linux-kernel - Re: [PATCH v5 8/9] iov_iter, block: Make bio structs pin pages rather than ref'ing if appropriate

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Message-ID: <Y7+6YVkhZsvdW+Hr@infradead.org>
Date:   Wed, 11 Jan 2023 23:44:33 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>, Jens Axboe <axboe@...nel.dk>,
        Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
        Matthew Wilcox <willy@...radead.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-block@...r.kernel.org, Christoph Hellwig <hch@...radead.org>,
        Jeff Layton <jlayton@...nel.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 8/9] iov_iter, block: Make bio structs pin pages
 rather than ref'ing if appropriate

On Wed, Jan 11, 2023 at 02:28:35PM +0000, David Howells wrote:
> [!] Note that this is tested a bit with ext4, but nothing else.

You probably want to also at least test it with block device I/O
as that is a slightly different I/O path from iomap.  More file systems
also never hurt, but aren't quite as important.

> +/*
> + * Clean up a page appropriately, where the page may be pinned, may have a
> + * ref taken on it or neither.
> + */
> +static void bio_release_page(struct bio *bio, struct page *page)
> +{
> +	if (bio_flagged(bio, BIO_PAGE_PINNED))
> +		unpin_user_page(page);
> +	if (bio_flagged(bio, BIO_PAGE_REFFED))
> +		put_page(page);
> +}
> +
>  void __bio_release_pages(struct bio *bio, bool mark_dirty)
>  {
>  	struct bvec_iter_all iter_all;
> @@ -1183,7 +1197,7 @@ 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);

So this does look correc an sensible, but given that the new pin/unpin
path has a significantly higher overhead I wonder if this might be a
good time to switch to folios here as soon as possible in a follow on
patch.


> +	size = iov_iter_extract_pages(iter, &pages,
> +				      UINT_MAX - bio->bi_iter.bi_size,
> +				      nr_pages, gup_flags,
> +				      &offset, &cleanup_mode);
>  	if (unlikely(size <= 0))
>  		return size ? size : -EFAULT;
>  
> +	bio_clear_flag(bio, BIO_PAGE_REFFED);
> +	bio_clear_flag(bio, BIO_PAGE_PINNED);
> +	if (cleanup_mode & FOLL_GET)
> +		bio_set_flag(bio, BIO_PAGE_REFFED);
> +	if (cleanup_mode & FOLL_PIN)
> +		bio_set_flag(bio, BIO_PAGE_PINNED);

The flags here must not change from one invocation to another, so
clearing and resetting them on every iteration seems dangerous.

This should probably be a:

	if (cleanup_mode & FOLL_GET) {
		WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_PINNED));
		bio_set_flag(bio, BIO_PAGE_REFFED);
	}
	if (cleanup_mode & FOLL_PIN) {
		WARN_ON_ONCE(bio_test_flag(bio, BIO_PAGE_REFFED));
		bio_set_flag(bio, BIO_PAGE_PINNED);
	}