Re: [PATCH 15/34] iomap: add an iomap-based readpage and readpages implementation

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Wed, May 23, 2018 at 04:43:38PM +0200, Christoph Hellwig wrote:
> Simply use iomap_apply to iterate over the file and a submit a bio for
> each non-uptodate but mapped region and zero everything else.  Note that
> as-is this can not be used for file systems with a blocksize smaller than
> the page size, but that support will be added later.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap.c            | 194 +++++++++++++++++++++++++++++++++++++++++-
>  include/linux/iomap.h |   4 +
>  2 files changed, 197 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/iomap.c b/fs/iomap.c
> index fa278ed338ce..78259a2249f4 100644
> --- a/fs/iomap.c
> +++ b/fs/iomap.c
> @@ -1,6 +1,6 @@
>  /*
>   * Copyright (C) 2010 Red Hat, Inc.
> - * Copyright (c) 2016 Christoph Hellwig.
> + * Copyright (c) 2016-2018 Christoph Hellwig.
>   *
>   * This program is free software; you can redistribute it and/or modify it
>   * under the terms and conditions of the GNU General Public License,
> @@ -18,6 +18,7 @@
>  #include <linux/uaccess.h>
>  #include <linux/gfp.h>
>  #include <linux/mm.h>
> +#include <linux/mm_inline.h>
>  #include <linux/swap.h>
>  #include <linux/pagemap.h>
>  #include <linux/pagevec.h>
> @@ -103,6 +104,197 @@ iomap_sector(struct iomap *iomap, loff_t pos)
>  	return (iomap->addr + pos - iomap->offset) >> SECTOR_SHIFT;
>  }
>  
> +static void
> +iomap_read_end_io(struct bio *bio)
> +{
> +	int error = blk_status_to_errno(bio->bi_status);
> +	struct bio_vec *bvec;
> +	int i;
> +
> +	bio_for_each_segment_all(bvec, bio, i)
> +		page_endio(bvec->bv_page, false, error);
> +	bio_put(bio);
> +}
> +
> +static struct bio *
> +iomap_read_bio_alloc(struct iomap *iomap, sector_t sector, loff_t length)
> +{
> +	int nr_vecs = (length + PAGE_SIZE - 1) >> PAGE_SHIFT;
> +	struct bio *bio = bio_alloc(GFP_NOFS, min(BIO_MAX_PAGES, nr_vecs));
> +
> +	bio->bi_opf = REQ_OP_READ;
> +	bio->bi_iter.bi_sector = sector;
> +	bio_set_dev(bio, iomap->bdev);
> +	bio->bi_end_io = iomap_read_end_io;
> +	return bio;
> +}
> +
> +struct iomap_readpage_ctx {
> +	struct page		*cur_page;
> +	bool			cur_page_in_bio;
> +	struct bio		*bio;
> +	struct list_head	*pages;
> +};
> +
> +static loff_t
> +iomap_readpage_actor(struct inode *inode, loff_t pos, loff_t length, void *data,
> +		struct iomap *iomap)
> +{
> +	struct iomap_readpage_ctx *ctx = data;
> +	struct page *page = ctx->cur_page;
> +	unsigned poff = pos & (PAGE_SIZE - 1);
> +	unsigned plen = min_t(loff_t, PAGE_SIZE - poff, length);
> +	bool is_contig = false;
> +	sector_t sector;
> +
> +	/* we don't support blocksize < PAGE_SIZE quite yet: */
> +	WARN_ON_ONCE(pos != page_offset(page));
> +	WARN_ON_ONCE(plen != PAGE_SIZE);
> +
> +	if (iomap->type != IOMAP_MAPPED || pos >= i_size_read(inode)) {
> +		zero_user(page, poff, plen);
> +		SetPageUptodate(page);
> +		goto done;
> +	}
> +
> +	ctx->cur_page_in_bio = true;
> +
> +	/*
> +	 * Try to merge into a previous segment if we can.
> +	 */
> +	sector = iomap_sector(iomap, pos);
> +	if (ctx->bio && bio_end_sector(ctx->bio) == sector) {
> +		if (__bio_try_merge_page(ctx->bio, page, plen, poff))
> +			goto done;
> +		is_contig = true;
> +	}
> +
> +	if (!ctx->bio || !is_contig || bio_full(ctx->bio)) {
> +		if (ctx->bio)
> +			submit_bio(ctx->bio);
> +		ctx->bio = iomap_read_bio_alloc(iomap, sector, length);
> +	}
> +
> +	__bio_add_page(ctx->bio, page, plen, poff);
> +done:
> +	return plen;
> +}
> +
> +int
> +iomap_readpage(struct page *page, const struct iomap_ops *ops)
> +{
> +	struct iomap_readpage_ctx ctx = { .cur_page = page };
> +	struct inode *inode = page->mapping->host;
> +	unsigned poff;
> +	loff_t ret;
> +
> +	WARN_ON_ONCE(page_has_buffers(page));
> +
> +	for (poff = 0; poff < PAGE_SIZE; poff += ret) {
> +		ret = iomap_apply(inode, page_offset(page) + poff,
> +				PAGE_SIZE - poff, 0, ops, &ctx,
> +				iomap_readpage_actor);
> +		if (ret <= 0) {
> +			WARN_ON_ONCE(ret == 0);
> +			SetPageError(page);
> +			break;
> +		}
> +	}
> +
> +	if (ctx.bio) {
> +		submit_bio(ctx.bio);
> +		WARN_ON_ONCE(!ctx.cur_page_in_bio);
> +	} else {
> +		WARN_ON_ONCE(ctx.cur_page_in_bio);
> +		unlock_page(page);
> +	}
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(iomap_readpage);
> +
> +static struct page *
> +iomap_next_page(struct inode *inode, struct list_head *pages, loff_t pos,
> +		loff_t length, loff_t *done)
> +{
> +	while (!list_empty(pages)) {
> +		struct page *page = lru_to_page(pages);
> +
> +		if (page_offset(page) >= (u64)pos + length)
> +			break;
> +
> +		list_del(&page->lru);
> +		if (!add_to_page_cache_lru(page, inode->i_mapping, page->index,
> +				GFP_NOFS))

I'm curious about this line -- if add_to_page_cache_lru returns an
error, why don't we want to send that back up the stack?  Is the idea
that the page doesn't become uptodate and something else notices?   It
seems a little odd that on error we just skip to the next page.

(If this /is/ correct then comment is needed here.)

--D

> +			return page;
> +
> +		*done += PAGE_SIZE;
> +		put_page(page);
> +	}
> +
> +	return NULL;
> +}
> +
> +static loff_t
> +iomap_readpages_actor(struct inode *inode, loff_t pos, loff_t length,
> +		void *data, struct iomap *iomap)
> +{
> +	struct iomap_readpage_ctx *ctx = data;
> +	loff_t done, ret;
> +
> +	for (done = 0; done < length; done += ret) {
> +		if (ctx->cur_page && ((pos + done) & (PAGE_SIZE - 1)) == 0) {
> +			if (!ctx->cur_page_in_bio)
> +				unlock_page(ctx->cur_page);
> +			put_page(ctx->cur_page);
> +			ctx->cur_page = NULL;
> +		}
> +		if (!ctx->cur_page) {
> +			ctx->cur_page = iomap_next_page(inode, ctx->pages,
> +					pos, length, &done);
> +			if (!ctx->cur_page)
> +				break;
> +			ctx->cur_page_in_bio = false;
> +		}
> +		ret = iomap_readpage_actor(inode, pos + done, length - done,
> +				ctx, iomap);
> +	}
> +
> +	return done;
> +}
> +
> +int
> +iomap_readpages(struct address_space *mapping, struct list_head *pages,
> +		unsigned nr_pages, const struct iomap_ops *ops)
> +{
> +	struct iomap_readpage_ctx ctx = { .pages = pages };
> +	loff_t pos = page_offset(list_entry(pages->prev, struct page, lru));
> +	loff_t last = page_offset(list_entry(pages->next, struct page, lru));
> +	loff_t length = last - pos + PAGE_SIZE, ret = 0;
> +
> +	while (length > 0) {
> +		ret = iomap_apply(mapping->host, pos, length, 0, ops,
> +				&ctx, iomap_readpages_actor);
> +		if (ret <= 0) {
> +			WARN_ON_ONCE(ret == 0);
> +			goto done;
> +		}
> +		pos += ret;
> +		length -= ret;
> +	}
> +	ret = 0;
> +done:
> +	if (ctx.bio)
> +		submit_bio(ctx.bio);
> +	if (ctx.cur_page) {
> +		if (!ctx.cur_page_in_bio)
> +			unlock_page(ctx.cur_page);
> +		put_page(ctx.cur_page);
> +	}
> +	WARN_ON_ONCE(!ret && !list_empty(ctx.pages));
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(iomap_readpages);
> +
>  static void
>  iomap_write_failed(struct inode *inode, loff_t pos, unsigned len)
>  {
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index a044a824da85..7300d30ca495 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -9,6 +9,7 @@ struct fiemap_extent_info;
>  struct inode;
>  struct iov_iter;
>  struct kiocb;
> +struct page;
>  struct vm_area_struct;
>  struct vm_fault;
>  
> @@ -88,6 +89,9 @@ struct iomap_ops {
>  
>  ssize_t iomap_file_buffered_write(struct kiocb *iocb, struct iov_iter *from,
>  		const struct iomap_ops *ops);
> +int iomap_readpage(struct page *page, const struct iomap_ops *ops);
> +int iomap_readpages(struct address_space *mapping, struct list_head *pages,
> +		unsigned nr_pages, const struct iomap_ops *ops);
>  int iomap_file_dirty(struct inode *inode, loff_t pos, loff_t len,
>  		const struct iomap_ops *ops);
>  int iomap_zero_range(struct inode *inode, loff_t pos, loff_t len,
> -- 
> 2.17.0
> 
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