On 2018年06月29日 20:46, David Sterba wrote:
> On Wed, Jun 27, 2018 at 04:38:24PM +0300, Nikolay Borisov wrote:
>> EXTENT_BUFFER_DUMMY is an awful name for this flag. Buffers which have
>> this flag set are not in any way dummy. Rather, they are private in
>> the sense that are not linked to the global buffer tree. This flag has
>> subtle implications to the way free_extent_buffer work for example, as
>> well as controls whether page->mapping->private_lock is held during
>> extent_buffer release. Pages for a private buffer cannot be under io,
>> nor can they be written by a 3rd party so taking the lock is
>> unnecessary.
>>
>> Signed-off-by: Nikolay Borisov <nbori...@suse.com>
>> ---
>> fs/btrfs/disk-io.c | 2 +-
>> fs/btrfs/extent_io.c | 10 +++++-----
>> fs/btrfs/extent_io.h | 2 +-
>> 3 files changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
>> index 8a469f70d5ee..1c655be92690 100644
>> --- a/fs/btrfs/disk-io.c
>> +++ b/fs/btrfs/disk-io.c
>> @@ -4093,7 +4093,7 @@ void btrfs_mark_buffer_dirty(struct extent_buffer *buf)
>> * enabled. Normal people shouldn't be marking dummy buffers as dirty
>> * outside of the sanity tests.
>> */
>> - if (unlikely(test_bit(EXTENT_BUFFER_DUMMY, &buf->bflags)))
>> + if (unlikely(test_bit(EXTENT_BUFFER_PRIVATE, &buf->bflags)))
>
> This is going to be confusing. There's page Private bit,
> PAGE_SET_PRIVATE2 and EXTENT_PAGE_PRIVATE, that are somehow logically
> connected.
>
> I'd suggest EXTENT_BUFFER_CLONED or _UNMAPPED as it's created by
> btrfs_clone_extent_buffer or used in the disconnected way (ie. without
> the mapping).
UNMAPPED looks good to me.
(Or ANONYMOUS?)
>
>> return;
>> #endif
>> root = BTRFS_I(buf->pages[0]->mapping->host)->root;
>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>> index 6ac15804bab1..6611207e8e1f 100644
>> --- a/fs/btrfs/extent_io.c
>> +++ b/fs/btrfs/extent_io.c
>> @@ -4642,7 +4642,7 @@ int extent_buffer_under_io(struct extent_buffer *eb)
>> static void btrfs_release_extent_buffer_page(struct extent_buffer *eb)
>> {
>> int i;
>> - int mapped = !test_bit(EXTENT_BUFFER_DUMMY, &eb->bflags);
>> + int mapped = !test_bit(EXTENT_BUFFER_PRIVATE, &eb->bflags);
>>
>> BUG_ON(extent_buffer_under_io(eb));
>>
>> @@ -4755,7 +4755,7 @@ struct extent_buffer *btrfs_clone_extent_buffer(struct
>> extent_buffer *src)
>> }
>>
>> set_bit(EXTENT_BUFFER_UPTODATE, &new->bflags);
>> - set_bit(EXTENT_BUFFER_DUMMY, &new->bflags);
>> + set_bit(EXTENT_BUFFER_PRIVATE, &new->bflags);
>>
>> return new;
>> }
>> @@ -4780,7 +4780,7 @@ struct extent_buffer
>> *__alloc_dummy_extent_buffer(struct btrfs_fs_info *fs_info,
>
> Would be good to sync the new name with the helpers:
> __alloc_dummy_extent_buffer and alloc_dummy_extent_buffer then.
>
>> }
>> set_extent_buffer_uptodate(eb);
>> btrfs_set_header_nritems(eb, 0);
>> - set_bit(EXTENT_BUFFER_DUMMY, &eb->bflags);
>> + set_bit(EXTENT_BUFFER_PRIVATE, &eb->bflags);
>>
>> return eb;
>> err:
>> @@ -5086,7 +5086,7 @@ static int release_extent_buffer(struct extent_buffer
>> *eb)
>> /* Should be safe to release our pages at this point */
>> btrfs_release_extent_buffer_page(eb);
>> #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
>> - if (unlikely(test_bit(EXTENT_BUFFER_DUMMY, &eb->bflags))) {
>> + if (unlikely(test_bit(EXTENT_BUFFER_PRIVATE, &eb->bflags))) {
>> __free_extent_buffer(eb);
>> return 1;
>> }
>> @@ -5117,7 +5117,7 @@ void free_extent_buffer(struct extent_buffer *eb)
>>
>> spin_lock(&eb->refs_lock);
>> if (atomic_read(&eb->refs) == 2 &&
>> - test_bit(EXTENT_BUFFER_DUMMY, &eb->bflags))
>> + test_bit(EXTENT_BUFFER_PRIVATE, &eb->bflags))
>> atomic_dec(&eb->refs);
Also discussed in off list mail, this extra atomic_dec for cloned eb
looks confusing.
(That also explains why after cloning the eb, we call
extent_buffer_get() and only frees it once, and still no eb leaking)
What about just removing such special handling?
Thanks,
Qu
>>
>> if (atomic_read(&eb->refs) == 2 &&
>> diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
>> index 0bfd4aeb822d..bfccaec2c89a 100644
>> --- a/fs/btrfs/extent_io.h
>> +++ b/fs/btrfs/extent_io.h
>> @@ -46,7 +46,7 @@
>> #define EXTENT_BUFFER_STALE 6
>> #define EXTENT_BUFFER_WRITEBACK 7
>> #define EXTENT_BUFFER_READ_ERR 8 /* read IO error */
>> -#define EXTENT_BUFFER_DUMMY 9
>> +#define EXTENT_BUFFER_PRIVATE 9
>> #define EXTENT_BUFFER_IN_TREE 10
>> #define EXTENT_BUFFER_WRITE_ERR 11 /* write IO error */
>>
>> --
>> 2.7.4
>>
>> --
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