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Date: Mon, 9 Jan 2023 18:35:13 +0100
From: Jan Kara <jack@...e.cz>
To: David Howells <dhowells@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...nel.dk>,
Christoph Hellwig <hch@....de>,
Matthew Wilcox <willy@...radead.org>,
Logan Gunthorpe <logang@...tatee.com>,
Christoph Hellwig <hch@...radead.org>,
Jeff Layton <jlayton@...nel.org>,
linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 7/7] iov_iter, block: Make bio structs pin pages
rather than ref'ing if appropriate
On Sat 07-01-23 00:34:21, David Howells wrote:
> Convert the block layer's bio code to use iov_iter_extract_pages() instead
> of iov_iter_get_pages(). This will pin pages or leave them unaltered
> rather than getting a ref on them as appropriate to the source iterator.
>
> A field, bi_cleanup_mode, is added to the bio struct that gets set by
> iov_iter_extract_pages() with FOLL_* flags indicating what cleanup is
> necessary. FOLL_GET -> put_page(), FOLL_PIN -> unpin_user_page(). Other
> flags could also be used in future.
>
> Newly allocated bio structs have bi_cleanup_mode set to FOLL_GET to
> indicate that attached pages are ref'd by default. Cloning sets it to 0.
> __bio_iov_iter_get_pages() overrides it to what iov_iter_extract_pages()
> indicates.
>
> [!] Note that this is tested a bit with ext4, but nothing else.
>
> Signed-off-by: David Howells <dhowells@...hat.com>
> cc: Al Viro <viro@...iv.linux.org.uk>
> cc: Jens Axboe <axboe@...nel.dk>
> cc: Christoph Hellwig <hch@....de>
> cc: Matthew Wilcox <willy@...radead.org>
> cc: Logan Gunthorpe <logang@...tatee.com>
So currently we already have BIO_NO_PAGE_REF flag and what you do in this
patch partially duplicates that. So either I'd drop that flag or instead of
bi_cleanup_mode variable (which honestly looks a bit wasteful given how we
microoptimize struct bio) just add another BIO_ flag...
Honza
> ---
>
> block/bio.c | 47 +++++++++++++++++++++++++++++++++------------
> include/linux/blk_types.h | 1 +
> 2 files changed, 35 insertions(+), 13 deletions(-)
>
> diff --git a/block/bio.c b/block/bio.c
> index 5f96fcae3f75..eafcbeba0bab 100644
> --- a/block/bio.c
> +++ b/block/bio.c
> @@ -243,6 +243,11 @@ static void bio_free(struct bio *bio)
> * Users of this function have their own bio allocation. Subsequently,
> * they must remember to pair any call to bio_init() with bio_uninit()
> * when IO has completed, or when the bio is released.
> + *
> + * We set the initial assumption that pages attached to the bio will be
> + * released with put_page() by setting bi_cleanup_mode to FOLL_GET, but this
> + * should be set to FOLL_PIN if the page should be unpinned instead; if the
> + * pages should not be put or unpinned, this should be set to 0
> */
> void bio_init(struct bio *bio, struct block_device *bdev, struct bio_vec *table,
> unsigned short max_vecs, blk_opf_t opf)
> @@ -274,6 +279,7 @@ void bio_init(struct bio *bio, struct block_device *bdev, struct bio_vec *table,
> #ifdef CONFIG_BLK_DEV_INTEGRITY
> bio->bi_integrity = NULL;
> #endif
> + bio->bi_cleanup_mode = FOLL_GET;
> bio->bi_vcnt = 0;
>
> atomic_set(&bio->__bi_remaining, 1);
> @@ -302,6 +308,7 @@ void bio_reset(struct bio *bio, struct block_device *bdev, blk_opf_t opf)
> {
> bio_uninit(bio);
> memset(bio, 0, BIO_RESET_BYTES);
> + bio->bi_cleanup_mode = FOLL_GET;
> atomic_set(&bio->__bi_remaining, 1);
> bio->bi_bdev = bdev;
> if (bio->bi_bdev)
> @@ -814,6 +821,7 @@ static int __bio_clone(struct bio *bio, struct bio *bio_src, gfp_t gfp)
> bio_set_flag(bio, BIO_CLONED);
> bio->bi_ioprio = bio_src->bi_ioprio;
> bio->bi_iter = bio_src->bi_iter;
> + bio->bi_cleanup_mode = 0;
>
> if (bio->bi_bdev) {
> if (bio->bi_bdev == bio_src->bi_bdev &&
> @@ -1168,6 +1176,18 @@ bool bio_add_folio(struct bio *bio, struct folio *folio, size_t len,
> return bio_add_page(bio, &folio->page, len, off) > 0;
> }
>
> +/*
> + * Clean up a page according to the mode indicated by iov_iter_extract_pages(),
> + * where the page is may be pinned or may have a ref taken on it.
> + */
> +static void bio_release_page(struct bio *bio, struct page *page)
> +{
> + if (bio->bi_cleanup_mode & FOLL_PIN)
> + unpin_user_page(page);
> + if (bio->bi_cleanup_mode & FOLL_GET)
> + put_page(page);
> +}
> +
> void __bio_release_pages(struct bio *bio, bool mark_dirty)
> {
> struct bvec_iter_all iter_all;
> @@ -1176,7 +1196,7 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
> bio_for_each_segment_all(bvec, bio, iter_all) {
> if (mark_dirty && !PageCompound(bvec->bv_page))
> set_page_dirty_lock(bvec->bv_page);
> - put_page(bvec->bv_page);
> + bio_release_page(bio, bvec->bv_page);
> }
> }
> EXPORT_SYMBOL_GPL(__bio_release_pages);
> @@ -1213,7 +1233,7 @@ static int bio_iov_add_page(struct bio *bio, struct page *page,
> }
>
> if (same_page)
> - put_page(page);
> + bio_release_page(bio, page);
> return 0;
> }
>
> @@ -1227,7 +1247,7 @@ static int bio_iov_add_zone_append_page(struct bio *bio, struct page *page,
> queue_max_zone_append_sectors(q), &same_page) != len)
> return -EINVAL;
> if (same_page)
> - put_page(page);
> + bio_release_page(bio, page);
> return 0;
> }
>
> @@ -1238,10 +1258,10 @@ static int bio_iov_add_zone_append_page(struct bio *bio, struct page *page,
> * @bio: bio to add pages to
> * @iter: iov iterator describing the region to be mapped
> *
> - * Pins pages from *iter and appends them to @bio's bvec array. The
> - * pages will have to be released using put_page() when done.
> - * For multi-segment *iter, this function only adds pages from the
> - * next non-empty segment of the iov iterator.
> + * Pins pages from *iter and appends them to @bio's bvec array. The pages will
> + * have to be released using put_page() or unpin_user_page() when done as
> + * according to bi_cleanup_mode. For multi-segment *iter, this function only
> + * adds pages from the next non-empty segment of the iov iterator.
> */
> static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> {
> @@ -1273,9 +1293,10 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> * result to ensure the bio's total size is correct. The remainder of
> * the iov data will be picked up in the next bio iteration.
> */
> - size = iov_iter_get_pages(iter, pages,
> - UINT_MAX - bio->bi_iter.bi_size,
> - nr_pages, &offset, gup_flags);
> + size = iov_iter_extract_pages(iter, &pages,
> + UINT_MAX - bio->bi_iter.bi_size,
> + nr_pages, gup_flags,
> + &offset, &bio->bi_cleanup_mode);
> if (unlikely(size <= 0))
> return size ? size : -EFAULT;
>
> @@ -1308,7 +1329,7 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> iov_iter_revert(iter, left);
> out:
> while (i < nr_pages)
> - put_page(pages[i++]);
> + bio_release_page(bio, pages[i++]);
>
> return ret;
> }
> @@ -1489,8 +1510,8 @@ void bio_set_pages_dirty(struct bio *bio)
> * the BIO and re-dirty the pages in process context.
> *
> * It is expected that bio_check_pages_dirty() will wholly own the BIO from
> - * here on. It will run one put_page() against each page and will run one
> - * bio_put() against the BIO.
> + * here on. It will run one put_page() or unpin_user_page() against each page
> + * and will run one bio_put() against the BIO.
> */
>
> static void bio_dirty_fn(struct work_struct *work);
> diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
> index 99be590f952f..883f873a01ef 100644
> --- a/include/linux/blk_types.h
> +++ b/include/linux/blk_types.h
> @@ -289,6 +289,7 @@ struct bio {
> #endif
> };
>
> + unsigned int bi_cleanup_mode; /* How to clean up pages */
> unsigned short bi_vcnt; /* how many bio_vec's */
>
> /*
>
>
--
Jan Kara <jack@...e.com>
SUSE Labs, CR
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