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    SubjectRe: [PATCH 2.5.66-mm3] New page_convert_anon

    --On Thursday, April 03, 2003 13:55:22 -0800 Andrew Morton <akpm@digeo.com>
    wrote:

    > OK. I'd still be more comfortable if that page was locked though. I
    > expect page->mapping is safe because truncate takes down pagetable before
    > pagecache, but it seems way cleaner.
    >
    > Is there any reason you didn't do that?

    Because my mind slipped a gear and I did it wrong. I tried to put it
    inside page_convert_anon and messed it up. How does this patch look?

    Dave

    ======================================================================
    Dave McCracken IBM Linux Base Kernel Team 1-512-838-3059
    dmccr@us.ibm.com T/L 678-3059
    --- 2.5.66-mm3/mm/rmap.c 2003-04-03 10:37:42.000000000 -0600
    +++ 2.5.66-mm3-objfix/mm/rmap.c 2003-04-03 16:11:03.000000000 -0600
    @@ -460,23 +460,6 @@ void page_remove_rmap(struct page * page
    }
    }
    }
    -#ifdef DEBUG_RMAP
    - /* Not found. This should NEVER happen! */
    - printk(KERN_ERR "page_remove_rmap: pte_chain %p not present.\n", ptep);
    - printk(KERN_ERR "page_remove_rmap: only found: ");
    - if (PageDirect(page)) {
    - printk("%llx", (u64)page->pte.direct);
    - } else {
    - for (pc = page->pte.chain; pc; pc = pc->next) {
    - int i;
    - for (i = 0; i < NRPTE; i++)
    - printk(" %d:%llx", i, (u64)pc->ptes[i]);
    - }
    - }
    - printk("\n");
    - printk(KERN_ERR "page_remove_rmap: driver cleared PG_reserved ?\n");
    -#endif
    -
    out:
    pte_chain_unlock(page);
    if (!page_mapped(page))
    @@ -781,152 +764,94 @@ out:
    * Find all the mappings for an object-based page and convert them
    * to 'anonymous', ie create a pte_chain and store all the pte pointers there.
    *
    - * This function takes the address_space->i_shared_sem and the pte_chain_lock
    - * for the page. It jumps through some hoops to preallocate the correct number
    - * of pte_chain structures to ensure that it can complete without releasing
    - * the lock.
    + * This function does a lock_page, takes the
    + * address_space->i_shared_sem, sets the PageAnon flag, then sets the
    + * mm->page_table_lock for each vma and calls page_add_rmap. This
    + * means there is a period when PageAnon is set, but still has some
    + * mappings with no pte_chain entry. This is in fact safe, since
    + * page_remove_rmap will simply not find it. try_to_unmap is safe because
    + * of the lock_page.
    */
    int page_convert_anon(struct page *page)
    {
    struct address_space *mapping = page->mapping;
    struct vm_area_struct *vma;
    - struct pte_chain *pte_chain = NULL, *ptec;
    + struct pte_chain *pte_chain = NULL;
    pte_t *pte;
    - pte_addr_t pte_paddr = 0;
    - int mapcount;
    - int ptecount;
    - int index = 1;
    int err = 0;

    + lock_page(page);
    down(&mapping->i_shared_sem);
    - pte_chain_lock(page);

    + /* Take this only during setup */
    + pte_chain_lock(page);
    /*
    * Has someone else done it for us before we got the lock?
    * If so, pte.direct or pte.chain has replaced pte.mapcount.
    */
    - if (PageAnon(page))
    + if (PageAnon(page)) {
    + pte_chain_unlock(page);
    goto out_unlock;
    + }

    - /*
    - * Preallocate the pte_chains outside the lock.
    - * If mapcount grows while we're allocating here, retry.
    - * If mapcount shrinks, we free the excess before returning.
    - */
    - mapcount = page->pte.mapcount;
    - while (index < mapcount) {
    + SetPageAnon(page);
    + if (page->pte.mapcount == 0) {
    pte_chain_unlock(page);
    - up(&mapping->i_shared_sem);
    - for (; index < mapcount; index += NRPTE) {
    - if (index == 1)
    - index = 0;
    - ptec = pte_chain_alloc(GFP_KERNEL);
    - if (!ptec) {
    - err = -ENOMEM;
    - goto out_free;
    - }
    - ptec->next = pte_chain;
    - pte_chain = ptec;
    - }
    - down(&mapping->i_shared_sem);
    - pte_chain_lock(page);
    - /*
    - * Has someone else done it while we were allocating?
    - */
    - if (PageAnon(page))
    - goto out_unlock;
    - mapcount = page->pte.mapcount;
    + goto out_unlock;
    }
    - if (!mapcount)
    - goto set_anon;
    + /* This is gonna get incremented by page_add_rmap */
    + dec_page_state(nr_mapped);
    + page->pte.mapcount = 0;

    -again:
    /*
    - * We don't try for page_table_lock (what would we do when a
    - * trylock fails?), therefore there's a small chance that we
    - * catch a vma just as it's being unmapped and its page tables
    - * freed. Our pte_chain_lock prevents that on vmas that really
    - * contain our page, but not on the others we look at. So we
    - * might locate junk that looks just like our page's pfn. It's
    - * a transient and very unlikely occurrence (much less likely
    - * than a trylock failing), so check how many maps we find,
    - * and if too many, start all over again.
    + * Now that the page is marked as anon, unlock it. page_add_rmap will lock
    + * it as necessary.
    */
    - ptecount = 0;
    - ptec = pte_chain;
    -
    - /*
    - * Arrange for the first pte_chain to be partially filled at
    - * the top, and the last (and any intermediates) to be full.
    - */
    - index = mapcount % NRPTE;
    - if (index)
    - index = NRPTE - index;
    + pte_chain_unlock(page);

    list_for_each_entry(vma, &mapping->i_mmap, shared) {
    if (vma->vm_pgoff > (page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT)))
    break;
    + if (!pte_chain) {
    + pte_chain = pte_chain_alloc(GFP_KERNEL);
    + if (!pte_chain) {
    + err = 1;
    + goto out_unlock;
    + }
    + }
    + spin_lock(&vma->vm_mm->page_table_lock);
    pte = find_pte(vma, page, NULL);
    if (pte) {
    - ptecount++;
    - if (unlikely(ptecount > mapcount))
    - goto again;
    - pte_paddr = ptep_to_paddr(pte);
    - pte_unmap(pte);
    - if (ptec) {
    - ptec->ptes[index] = pte_paddr;
    - index++;
    - if (index == NRPTE) {
    - ptec = ptec->next;
    - index = 0;
    - }
    - }
    + /* Make sure this isn't a duplicate */
    + page_remove_rmap(page, pte);
    + pte_chain = page_add_rmap(page, pte, pte_chain);
    }
    + spin_unlock(&vma->vm_mm->page_table_lock);
    }
    list_for_each_entry(vma, &mapping->i_mmap_shared, shared) {
    if (vma->vm_pgoff > (page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT)))
    break;
    + if (!pte_chain) {
    + pte_chain = pte_chain_alloc(GFP_KERNEL);
    + if (!pte_chain) {
    + err = 1;
    + goto out_unlock;
    + }
    + }
    + spin_lock(&vma->vm_mm->page_table_lock);
    pte = find_pte(vma, page, NULL);
    if (pte) {
    - ptecount++;
    - if (unlikely(ptecount > mapcount))
    - goto again;
    - pte_paddr = ptep_to_paddr(pte);
    - pte_unmap(pte);
    - if (ptec) {
    - ptec->ptes[index] = pte_paddr;
    - index++;
    - if (index == NRPTE) {
    - ptec = ptec->next;
    - index = 0;
    - }
    - }
    + /* Make sure this isn't a duplicate */
    + page_remove_rmap(page, pte);
    + pte_chain = page_add_rmap(page, pte, pte_chain);
    }
    + spin_unlock(&vma->vm_mm->page_table_lock);
    }

    - BUG_ON(ptecount != mapcount);
    - if (mapcount == 1) {
    - SetPageDirect(page);
    - page->pte.direct = pte_paddr;
    - /* If pte_chain is set, it's all excess to be freed */
    - } else {
    - page->pte.chain = pte_chain;
    - /* Point pte_chain to any excess to be freed */
    - pte_chain = ptec;
    - BUG_ON(index);
    - }
    -
    -set_anon:
    - SetPageAnon(page);
    out_unlock:
    - pte_chain_unlock(page);
    + pte_chain_free(pte_chain);
    up(&mapping->i_shared_sem);
    -out_free:
    - while (pte_chain) {
    - ptec = pte_chain->next;
    - pte_chain_free(pte_chain);
    - pte_chain = ptec;
    - }
    + unlock_page(page);
    return err;
    }
    \
     
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