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Date: Thu, 29 Feb 2024 19:53:29 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Tony Battersby <tonyb@cybernetics.com>
Cc: Jens Axboe <axboe@kernel.dk>, Andrew Morton <akpm@linux-foundation.org>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
	Hugh Dickins <hughd@google.com>, Hannes Reinecke <hare@suse.de>,
	Keith Busch <kbusch@kernel.org>, linux-mm <linux-mm@kvack.org>,
	linux-block@vger.kernel.org,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] block: Fix page refcounts for unaligned buffers in
 __bio_release_pages()
Message-ID: <ZeDguZujxets0KtD@casper.infradead.org>
References: <86e592a9-98d4-4cff-a646-0c0084328356@cybernetics.com>
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In-Reply-To: <86e592a9-98d4-4cff-a646-0c0084328356@cybernetics.com>

On Thu, Feb 29, 2024 at 01:08:09PM -0500, Tony Battersby wrote:
> Fix an incorrect number of pages being released for buffers that do not
> start at the beginning of a page.

Oh, I see what I did.  Wouldn't a simpler fix be to just set "done" to
offset_in_page(fi.offset)?

> @@ -1152,7 +1152,7 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
>  
>  	bio_for_each_folio_all(fi, bio) {
>  		struct page *page;
> -		size_t done = 0;
> +		size_t nr_pages;
>  
>  		if (mark_dirty) {
>  			folio_lock(fi.folio);
> @@ -1160,10 +1160,11 @@ void __bio_release_pages(struct bio *bio, bool mark_dirty)
>  			folio_unlock(fi.folio);
>  		}
>  		page = folio_page(fi.folio, fi.offset / PAGE_SIZE);
> +		nr_pages = (fi.offset + fi.length - 1) / PAGE_SIZE -
> +			   fi.offset / PAGE_SIZE + 1;
>  		do {
>  			bio_release_page(bio, page++);
> -			done += PAGE_SIZE;
> -		} while (done < fi.length);
> +		} while (--nr_pages != 0);
>  	}
>  }
>  EXPORT_SYMBOL_GPL(__bio_release_pages);

The long-term path here, I think, is to replace this bio_release_page()
with a bio_release_folio(folio, offset, length) which calls into
a new unpin_user_folio(folio, nr) which calls gup_put_folio().