Re: [PATCH 20/23] thp: handle file pages in split_huge_page()

[Ning Qu <quning@google.com>]

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I am probably running into a deadlock case for this patch.

When splitting the file huge page, we hold the i_mmap_mutex.

However, when coming from the call path in vma_adjust as following, we will
grab the i_mmap_mutex already before doing vma_adjust_trans_huge, which
will eventually calls the split_huge_page then split_file_huge_page ....

https://git.kernel.org/cgit/linux/kernel/git/kas/linux.git/tree/mm/mmap.c?h=thp/pagecache#n753




Best wishes,
-- 
Ning Qu (曲宁) | Software Engineer | quning@google.com | +1-408-418-6066


On Sat, Aug 3, 2013 at 7:17 PM, Kirill A. Shutemov <
kirill.shutemov@linux.intel.com> wrote:

> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
>
> The base scheme is the same as for anonymous pages, but we walk by
> mapping->i_mmap rather then anon_vma->rb_root.
>
> When we add a huge page to page cache we take only reference to head
> page, but on split we need to take addition reference to all tail pages
> since they are still in page cache after splitting.
>
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  mm/huge_memory.c | 89
> +++++++++++++++++++++++++++++++++++++++++++++++---------
>  1 file changed, 76 insertions(+), 13 deletions(-)
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 523946c..d7c6830 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1580,6 +1580,7 @@ static void __split_huge_page_refcount(struct page
> *page,
>         struct zone *zone = page_zone(page);
>         struct lruvec *lruvec;
>         int tail_count = 0;
> +       int initial_tail_refcount;
>
>         /* prevent PageLRU to go away from under us, and freeze lru stats
> */
>         spin_lock_irq(&zone->lru_lock);
> @@ -1589,6 +1590,13 @@ static void __split_huge_page_refcount(struct page
> *page,
>         /* complete memcg works before add pages to LRU */
>         mem_cgroup_split_huge_fixup(page);
>
> +       /*
> +        * When we add a huge page to page cache we take only reference to
> head
> +        * page, but on split we need to take addition reference to all
> tail
> +        * pages since they are still in page cache after splitting.
> +        */
> +       initial_tail_refcount = PageAnon(page) ? 0 : 1;
> +
>         for (i = HPAGE_PMD_NR - 1; i >= 1; i--) {
>                 struct page *page_tail = page + i;
>
> @@ -1611,8 +1619,9 @@ static void __split_huge_page_refcount(struct page
> *page,
>                  * atomic_set() here would be safe on all archs (and
>                  * not only on x86), it's safer to use atomic_add().
>                  */
> -               atomic_add(page_mapcount(page) + page_mapcount(page_tail)
> + 1,
> -                          &page_tail->_count);
> +               atomic_add(initial_tail_refcount + page_mapcount(page) +
> +                               page_mapcount(page_tail) + 1,
> +                               &page_tail->_count);
>
>                 /* after clearing PageTail the gup refcount can be
> released */
>                 smp_mb();
> @@ -1651,23 +1660,23 @@ static void __split_huge_page_refcount(struct page
> *page,
>                 */
>                 page_tail->_mapcount = page->_mapcount;
>
> -               BUG_ON(page_tail->mapping);
>                 page_tail->mapping = page->mapping;
>
>                 page_tail->index = page->index + i;
>                 page_nid_xchg_last(page_tail, page_nid_last(page));
>
> -               BUG_ON(!PageAnon(page_tail));
>                 BUG_ON(!PageUptodate(page_tail));
>                 BUG_ON(!PageDirty(page_tail));
> -               BUG_ON(!PageSwapBacked(page_tail));
>
>                 lru_add_page_tail(page, page_tail, lruvec, list);
>         }
>         atomic_sub(tail_count, &page->_count);
>         BUG_ON(atomic_read(&page->_count) <= 0);
>
> -       __mod_zone_page_state(zone, NR_ANON_TRANSPARENT_HUGEPAGES, -1);
> +       if (PageAnon(page))
> +               __mod_zone_page_state(zone, NR_ANON_TRANSPARENT_HUGEPAGES,
> -1);
> +       else
> +               __mod_zone_page_state(zone, NR_FILE_TRANSPARENT_HUGEPAGES,
> -1);
>
>         ClearPageCompound(page);
>         compound_unlock(page);
> @@ -1767,7 +1776,7 @@ static int __split_huge_page_map(struct page *page,
>  }
>
>  /* must be called with anon_vma->root->rwsem held */
> -static void __split_huge_page(struct page *page,
> +static void __split_anon_huge_page(struct page *page,
>                               struct anon_vma *anon_vma,
>                               struct list_head *list)
>  {
> @@ -1791,7 +1800,7 @@ static void __split_huge_page(struct page *page,
>          * and establishes a child pmd before
>          * __split_huge_page_splitting() freezes the parent pmd (so if
>          * we fail to prevent copy_huge_pmd() from running until the
> -        * whole __split_huge_page() is complete), we will still see
> +        * whole __split_anon_huge_page() is complete), we will still see
>          * the newly established pmd of the child later during the
>          * walk, to be able to set it as pmd_trans_splitting too.
>          */
> @@ -1822,14 +1831,11 @@ static void __split_huge_page(struct page *page,
>   * from the hugepage.
>   * Return 0 if the hugepage is split successfully otherwise return 1.
>   */
> -int split_huge_page_to_list(struct page *page, struct list_head *list)
> +static int split_anon_huge_page(struct page *page, struct list_head *list)
>  {
>         struct anon_vma *anon_vma;
>         int ret = 1;
>
> -       BUG_ON(is_huge_zero_page(page));
> -       BUG_ON(!PageAnon(page));
> -
>         /*
>          * The caller does not necessarily hold an mmap_sem that would
> prevent
>          * the anon_vma disappearing so we first we take a reference to it
> @@ -1847,7 +1853,7 @@ int split_huge_page_to_list(struct page *page,
> struct list_head *list)
>                 goto out_unlock;
>
>         BUG_ON(!PageSwapBacked(page));
> -       __split_huge_page(page, anon_vma, list);
> +       __split_anon_huge_page(page, anon_vma, list);
>         count_vm_event(THP_SPLIT);
>
>         BUG_ON(PageCompound(page));
> @@ -1858,6 +1864,63 @@ out:
>         return ret;
>  }
>
> +static int split_file_huge_page(struct page *page, struct list_head *list)
> +{
> +       struct address_space *mapping = page->mapping;
> +       pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
> +       struct vm_area_struct *vma;
> +       int mapcount, mapcount2;
> +
> +       BUG_ON(!PageHead(page));
> +       BUG_ON(PageTail(page));
> +
> +       mutex_lock(&mapping->i_mmap_mutex);
> +       mapcount = 0;
> +       vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
> +               unsigned long addr = vma_address(page, vma);
> +               mapcount += __split_huge_page_splitting(page, vma, addr);
> +       }
> +
> +       if (mapcount != page_mapcount(page))
> +               printk(KERN_ERR "mapcount %d page_mapcount %d\n",
> +                      mapcount, page_mapcount(page));
> +       BUG_ON(mapcount != page_mapcount(page));
> +
> +       __split_huge_page_refcount(page, list);
> +
> +       mapcount2 = 0;
> +       vma_interval_tree_foreach(vma, &mapping->i_mmap, pgoff, pgoff) {
> +               unsigned long addr = vma_address(page, vma);
> +               mapcount2 += __split_huge_page_map(page, vma, addr);
> +       }
> +
> +       if (mapcount != mapcount2)
> +               printk(KERN_ERR "mapcount %d mapcount2 %d page_mapcount
> %d\n",
> +                      mapcount, mapcount2, page_mapcount(page));
> +       BUG_ON(mapcount != mapcount2);
> +       count_vm_event(THP_SPLIT);
> +       mutex_unlock(&mapping->i_mmap_mutex);
> +
> +       /*
> +        * Drop small pages beyond i_size if any.
> +        *
> +        * XXX: do we need to serialize over i_mutex here?
> +        * If yes, how to get mmap_sem vs. i_mutex ordering fixed?
> +        */
> +       truncate_inode_pages(mapping, i_size_read(mapping->host));
> +       return 0;
> +}
> +
> +int split_huge_page_to_list(struct page *page, struct list_head *list)
> +{
> +       BUG_ON(is_huge_zero_page(page));
> +
> +       if (PageAnon(page))
> +               return split_anon_huge_page(page, list);
> +       else
> +               return split_file_huge_page(page, list);
> +}
> +
>  #define VM_NO_THP
> (VM_SPECIAL|VM_MIXEDMAP|VM_HUGETLB|VM_SHARED|VM_MAYSHARE)
>
>  int hugepage_madvise(struct vm_area_struct *vma,
> --
> 1.8.3.2
>
>

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