Re: [PATCH 13/16] iomap: make buffered writes work with RWF_UNCACHED

[Jens Axboe <axboe@kernel.dk>]

On 11/11/24 6:01 PM, Darrick J. Wong wrote:
> On Mon, Nov 11, 2024 at 04:37:40PM -0700, Jens Axboe wrote:
>> Add iomap buffered write support for RWF_UNCACHED. If RWF_UNCACHED is
>> set for a write, mark the folios being written with drop_writeback. Then
>> writeback completion will drop the pages. The write_iter handler simply
>> kicks off writeback for the pages, and writeback completion will take
>> care of the rest.
>>
>> This still needs the user of the iomap buffered write helpers to call
>> iocb_uncached_write() upon successful issue of the writes.
>>
>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
>> ---
>>  fs/iomap/buffered-io.c | 15 +++++++++++++--
>>  include/linux/iomap.h  |  4 +++-
>>  2 files changed, 16 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/iomap/buffered-io.c b/fs/iomap/buffered-io.c
>> index ef0b68bccbb6..2f2a5db04a68 100644
>> --- a/fs/iomap/buffered-io.c
>> +++ b/fs/iomap/buffered-io.c
>> @@ -603,6 +603,8 @@ struct folio *iomap_get_folio(struct iomap_iter *iter, loff_t pos, size_t len)
>>  
>>  	if (iter->flags & IOMAP_NOWAIT)
>>  		fgp |= FGP_NOWAIT;
>> +	if (iter->flags & IOMAP_UNCACHED)
>> +		fgp |= FGP_UNCACHED;
>>  	fgp |= fgf_set_order(len);
>>  
>>  	return __filemap_get_folio(iter->inode->i_mapping, pos >> PAGE_SHIFT,
>> @@ -1023,8 +1025,9 @@ ssize_t
>>  iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
>>  		const struct iomap_ops *ops, void *private)
>>  {
>> +	struct address_space *mapping = iocb->ki_filp->f_mapping;
>>  	struct iomap_iter iter = {
>> -		.inode		= iocb->ki_filp->f_mapping->host,
>> +		.inode		= mapping->host,
>>  		.pos		= iocb->ki_pos,
>>  		.len		= iov_iter_count(i),
>>  		.flags		= IOMAP_WRITE,
>> @@ -1034,9 +1037,14 @@ iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *i,
>>  
>>  	if (iocb->ki_flags & IOCB_NOWAIT)
>>  		iter.flags |= IOMAP_NOWAIT;
>> +	if (iocb->ki_flags & IOCB_UNCACHED)
>> +		iter.flags |= IOMAP_UNCACHED;
>>  
>> -	while ((ret = iomap_iter(&iter, ops)) > 0)
>> +	while ((ret = iomap_iter(&iter, ops)) > 0) {
>> +		if (iocb->ki_flags & IOCB_UNCACHED)
>> +			iter.iomap.flags |= IOMAP_F_UNCACHED;
>>  		iter.processed = iomap_write_iter(&iter, i);
>> +	}
>>  
>>  	if (unlikely(iter.pos == iocb->ki_pos))
>>  		return ret;
>> @@ -1770,6 +1778,9 @@ static int iomap_add_to_ioend(struct iomap_writepage_ctx *wpc,
>>  	size_t poff = offset_in_folio(folio, pos);
>>  	int error;
>>  
>> +	if (folio_test_uncached(folio))
>> +		wpc->iomap.flags |= IOMAP_F_UNCACHED;
>> +
>>  	if (!wpc->ioend || !iomap_can_add_to_ioend(wpc, pos)) {
>>  new_ioend:
>>  		error = iomap_submit_ioend(wpc, 0);
>> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
>> index f61407e3b121..2efc72df19a2 100644
>> --- a/include/linux/iomap.h
>> +++ b/include/linux/iomap.h
>> @@ -64,6 +64,7 @@ struct vm_fault;
>>  #define IOMAP_F_BUFFER_HEAD	0
>>  #endif /* CONFIG_BUFFER_HEAD */
>>  #define IOMAP_F_XATTR		(1U << 5)
>> +#define IOMAP_F_UNCACHED	(1U << 6)
> 
> This value ^^^ is set only by the core iomap code, right?

Correct

>>  /*
>>   * Flags set by the core iomap code during operations:
> 
> ...in which case it should be set down here.  It probably ought to have
> a description of what it does, too:

Ah yes indeed, good point. I'll move it and add a description.

> "IOMAP_F_UNCACHED is set to indicate that writes to the page cache (and
> hence writeback) will result in folios being evicted as soon as the
> updated bytes are written back to the storage."

Excellent, I'll go with that.

> If the writeback fails, does that mean that the dirty data will /not/ be
> retained in the page cache?  IIRC we finally got to the point where the
> major filesystems leave pagecache alone after writeback EIO.

Good question - didn't change any of those bits. It currently relies on
writeback completion to prune the ranges. So if an EIO completion
triggers writeback completion, then it'll get pruned. But for that case,
I suspect the range is still dirty, and hence the pruning would not
succeed, for obvious reasons. So you'd need further things on top of
that, I'm afraid.

> The rest of the mechanics looks nifty to me; there's plenty of places
> where this could be useful to me personally. :)

For sure, I think there are tons of use cases for this as well. Thanks
for taking a look!

-- 
Jens Axboe