| Message ID | 20200908075230.86856-9-wqu@suse.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | btrfs: add read-only support for subpage sector size | expand |
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); >
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; | + * + * 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)) { spin_unlock(&mapping->private_lock); unlock_page(p); put_page(p);
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(-)