Re: [PATCH v2 09/18] btrfs: subpage: introduce helper for subpage uptodate status

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

On 2020/12/11 下午7:41, Nikolay Borisov wrote:
>
>
> On 11.12.20 г. 12:48 ч., Qu Wenruo wrote:
>>
>>
>> On 2020/12/11 下午6:10, Nikolay Borisov wrote:
>>>
>>>
>>> On 10.12.20 г. 8:38 ч., 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.
>>>>
>>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>>> ---
>>>>  fs/btrfs/subpage.h | 98 ++++++++++++++++++++++++++++++++++++++++++++++
>>>>  1 file changed, 98 insertions(+)
>>>>
>>>> diff --git a/fs/btrfs/subpage.h b/fs/btrfs/subpage.h
>>>> index 87b4e028ae18..b3cf9171ec98 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 */
>>>>  		struct {
>>>> @@ -35,6 +36,17 @@ struct btrfs_subpage {
>>>>  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);
>>>>
>>>> +static inline void btrfs_subpage_clamp_range(struct page *page,
>>>> +					     u64 *start, u32 *len)
>>>> +{
>>>> +	u64 orig_start = *start;
>>>> +	u32 orig_len = *len;
>>>> +
>>>> +	*start = max_t(u64, page_offset(page), orig_start);
>>>> +	*len = min_t(u64, page_offset(page) + PAGE_SIZE,
>>>> +		     orig_start + orig_len) - *start;
>>>> +}
>>>
>>> This handles EB's which span pages, right? If so - a comment is in order
>>> since there is no design document specifying whether eb can or cannot
>>> span multiple pages.
>>
>> Didn't I have already stated that in the subpage eb accessors patch?
>>
>> No subpage eb can across page bounday.
>>
>
> As just discussed during the whiteboard session this function is really
> dead code for eb's because they are guaranteed to not span pages. Even
> for RW support it seems there is only btrfs_dirty_pages which changes
> page flags without having clamped the data i.e. there's only 1
> exception. In light of this I think it would be better to replace this
> function with ASSERTS and handle the only exception at the call site.

You're completely right.

I'll definite change these in next update.

Thanks,
Qu
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