Re: [PATCH v4 08/18] btrfs: introduce helper for subpage uptodate status

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

On 2021/1/20 下午10:55, Josef Bacik wrote:
> On 1/16/21 2:15 AM, Qu Wenruo wrote:
>> This patch introduce the following functions to handle btrfs subpage
>> uptodate status:
>> - btrfs_subpage_set_uptodate()
>> - btrfs_subpage_clear_uptodate()
>> - btrfs_subpage_test_uptodate()
>>    Those helpers can only be called when the range is ensured to be
>>    inside the page.
>>
>> - btrfs_page_set_uptodate()
>> - btrfs_page_clear_uptodate()
>> - btrfs_page_test_uptodate()
>>    Those helpers can handle both regular sector size and subpage without
>>    problem.
>>    Although caller should still ensure that the range is inside the page.
>>
>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>> ---
>>   fs/btrfs/subpage.h | 115 +++++++++++++++++++++++++++++++++++++++++++++
>>   1 file changed, 115 insertions(+)
>>
>> diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
>> index d8b34879368d..3373ef4ffec1 100644
>> --- a/fs/btrfs/subpage.h
>> +++ b/fs/btrfs/subpage.h
>> @@ -23,6 +23,7 @@
>>   struct btrfs_subpage {
>>       /* Common members for both data and metadata pages */
>>       spinlock_t lock;
>> +    u16 uptodate_bitmap;
>>       union {
>>           /* Structures only used by metadata */
>>           bool under_alloc;
>> @@ -78,4 +79,118 @@ static inline void
>> btrfs_page_end_meta_alloc(struct btrfs_fs_info *fs_info,
>>   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);
>> +/*
>> + * Convert the [start, start + len) range into a u16 bitmap
>> + *
>> + * E.g. if start == page_offset() + 16K, len = 16K, we get 0x00f0.
>> + */
>> +static inline u16 btrfs_subpage_calc_bitmap(struct btrfs_fs_info
>> *fs_info,
>> +            struct page *page, u64 start, u32 len)
>> +{
>> +    int bit_start = offset_in_page(start) >> fs_info->sectorsize_bits;
>> +    int nbits = len >> fs_info->sectorsize_bits;
>> +
>> +    /* Basic checks */
>> +    ASSERT(PagePrivate(page) && page->private);
>> +    ASSERT(IS_ALIGNED(start, fs_info->sectorsize) &&
>> +           IS_ALIGNED(len, fs_info->sectorsize));
>> +
>> +    /*
>> +     * The range check only works for mapped page, we can
>> +     * still have unampped page like dummy extent buffer pages.
>> +     */
>> +    if (page->mapping)
>> +        ASSERT(page_offset(page) <= start &&
>> +            start + len <= page_offset(page) + PAGE_SIZE);
>
> Once you gate the helpers on UNMAPPED you'll always have page->mapping
> set and you can drop the if statement.  Thanks,
>

I'd say, if we make ASSERT() to really do nothing if CONFIG_BTRFS_ASSERT
is not selected, we won't really need to bother then.

As in that case, the function will do nothing.

For now, it's a mixed bag as we can still have subpage UNMAPPED eb go
into such subpage helpers, and doing the UNMAPPED checks in so many
helpers itself can be a big load.

Thus I prefer to keep the if here.

Thanks,
Qu

> Josef