Re: [PATCH v4 12/18] btrfs: implement try_release_extent_buffer() for subpage metadata support

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

On 2021/1/20 下午11:05, Josef Bacik wrote:
> On 1/16/21 2:15 AM, Qu Wenruo wrote:
>> Unlike the original try_release_extent_buffer(),
>> try_release_subpage_extent_buffer() will iterate through all the ebs in
>> the page, and try to release each eb.
>>
>> And only if the page and no private attached, which implies we have
>> released all ebs of the page, then we can release the full page.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>   fs/btrfs/extent_io.c | 106 ++++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 104 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/btrfs/extent_io.c b/fs/btrfs/extent_io.c
>> index 74a37eec921f..9414219fa28b 100644
>> --- a/fs/btrfs/extent_io.c
>> +++ b/fs/btrfs/extent_io.c
>> @@ -6335,13 +6335,115 @@ void memmove_extent_buffer(const struct
>> extent_buffer *dst,
>>       }
>>   }
>> +static struct extent_buffer *get_next_extent_buffer(
>> +        struct btrfs_fs_info *fs_info, struct page *page, u64 bytenr)
>> +{
>> +    struct extent_buffer *gang[BTRFS_SUBPAGE_BITMAP_SIZE];
>> +    struct extent_buffer *found = NULL;
>> +    u64 page_start = page_offset(page);
>> +    int ret;
>> +    int i;
>> +
>> +    ASSERT(in_range(bytenr, page_start, PAGE_SIZE));
>> +    ASSERT(PAGE_SIZE / fs_info->nodesize <= BTRFS_SUBPAGE_BITMAP_SIZE);
>> +    lockdep_assert_held(&fs_info->buffer_lock);
>> +
>> +    ret = radix_tree_gang_lookup(&fs_info->buffer_radix, (void **)gang,
>> +            bytenr >> fs_info->sectorsize_bits,
>> +            PAGE_SIZE / fs_info->nodesize);
>> +    for (i = 0; i < ret; i++) {
>> +        /* Already beyond page end */
>> +        if (gang[i]->start >= page_start + PAGE_SIZE)
>> +            break;
>> +        /* Found one */
>> +        if (gang[i]->start >= bytenr) {
>> +            found = gang[i];
>> +            break;
>> +        }
>> +    }
>> +    return found;
>> +}
>> +
>> +static int try_release_subpage_extent_buffer(struct page *page)
>> +{
>> +    struct btrfs_fs_info *fs_info = btrfs_sb(page->mapping->host->i_sb);
>> +    u64 cur = page_offset(page);
>> +    const u64 end = page_offset(page) + PAGE_SIZE;
>> +    int ret;
>> +
>> +    while (cur < end) {
>> +        struct extent_buffer *eb = NULL;
>> +
>> +        /*
>> +         * Unlike try_release_extent_buffer() which uses page->private
>> +         * to grab buffer, for subpage case we rely on radix tree, thus
>> +         * we need to ensure radix tree consistency.
>> +         *
>> +         * We also want an atomic snapshot of the radix tree, thus go
>> +         * spinlock other than RCU.
>> +         */
>> +        spin_lock(&fs_info->buffer_lock);
>> +        eb = get_next_extent_buffer(fs_info, page, cur);
>> +        if (!eb) {
>> +            /* No more eb in the page range after or at @cur */
>> +            spin_unlock(&fs_info->buffer_lock);
>> +            break;
>> +        }
>> +        cur = eb->start + eb->len;
>> +
>> +        /*
>> +         * The same as try_release_extent_buffer(), to ensure the eb
>> +         * won't disappear out from under us.
>> +         */
>> +        spin_lock(&eb->refs_lock);
>> +        if (atomic_read(&eb->refs) != 1 || extent_buffer_under_io(eb)) {
>> +            spin_unlock(&eb->refs_lock);
>> +            spin_unlock(&fs_info->buffer_lock);
>
> Why continue at this point?  We know we can't drop this thing, break here.
>
> <snip>
>
>> +}
>> +
>>   int try_release_extent_buffer(struct page *page)
>>   {
>>       struct extent_buffer *eb;
>> +    if (btrfs_sb(page->mapping->host->i_sb)->sectorsize < PAGE_SIZE)
>> +        return try_release_subpage_extent_buffer(page);
>
> You're using sectorsize again here.  I realize the problem is sectorsize
> != PAGE_SIZE, but sectorsize != nodesize all the time, so please change
> all of the patches to check the actual relevant size for the
> data/metadata type.  Thanks,

Again, nodesize >= sectorsize is the requirement for current mkfs.

As sectorsize is the minimal unit for both data and metadata.
(We don't have separate data unit size and metadata unit size, they
share sector size for now).

Thanks,
Qu

>
> Josef