = p->mm->swap_cnt;
                                pbest = p;
                        }
                }
                read_unlock(&tasklist_lock);
                if (assign == 1)
                        assign = 2;
                if (!pbest) {
                        if (!assign) {
                                assign = 1;
                                goto select;
                        }
                        goto out;
                }

                switch (swap_out_process(pbest, gfp_mask)) {
                case 1:
                        return 1;
                case 2:
                        current->state = TASK_RUNNING;
                        schedule();
                }
        }
out:
        return 0;
}

/*
 * We need to make the locks finer granularity, but right
 * now we need this so that we can do page allocations
 * without holding the kernel lock etc.
 *
 * We want to try to free "count" pages, and we need to 
 * cluster them so that we get good swap-out behaviour. See
 * the "free_memory()" macro for details.
 */
int try_to_free_pages(unsigned int gfp_mask)
{
        int priority;
        int count = SWAP_CLUSTER_MAX;

        lock_kernel();

        /* Always trim SLAB caches when memory gets low. */
        kmem_cache_reap(gfp_mask);

        priority = 5;
        do {
                /* Always pick on the dcache even if we didnt need to,
                   without this some workloads cause excessive dcache
                   growth */

                shrink_dcache_memory(priority, gfp_mask);
        
                while (shrink_mmap(priority, gfp_mask)) {
                        if (!--count)
                                goto done;
                }

                /* Try to get rid of some shared memory pages.. */
                if (gfp_mask & __GFP_IO && !current->fs_locks) {
                        while (shm_swap(priority, gfp_mask)) {
                                if (!--count)
                                        goto done;
                        }
                }

                /* Then, try to page stuff out.. */
                while (swap_out(priority, gfp_mask)) {
                        if (!--count)
                                goto done;
                }

        } while (--priority > 0);
done:
        unlock_kernel();

        /* Return success if we freed a page. */
        return priority > 0;
}

/*
 * Before we start the kernel thread, print out the 
 * kswapd initialization message (otherwise the init message 
 * may be printed in the middle of another driver's init 
 * message).  It looks very bad when that happens.
 */
void __init kswapd_setup(void)
{
       int i;
       char *revision="$Revision: 1.5 $", *s, *e;

       swap_setup();
       
       if ((s = strchr(revision, ':')) &&
           (e = strchr(s, '$')))
               s++, i = e - s;
       else
               s = revision, i = -1;
       printk ("Starting kswapd v%.*s\n", i, s);
}

struct wait_queue * kswapd_wait;

/*
 * The background pageout daemon, started as a kernel thread
 * from the init process. 
 *
 * This basically executes once a second, trickling out pages
 * so that we have _some_ free memory available even if there
 * is no other activity that frees anything up. This is needed
 * for things like routing etc, where we otherwise might have
 * all activity going on in asynchronous contexts that cannot
 * page things out.
 *
 * If there are applications that are active memory-allocators
 * (most normal use), this basically shouldn't matter.
 */
int kswapd(void *unused)
{
        struct task_struct *tsk = current;

        tsk->session = 1;
        tsk->pgrp = 1;
        strcpy(tsk->comm, "kswapd");
        sigfillset(&tsk->blocked);
        
        /*
         * Tell the memory management that we're a "memory allocator",
         * and that if we need more memory we should get access to it
         * regardless (see "__get_free_pages()"). "kswapd" should
         * never get caught in the normal page freeing logic.
         *
         * (Kswapd normally doesn't need memory anyway, but sometimes
         * you need a small amount of memory in order to be able to
         * page out something else, and this flag essentially protects
         * us from recursively trying to free more memory as we're
         * trying to free the first piece of memory in the first place).
         */
        tsk->flags |= PF_MEMALLOC;

        while (1) {
                /*
                 * Wake up once a second to see if we need to make
                 * more memory available.
                 *
                 * If we actually get into a low-memory situation,
                 * the processes needing more memory will wake us
                 * up on a more timely basis.
                 */
                interruptible_sleep_on(&kswapd_wait);

                while (nr_free_pages < freepages.high)
                {
                        if (try_to_free_pages(GFP_KSWAPD))
                        {
                                if (tsk->need_resched)
                                        schedule();
                                continue;
                        }
                        tsk->state = TASK_INTERRUPTIBLE;
                        schedule_timeout(10*HZ);
                }
        }
}