Re: [f2fs-dev] [PATCH] f2fs: simplify accounting inflight directIO request

[Chao Yu]

On 2021/7/22 23:48, Eric Biggers wrote:
> On Thu, Jul 22, 2021 at 09:16:17PM +0800, Chao Yu wrote:
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index ba120d55e9b1..d0a1ca6ae38e 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -1720,6 +1720,9 @@ static int __get_data_block(struct inode *inode, sector_t iblock,
> >   		map_bh(bh, inode->i_sb, map.m_pblk);
> >   		bh->b_state = (bh->b_state & ~F2FS_MAP_FLAGS) | map.m_flags;
> >   		bh->b_size = blks_to_bytes(inode, map.m_len);
> > +
> > +		if (flag == F2FS_GET_BLOCK_DIO)
> > +			bh->b_private = (void *)may_write;
>
> Why is this hunk needed?

For passing rw info from get_block() to dio->private.

> > +static int f2fs_dio_end_io(struct kiocb *iocb, loff_t offset,
> > +					ssize_t bytes, void *private)
> >   {
> > -	struct f2fs_private_dio *dio = bio->bi_private;
> > -
> > -	dec_page_count(F2FS_I_SB(dio->inode),
> > -			dio->write ? F2FS_DIO_WRITE : F2FS_DIO_READ);
> > -
> > -	bio->bi_private = dio->orig_private;
> > -	bio->bi_end_io = dio->orig_end_io;
> > -
> > -	kfree(dio);
> > +	struct inode *inode = file_inode(iocb->ki_filp);
> > +	bool may_write = private;
> >   
> > -	bio_endio(bio);
> > +	dec_dio_req_count(F2FS_I_SB(inode), may_write ? WRITE : READ);
> > +	return 0;
> >   }
> >   
> >   static void f2fs_dio_submit_bio(struct bio *bio, struct inode *inode,
> >   							loff_t file_offset)
> >   {
> > -	struct f2fs_private_dio *dio;
> > -	bool write = (bio_op(bio) == REQ_OP_WRITE);
> > -
> > -	dio = f2fs_kzalloc(F2FS_I_SB(inode),
> > -			sizeof(struct f2fs_private_dio), GFP_NOFS);
> > -	if (!dio)
> > -		goto out;
> > -
> > -	dio->inode = inode;
> > -	dio->orig_end_io = bio->bi_end_io;
> > -	dio->orig_private = bio->bi_private;
> > -	dio->write = write;
> > -
> > -	bio->bi_end_io = f2fs_dio_end_io;
> > -	bio->bi_private = dio;
> > -
> > -	inc_page_count(F2FS_I_SB(inode),
> > -			write ? F2FS_DIO_WRITE : F2FS_DIO_READ);
> > +	inc_dio_req_count(F2FS_I_SB(inode),
> > +			op_is_write(bio_op(bio)) ? WRITE : READ);
> >   
> >   	submit_bio(bio);
> > -	return;
> > -out:
> > -	bio->bi_status = BLK_STS_IOERR;
> > -	bio_endio(bio);
> >   }
>
> The inc and dec here aren't correctly paired, since f2fs_dio_submit_bio() is
> called once per bio whereas f2fs_dio_end_io() is called when the entire direct
> I/O request (which may have consisted of multiple bios) has completed.

Correct, it still needs to hook bio->bi_end_io rather than hooking dio->end_io,
however, w/o one extra allocated memory, we have no way to pass sbi to
f2fs_dio_end_io() for dec_dio_req_count(sbi, ...) invoking.

> The correct way to do this would be to do the inc before calling
> __blockdev_direct_IO(), and do the dec in end_io or if __blockdev_direct_IO()
> returned without actually issuing any I/O.
>
> But I think you shouldn't bother with this part of the change before we switch
> to iomap, as it will then need to be changed again anyway.

Agreed.

Thanks,

> - Eric