Re: [PATCH v1 07/14] fs: Add aop_flags parameter to create_page_buffers()

[Stefan Roesch <shr@fb.com>]

On 2/14/22 10:14 AM, Matthew Wilcox wrote:
> On Mon, Feb 14, 2022 at 09:43:56AM -0800, Stefan Roesch wrote:
>> This adds the aop_flags parameter to the create_page_buffers function.
>> When AOP_FLAGS_NOWAIT parameter is set, the atomic allocation flag is
>> set. The AOP_FLAGS_NOWAIT flag is set, when async buffered writes are
>> enabled.
> 
> Why is this better than passing in gfp flags directly?
> 

I don't think that gfp flags are a great fit here. We only want to pass in
a nowait flag and this does not map nicely to a gfp flag. 

Instead of passing in a flag parameter we can also pass in a bool parameter,
however that has its limitations as it can't be extended in the future.

>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> ---
>>  fs/buffer.c | 28 +++++++++++++++++++++-------
>>  1 file changed, 21 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/buffer.c b/fs/buffer.c
>> index 948505480b43..5e3067173580 100644
>> --- a/fs/buffer.c
>> +++ b/fs/buffer.c
>> @@ -1682,13 +1682,27 @@ static inline int block_size_bits(unsigned int blocksize)
>>  	return ilog2(blocksize);
>>  }
>>  
>> -static struct buffer_head *create_page_buffers(struct page *page, struct inode *inode, unsigned int b_state)
>> +static struct buffer_head *create_page_buffers(struct page *page,
>> +					struct inode *inode,
>> +					unsigned int b_state,
>> +					unsigned int aop_flags)
>>  {
>>  	BUG_ON(!PageLocked(page));
>>  
>> -	if (!page_has_buffers(page))
>> -		create_empty_buffers(page, 1 << READ_ONCE(inode->i_blkbits),
>> -				     b_state);
>> +	if (!page_has_buffers(page)) {
>> +		gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT;
>> +
>> +		if (aop_flags & AOP_FLAGS_NOWAIT) {
>> +			gfp |= GFP_ATOMIC | __GFP_NOWARN;
>> +			gfp &= ~__GFP_DIRECT_RECLAIM;
>> +		} else {
>> +			gfp |= __GFP_NOFAIL;
>> +		}
>> +
>> +		__create_empty_buffers(page, 1 << READ_ONCE(inode->i_blkbits),
>> +				     b_state, gfp);
>> +	}
>> +
>>  	return page_buffers(page);
>>  }
>>  
>> @@ -1734,7 +1748,7 @@ int __block_write_full_page(struct inode *inode, struct page *page,
>>  	int write_flags = wbc_to_write_flags(wbc);
>>  
>>  	head = create_page_buffers(page, inode,
>> -					(1 << BH_Dirty)|(1 << BH_Uptodate));
>> +					(1 << BH_Dirty)|(1 << BH_Uptodate), 0);
>>  
>>  	/*
>>  	 * Be very careful.  We have no exclusion from __set_page_dirty_buffers
>> @@ -2000,7 +2014,7 @@ int __block_write_begin_int(struct folio *folio, loff_t pos, unsigned len,
>>  	BUG_ON(to > PAGE_SIZE);
>>  	BUG_ON(from > to);
>>  
>> -	head = create_page_buffers(&folio->page, inode, 0);
>> +	head = create_page_buffers(&folio->page, inode, 0, flags);
>>  	blocksize = head->b_size;
>>  	bbits = block_size_bits(blocksize);
>>  
>> @@ -2280,7 +2294,7 @@ int block_read_full_page(struct page *page, get_block_t *get_block)
>>  	int nr, i;
>>  	int fully_mapped = 1;
>>  
>> -	head = create_page_buffers(page, inode, 0);
>> +	head = create_page_buffers(page, inode, 0, 0);
>>  	blocksize = head->b_size;
>>  	bbits = block_size_bits(blocksize);
>>  
>> -- 
>> 2.30.2
>>