Re: [PATCH 5/5] block: truncate page cache only when necessary on fallocate

[Dmitry Monakhov]

Christoph Hellwig <hch@xxxxxxxxxxxxx> writes:

> why?
because it is not good thing to truncate page cache and fiew lines later
realize that feature is not supported !blk_queue_discard(q) and return ENOTSUPP.

Event more:  if mode ==  FALLOC_FL_KEEP_SIZE then we do nothing and
return ENOTSUPP unconditionally.

IMHO (mode ==  FALLOC_FL_KEEP_SIZE) is sane API for thin-provision blkdevs
to preallocate space in advance. Nobody use it at the moment, but it may
be usefull in future.
>
> On Thu, Apr 06, 2017 at 04:02:49PM +0400, Dmitry Monakhov wrote:
>> Signed-off-by: Dmitry Monakhov <dmonakhov@xxxxxxxxxx>
>> ---
>>  fs/block_dev.c | 9 ++++-----
>>  1 file changed, 4 insertions(+), 5 deletions(-)
>> 
>> diff --git a/fs/block_dev.c b/fs/block_dev.c
>> index 2eca00e..f4b13e1 100644
>> --- a/fs/block_dev.c
>> +++ b/fs/block_dev.c
>> @@ -2075,7 +2075,7 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
>>  {
>>  	struct block_device *bdev = I_BDEV(bdev_file_inode(file));
>>  	struct request_queue *q = bdev_get_queue(bdev);
>> -	struct address_space *mapping;
>> +	struct address_space *mapping = bdev->bd_inode->i_mapping;
>>  	loff_t end = start + len - 1;
>>  	loff_t isize;
>>  	int error;
>> @@ -2102,13 +2102,10 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
>>  	if ((start | len) & (bdev_logical_block_size(bdev) - 1))
>>  		return -EINVAL;
>>  
>> -	/* Invalidate the page cache, including dirty pages. */
>> -	mapping = bdev->bd_inode->i_mapping;
>> -	truncate_inode_pages_range(mapping, start, end);
>> -
>>  	switch (mode) {
>>  	case FALLOC_FL_ZERO_RANGE:
>>  	case FALLOC_FL_ZERO_RANGE | FALLOC_FL_KEEP_SIZE:
>> +		truncate_inode_pages_range(mapping, start, end);
>>  		error = blkdev_issue_zeroout(bdev, start >> 9, len >> 9,
>>  					    GFP_KERNEL, false);
>>  		break;
>> @@ -2116,12 +2113,14 @@ static long blkdev_fallocate(struct file *file, int mode, loff_t start,
>>  		/* Only punch if the device can do zeroing discard. */
>>  		if (!blk_queue_discard(q) || !q->limits.discard_zeroes_data)
>>  			return -EOPNOTSUPP;
>> +		truncate_inode_pages_range(mapping, start, end);
>>  		error = blkdev_issue_discard(bdev, start >> 9, len >> 9,
>>  					     GFP_KERNEL, 0);
>>  		break;
>>  	case FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE | FALLOC_FL_NO_HIDE_STALE:
>>  		if (!blk_queue_discard(q))
>>  			return -EOPNOTSUPP;
>> +		truncate_inode_pages_range(mapping, start, end);
>>  		error = blkdev_issue_discard(bdev, start >> 9, len >> 9,
>>  					     GFP_KERNEL, 0);
>>  		break;
>> -- 
>> 2.9.3
>> 
> ---end quoted text---