Re: [PATCH v4 03/18] btrfs: introduce the skeleton of btrfs_subpage structure

[Qu Wenruo <quwenruo.btrfs@gmx.com>]

On 2021/1/19 上午6:46, David Sterba wrote:
> On Sat, Jan 16, 2021 at 03:15:18PM +0800, Qu Wenruo wrote:
>> For btrfs subpage support, we need a structure to record extra info for
>> the status of each sectors of a page.
>>
>> This patch will introduce the skeleton structure for future btrfs
>> subpage support.
>> All subpage related code would go to subpage.[ch] to avoid populating
>> the existing code base.
>>
>> Reviewed-by: Josef Bacik <josef@toxicpanda.com>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>   fs/btrfs/Makefile  |  3 ++-
>>   fs/btrfs/subpage.c | 39 +++++++++++++++++++++++++++++++++++++++
>>   fs/btrfs/subpage.h | 31 +++++++++++++++++++++++++++++++
>>   3 files changed, 72 insertions(+), 1 deletion(-)
>>   create mode 100644 fs/btrfs/subpage.c
>>   create mode 100644 fs/btrfs/subpage.h
>>
>> diff --git a/fs/btrfs/Makefile b/fs/btrfs/Makefile
>> index 9f1b1a88e317..942562e11456 100644
>> --- a/fs/btrfs/Makefile
>> +++ b/fs/btrfs/Makefile
>> @@ -11,7 +11,8 @@ btrfs-y += super.o ctree.o extent-tree.o print-tree.o root-tree.o dir-item.o \
>>   	   compression.o delayed-ref.o relocation.o delayed-inode.o scrub.o \
>>   	   reada.o backref.o ulist.o qgroup.o send.o dev-replace.o raid56.o \
>>   	   uuid-tree.o props.o free-space-tree.o tree-checker.o space-info.o \
>> -	   block-rsv.o delalloc-space.o block-group.o discard.o reflink.o
>> +	   block-rsv.o delalloc-space.o block-group.o discard.o reflink.o \
>> +	   subpage.o
>>
>>   btrfs-$(CONFIG_BTRFS_FS_POSIX_ACL) += acl.o
>>   btrfs-$(CONFIG_BTRFS_FS_CHECK_INTEGRITY) += check-integrity.o
>> diff --git a/fs/btrfs/subpage.c b/fs/btrfs/subpage.c
>> new file mode 100644
>> index 000000000000..c6ab32db3995
>> --- /dev/null
>> +++ b/fs/btrfs/subpage.c
>> @@ -0,0 +1,39 @@
>> +// SPDX-License-Identifier: GPL-2.0
>> +
>> +#include "subpage.h"
>> +
>> +int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct page *page)
>> +{
>> +	struct btrfs_subpage *subpage;
>> +
>> +	/*
>> +	 * We have cases like a dummy extent buffer page, which is not
>> +	 * mappped and doesn't need to be locked.
>> +	 */
>> +	if (page->mapping)
>> +		ASSERT(PageLocked(page));
>> +	/* Either not subpage, or the page already has private attached */
>> +	if (fs_info->sectorsize == PAGE_SIZE || PagePrivate(page))
>> +		return 0;
>> +
>> +	subpage = kzalloc(sizeof(*subpage), GFP_NOFS);
>> +	if (!subpage)
>> +		return -ENOMEM;
>> +
>> +	spin_lock_init(&subpage->lock);
>> +	attach_page_private(page, subpage);
>> +	return 0;
>> +}
>> +
>> +void btrfs_detach_subpage(struct btrfs_fs_info *fs_info, struct page *page)
>> +{
>> +	struct btrfs_subpage *subpage;
>> +
>> +	/* Either not subpage, or already detached */
>> +	if (fs_info->sectorsize == PAGE_SIZE || !PagePrivate(page))
>> +		return;
>> +
>> +	subpage = (struct btrfs_subpage *)detach_page_private(page);
>> +	ASSERT(subpage);
>> +	kfree(subpage);
>> +}
>> diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
>> new file mode 100644
>> index 000000000000..96f3b226913e
>> --- /dev/null
>> +++ b/fs/btrfs/subpage.h
>> @@ -0,0 +1,31 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +
>> +#ifndef BTRFS_SUBPAGE_H
>> +#define BTRFS_SUBPAGE_H
>> +
>> +#include <linux/spinlock.h>
>> +#include "ctree.h"
>
> So subpage.h would pull the whole ctree.h, that's not very nice. If
> anything, the .c could include ctree.h because there are lots of the
> common structure and function definitions, but not the .h. This creates
> unnecessary include dependencies.
>
> Any pointer type you'd need in structures could be forward declared.

Unfortunately, the main needed pointer is fs_info, and we're accessing
it pretty frequently (mostly for sector/node size).

I don't believe forward declaration would help in this case.

Thanks,
Qu
>
>> +
>> +/*
>> + * Since the maximum page size btrfs is going to support is 64K while the
>> + * minimum sectorsize is 4K, this means a u16 bitmap is enough.
>> + *
>> + * The regular bitmap requires 32 bits as minimal bitmap size, so we can't use
>> + * existing bitmap_* helpers here.
>> + */
>> +#define BTRFS_SUBPAGE_BITMAP_SIZE	16
>> +
>> +/*
>> + * Structure to trace status of each sector inside a page.
>> + *
>> + * Will be attached to page::private for both data and metadata inodes.
>> + */
>> +struct btrfs_subpage {
>> +	/* Common members for both data and metadata pages */
>> +	spinlock_t lock;
>> +};
>> +
>> +int btrfs_attach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
>> +void btrfs_detach_subpage(struct btrfs_fs_info *fs_info, struct page *page);
>> +
>> +#endif /* BTRFS_SUBPAGE_H */
>> --
>> 2.30.0