Re: [PATCH 08/17] btrfs: refactor how we extract extent buffer from page for alloc_extent_buffer()

Nikolay Borisov
2020-09-11T11:14:31Z

On 8.09.20 г. 10:52 ч., Qu Wenruo wrote:
> This patch will extract the code to extract extent_buffer from
> page::private into its own function, grab_extent_buffer_from_page().
>
> Although it's just one line, for later sub-page size support it will
> become way more larger.
>
> Also add some extra comments why we need to do such page::private
> dancing.
>
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> fs/btrfs/extent_io.c | 49 ++++++++++++++++++++++++++++++++++++--------
> 1 file changed, 40 insertions(+), 9 deletions(-)
>
> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
> index 6fafbc1d047b..3c8fe40f67fa 100644
> --- a/fs/btrfs/extent_io.c
> +++ b/fs/btrfs/extent_io.c
> @@ -5214,6 +5214,44 @@ struct extent_buffer *alloc_test_extent_buffer(struct btrfs_fs_info *fs_info,
> }
> #endif
>
> +/*
> + * A helper to grab the exist extent buffer from a page.
> + *
> + * There is a small race window where two callers of alloc_extent_buffer():
> + * Thread 1 | Thread 2
> + * -------------------------------------+---------------------------------------
> + * alloc_extent_buffer() | alloc_extent_buffer()
> + * |- eb = __alloc_extent_buffer() | |- eb = __alloc_extent_buffer()
> + * |- p = find_or_create_page() | |- p = find_or_create_page()
> + *
> + * In above case, two ebs get allocated for the same bytenr, and got the same
> + * page.
> + * We have to rely on the page->mapping->private_lock to make one of them to
> + * give up and reuse the allocated eb:
> + *
> + * | |- grab_extent_buffer_from_page()
> + * | |- get nothing
> + * | |- attach_extent_buffer_page()
> + * | | |- Now page->private is set
> + * | |- spin_unlock(&mapping->private_lock);
> + * |- spin_lock(private_lock); | |- Continue to insert radix tree.
> + * |- grab_extent_buffer_from_page() |
> + * |- got eb from thread 2 |
> + * |- spin_unlock(private_lock); |
> + * |- goto free_eb; |

This comment is slightly misleading because it's not
graB_extent_buffer_form_page's return value which decides who wins the
race but rather the retval of PagePrivate which invoked _before_
grab_extent_buffer_from_page.

> + *
> + * The function here is to ensure we have proper locking and detect such race
> + * so we won't allocating an eb twice.
> + */
> +static struct extent_buffer *grab_extent_buffer_from_page(struct page *page)
> +{
> + /*
> + * For PAGE_SIZE == sectorsize case, a btree_inode page should have its
> + * private pointer as extent buffer who owns this page.
> + */
> + return (struct extent_buffer *)page->private;
> +}
> +
> struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
> u64 start)
> {
> @@ -5258,15 +5296,8 @@ struct extent_buffer *alloc_extent_buffer(struct btrfs_fs_info *fs_info,
>
> spin_lock(&mapping->private_lock);
> if (PagePrivate(p)) {
> - /*
> - * We could have already allocated an eb for this page
> - * and attached one so lets see if we can get a ref on
> - * the existing eb, and if we can we know it's good and
> - * we can just return that one, else we know we can just
> - * overwrite page->private.
> - */
> - exists = (struct extent_buffer *)p->private;
> - if (atomic_inc_not_zero(&exists->refs)) {
> + exists = grab_extent_buffer_from_page(p);
> + if (exists && atomic_inc_not_zero(&exists->refs)) {

grab_extent_buffer_from_page is guaranteed to return an extent buffer
due to the PagePrivate() check above. So simply doing atomic_in_not_zero
is fine.
> spin_unlock(&mapping->private_lock);
> unlock_page(p);
> put_page(p);
>