Re: [PATCH v4 01/18] btrfs: update locked page dirty/writeback/error bits in __process_pages_contig()

[Qu Wenruo <wqu@suse.com>]

On 2021/1/21 下午2:32, Qu Wenruo wrote:
> 
> 
> On 2021/1/20 上午5:41, Josef Bacik wrote:
>> On 1/16/21 2:15 AM, Qu Wenruo wrote:
>>> When __process_pages_contig() get called for
>>> extent_clear_unlock_delalloc(), if we hit the locked page, only Private2
>>> bit is updated, but dirty/writeback/error bits are all skipped.
>>>
>>> There are several call sites call extent_clear_unlock_delalloc() with
>>> @locked_page and PAGE_CLEAR_DIRTY/PAGE_SET_WRITEBACK/PAGE_END_WRITEBACK
>>>
>>> - cow_file_range()
>>> - run_delalloc_nocow()
>>> - cow_file_range_async()
>>>    All for their error handling branches.
>>>
>>> For those call sites, since we skip the locked page for
>>> dirty/error/writeback bit update, the locked page will still have its
>>> dirty bit remaining.
>>>
>>> Thankfully, since all those call sites can only be hit with various
>>> serious errors, it's pretty hard to hit and shouldn't affect regular
>>> btrfs operations.
>>>
>>> But still, we shouldn't leave the locked_page with its
>>> dirty/error/writeback bits untouched.
>>>
>>> Fix this by only skipping lock/unlock page operations for locked_page.
>>>
>>> Signed-off-by: Qu Wenruo <wqu@suse.com>
>>
>> Except this is handled by the callers.  We clear_page_dirty_for_io() the
>> page before calling btrfs_run_delalloc_range(), so we don't need the
>> PAGE_CLEAR_DIRTY, it's already cleared.  The SetPageError() is handled
>> in the error path for locked_page, as is the
>> set_writeback/end_writeback.  Now I don't think this patch causes
>> problems specifically, but the changelog is at least wrong, and I'd
>> rather we'd skip the handling of the locked_page here and leave it in
>> the proper error handling.  If you need to do this for some other reason
>> that I haven't gotten to yet then you need to make that clear in the
>> changelog, because as of right now I don't see why this is needed.  
>> Thanks,
> 
> This is mostly to co-operate with a later patch on
> __process_pages_contig(), where we need to make sure page locked by
> __process_pages_contig() is only unlocked by __process_pages_contig() too.
> 
> The exception is after cow_file_inline(), we call
> __process_pages_contig() on the locked page, making it to clear page
> writeback and unlock it.

To be more clear, we call extent_clear_unlock_delalloc() with 
locked_page == NULL, to allow __process_pages_contig() to unlock the 
locked page (while the locked page isn't locked by 
__process_pages_contig()).

For subpage data, we need writers accounting for subpage, but that 
accounting only happens in __process_pages_contig(), thus we don't want 
pages without the accounting to be unlocked by __process_pages_contig().

I can do extra page unlock/clear_dirty/end_writeback just for that 
exception, but it would definitely needs more comments.

Thanks,
Qu

> 
> That is going to cause problems for subpage.
> 
> Thus I prefer to make __process_pages_contig() to clear page dirty/end
> writeback for locked page.
> 
> Thanks,
> Qu
>>
>> Josef
>