Re: [PATCH] block: add bio_iov_iter_nvecs for figuring out nr_vecs

[Pavel Begunkov <asml.silence@gmail.com>]

On 01/12/2020 12:06, Ming Lei wrote:
> Pavel reported that iov_iter_npages is a bit heavy in case of bvec
> iter.
> 
> Turns out it isn't necessary to iterate every page in the bvec iter,
> and we call iov_iter_npages() just for figuring out how many bio
> vecs need to be allocated. And we can simply map each vector in bvec iter
> to bio's vec, so just return iter->nr_segs from bio_iov_iter_nvecs() for
> bvec iter.
> 
> Also rename local variable 'nr_pages' as 'nr_vecs' which exactly matches its
> real usage.
> 
> This patch is based on Mathew's post:

Tried this, the system didn't boot + discovered a filesystem blowned after
booting with a stable kernel. That's on top of 4498a8536c816 ("block: use
an xarray for disk->part_tbl"), which works fine. Ideas?

> https://lore.kernel.org/linux-block/20201120123931.GN29991@casper.infradead.org/
> 
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Pavel Begunkov <asml.silence@gmail.com>
> Cc: Christoph Hellwig <hch@infradead.org>
> Cc: linux-fsdevel@vger.kernel.org
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  fs/block_dev.c       | 30 +++++++++++++++---------------
>  fs/iomap/direct-io.c | 14 +++++++-------
>  include/linux/bio.h  | 10 ++++++++++
>  3 files changed, 32 insertions(+), 22 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index d8664f5c1ff6..4fd9bb4306db 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -218,7 +218,7 @@ static void blkdev_bio_end_io_simple(struct bio *bio)
>  
>  static ssize_t
>  __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
> -		int nr_pages)
> +		int nr_vecs)
>  {
>  	struct file *file = iocb->ki_filp;
>  	struct block_device *bdev = I_BDEV(bdev_file_inode(file));
> @@ -233,16 +233,16 @@ __blkdev_direct_IO_simple(struct kiocb *iocb, struct iov_iter *iter,
>  	    (bdev_logical_block_size(bdev) - 1))
>  		return -EINVAL;
>  
> -	if (nr_pages <= DIO_INLINE_BIO_VECS)
> +	if (nr_vecs <= DIO_INLINE_BIO_VECS)
>  		vecs = inline_vecs;
>  	else {
> -		vecs = kmalloc_array(nr_pages, sizeof(struct bio_vec),
> +		vecs = kmalloc_array(nr_vecs, sizeof(struct bio_vec),
>  				     GFP_KERNEL);
>  		if (!vecs)
>  			return -ENOMEM;
>  	}
>  
> -	bio_init(&bio, vecs, nr_pages);
> +	bio_init(&bio, vecs, nr_vecs);
>  	bio_set_dev(&bio, bdev);
>  	bio.bi_iter.bi_sector = pos >> 9;
>  	bio.bi_write_hint = iocb->ki_hint;
> @@ -353,7 +353,7 @@ static void blkdev_bio_end_io(struct bio *bio)
>  }
>  
>  static ssize_t
> -__blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
> +__blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_vecs)
>  {
>  	struct file *file = iocb->ki_filp;
>  	struct inode *inode = bdev_file_inode(file);
> @@ -371,7 +371,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  	    (bdev_logical_block_size(bdev) - 1))
>  		return -EINVAL;
>  
> -	bio = bio_alloc_bioset(GFP_KERNEL, nr_pages, &blkdev_dio_pool);
> +	bio = bio_alloc_bioset(GFP_KERNEL, nr_vecs, &blkdev_dio_pool);
>  
>  	dio = container_of(bio, struct blkdev_dio, bio);
>  	dio->is_sync = is_sync = is_sync_kiocb(iocb);
> @@ -420,8 +420,8 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  		dio->size += bio->bi_iter.bi_size;
>  		pos += bio->bi_iter.bi_size;
>  
> -		nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES);
> -		if (!nr_pages) {
> +		nr_vecs = bio_iov_iter_nvecs(iter, BIO_MAX_PAGES);
> +		if (!nr_vecs) {
>  			bool polled = false;
>  
>  			if (iocb->ki_flags & IOCB_HIPRI) {
> @@ -451,7 +451,7 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  		}
>  
>  		submit_bio(bio);
> -		bio = bio_alloc(GFP_KERNEL, nr_pages);
> +		bio = bio_alloc(GFP_KERNEL, nr_vecs);
>  	}
>  
>  	if (!is_poll)
> @@ -483,15 +483,15 @@ __blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter, int nr_pages)
>  static ssize_t
>  blkdev_direct_IO(struct kiocb *iocb, struct iov_iter *iter)
>  {
> -	int nr_pages;
> +	int nr_vecs;
>  
> -	nr_pages = iov_iter_npages(iter, BIO_MAX_PAGES + 1);
> -	if (!nr_pages)
> +	nr_vecs = bio_iov_iter_nvecs(iter, BIO_MAX_PAGES + 1);
> +	if (!nr_vecs)
>  		return 0;
> -	if (is_sync_kiocb(iocb) && nr_pages <= BIO_MAX_PAGES)
> -		return __blkdev_direct_IO_simple(iocb, iter, nr_pages);
> +	if (is_sync_kiocb(iocb) && nr_vecs <= BIO_MAX_PAGES)
> +		return __blkdev_direct_IO_simple(iocb, iter, nr_vecs);
>  
> -	return __blkdev_direct_IO(iocb, iter, min(nr_pages, BIO_MAX_PAGES));
> +	return __blkdev_direct_IO(iocb, iter, min(nr_vecs, BIO_MAX_PAGES));
>  }
>  
>  static __init int blkdev_init(void)
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 933f234d5bec..cc779ecc8144 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -211,7 +211,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  	struct bio *bio;
>  	bool need_zeroout = false;
>  	bool use_fua = false;
> -	int nr_pages, ret = 0;
> +	int nr_vecs, ret = 0;
>  	size_t copied = 0;
>  	size_t orig_count;
>  
> @@ -250,9 +250,9 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  	orig_count = iov_iter_count(dio->submit.iter);
>  	iov_iter_truncate(dio->submit.iter, length);
>  
> -	nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> -	if (nr_pages <= 0) {
> -		ret = nr_pages;
> +	nr_vecs = bio_iov_iter_nvecs(dio->submit.iter, BIO_MAX_PAGES);
> +	if (nr_vecs <= 0) {
> +		ret = nr_vecs;
>  		goto out;
>  	}
>  
> @@ -271,7 +271,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  			goto out;
>  		}
>  
> -		bio = bio_alloc(GFP_KERNEL, nr_pages);
> +		bio = bio_alloc(GFP_KERNEL, nr_vecs);
>  		bio_set_dev(bio, iomap->bdev);
>  		bio->bi_iter.bi_sector = iomap_sector(iomap, pos);
>  		bio->bi_write_hint = dio->iocb->ki_hint;
> @@ -308,10 +308,10 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		dio->size += n;
>  		copied += n;
>  
> -		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
> +		nr_vecs = bio_iov_iter_nvecs(dio->submit.iter, BIO_MAX_PAGES);
>  		iomap_dio_submit_bio(dio, iomap, bio, pos);
>  		pos += n;
> -	} while (nr_pages);
> +	} while (nr_vecs);
>  
>  	/*
>  	 * We need to zeroout the tail of a sub-block write if the extent type
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index ecf67108f091..b985857ce9d1 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -10,6 +10,7 @@
>  #include <linux/ioprio.h>
>  /* struct bio, bio_vec and BIO_* flags are defined in blk_types.h */
>  #include <linux/blk_types.h>
> +#include <linux/uio.h>
>  
>  #define BIO_DEBUG
>  
> @@ -807,4 +808,13 @@ static inline void bio_set_polled(struct bio *bio, struct kiocb *kiocb)
>  		bio->bi_opf |= REQ_NOWAIT;
>  }
>  
> +static inline int bio_iov_iter_nvecs(const struct iov_iter *i, int maxvecs)
> +{
> +	if (!iov_iter_count(i))
> +		return 0;
> +	if (iov_iter_is_bvec(i))
> +               return min_t(int, maxvecs, i->nr_segs);
> +	return iov_iter_npages(i, maxvecs);
> +}
> +
>  #endif /* __LINUX_BIO_H */
> 

-- 
Pavel Begunkov