Re: [PATCH v6 04/16] iomap: Add flags parameter to iomap_page_create()

[Stefan Roesch <shr@fb.com>]

On 5/26/22 11:25 AM, Darrick J. Wong wrote:
> On Thu, May 26, 2022 at 10:38:28AM -0700, Stefan Roesch wrote:
>> Add the kiocb flags parameter to the function iomap_page_create().
>> Depending on the value of the flags parameter it enables different gfp
>> flags.
>>
>> No intended functional changes in this patch.
>>
>> Signed-off-by: Stefan Roesch <shr@fb.com>
>> Reviewed-by: Jan Kara <jack@suse.cz>
>> ---
>>  fs/iomap/buffered-io.c | 19 +++++++++++++------
>>  1 file changed, 13 insertions(+), 6 deletions(-)
>>
>> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
>> index 8ce8720093b9..d6ddc54e190e 100644
>> --- a/fs/iomap/buffered-io.c
>> +++ b/fs/iomap/buffered-io.c
>> @@ -44,16 +44,21 @@ static inline struct iomap_page *to_iomap_page(struct folio *folio)
>>  static struct bio_set iomap_ioend_bioset;
>>  
>>  static struct iomap_page *
>> -iomap_page_create(struct inode *inode, struct folio *folio)
>> +iomap_page_create(struct inode *inode, struct folio *folio, unsigned int flags)
>>  {
>>  	struct iomap_page *iop = to_iomap_page(folio);
>>  	unsigned int nr_blocks = i_blocks_per_folio(inode, folio);
>> +	gfp_t gfp = GFP_NOFS | __GFP_NOFAIL;
>>  
>>  	if (iop || nr_blocks <= 1)
>>  		return iop;
>>  
>> +	if (flags & IOMAP_NOWAIT)
>> +		gfp = GFP_NOWAIT;
> 
> Hmm.  GFP_NOWAIT means we don't wait for reclaim or IO or filesystem
> callbacks, and NOFAIL means we retry indefinitely.  What happens in the
> NOWAIT|NOFAIL case?  Does that imply that the kzalloc loops without
> triggering direct reclaim until someone else frees enough memory?
> 

Before this patch all requests allocate memory with the GFP_NOFS | __GFP_NOFAIL
flags. With this patch no_wait requests will be allocated with GFP_NOWAIT.
I don't see how allocations will happen with  GFP_NOWAIT| __GFP_NOFAIL.

If an allocation with GFP_NOWAIT fails. It will return -EAGAIN, for the write
code path. In io-uring, the write request will be punted to the io-worker.
The io-worker will process the write request, but nowait will not be specified.

> --D
> 
>> +
>>  	iop = kzalloc(struct_size(iop, uptodate, BITS_TO_LONGS(nr_blocks)),
>> -			GFP_NOFS | __GFP_NOFAIL);
>> +		      gfp);
>> +
>>  	spin_lock_init(&iop->uptodate_lock);
>>  	if (folio_test_uptodate(folio))
>>  		bitmap_fill(iop->uptodate, nr_blocks);
>> @@ -226,7 +231,7 @@ static int iomap_read_inline_data(const struct iomap_iter *iter,
>>  	if (WARN_ON_ONCE(size > iomap->length))
>>  		return -EIO;
>>  	if (offset > 0)
>> -		iop = iomap_page_create(iter->inode, folio);
>> +		iop = iomap_page_create(iter->inode, folio, iter->flags);
>>  	else
>>  		iop = to_iomap_page(folio);
>>  
>> @@ -264,7 +269,7 @@ static loff_t iomap_readpage_iter(const struct iomap_iter *iter,
>>  		return iomap_read_inline_data(iter, folio);
>>  
>>  	/* zero post-eof blocks as the page may be mapped */
>> -	iop = iomap_page_create(iter->inode, folio);
>> +	iop = iomap_page_create(iter->inode, folio, iter->flags);
>>  	iomap_adjust_read_range(iter->inode, folio, &pos, length, &poff, &plen);
>>  	if (plen == 0)
>>  		goto done;
>> @@ -550,7 +555,7 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>>  		size_t len, struct folio *folio)
>>  {
>>  	const struct iomap *srcmap = iomap_iter_srcmap(iter);
>> -	struct iomap_page *iop = iomap_page_create(iter->inode, folio);
>> +	struct iomap_page *iop;
>>  	loff_t block_size = i_blocksize(iter->inode);
>>  	loff_t block_start = round_down(pos, block_size);
>>  	loff_t block_end = round_up(pos + len, block_size);
>> @@ -561,6 +566,8 @@ static int __iomap_write_begin(const struct iomap_iter *iter, loff_t pos,
>>  		return 0;
>>  	folio_clear_error(folio);
>>  
>> +	iop = iomap_page_create(iter->inode, folio, iter->flags);
>> +
>>  	do {
>>  		iomap_adjust_read_range(iter->inode, folio, &block_start,
>>  				block_end - block_start, &poff, &plen);
>> @@ -1332,7 +1339,7 @@ iomap_writepage_map(struct iomap_writepage_ctx *wpc,
>>  		struct writeback_control *wbc, struct inode *inode,
>>  		struct folio *folio, u64 end_pos)
>>  {
>> -	struct iomap_page *iop = iomap_page_create(inode, folio);
>> +	struct iomap_page *iop = iomap_page_create(inode, folio, 0);
>>  	struct iomap_ioend *ioend, *next;
>>  	unsigned len = i_blocksize(inode);
>>  	unsigned nblocks = i_blocks_per_folio(inode, folio);
>> -- 
>> 2.30.2
>>