linux-kernel - Re: [PATCH v8 07/10] block: Switch to pinning pages.

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Date:   Tue, 24 Jan 2023 15:03:53 +0000
From:   David Howells <dhowells@...hat.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     dhowells@...hat.com, David Hildenbrand <david@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>,
        Jens Axboe <axboe@...nel.dk>, Jan Kara <jack@...e.cz>,
        Jeff Layton <jlayton@...nel.org>,
        Logan Gunthorpe <logang@...tatee.com>,
        linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v8 07/10] block: Switch to pinning pages.

Christoph Hellwig <hch@...radead.org> wrote:

> It can't.  Per your latest branch:

Yes it can.  Patch 6:

--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -282,6 +282,7 @@ static int bio_map_user_iov(struct request *rq, struct iov_iter *iter,
 	if (blk_queue_pci_p2pdma(rq->q))
 		extract_flags |= ITER_ALLOW_P2PDMA;
 
+	bio_set_flag(bio, BIO_PAGE_REFFED);
 	while (iov_iter_count(iter)) {
 		struct page **pages, *stack_pages[UIO_FASTIOV];
 		ssize_t bytes;

Patch 7:

+static inline unsigned int bio_to_gup_flags(struct bio *bio)
+{
+	return (bio_flagged(bio, BIO_PAGE_REFFED) ? FOLL_GET : 0) |
+		(bio_flagged(bio, BIO_PAGE_PINNED) ? FOLL_PIN : 0);
+}
+
+/*
+ * Clean up a page appropriately, where the page may be pinned, may have a
+ * ref taken on it or neither.
+ */
+static inline void bio_release_page(struct bio *bio, struct page *page)
+{
+	page_put_unpin(page, bio_to_gup_flags(bio));
+}

At patch 8, you can get either FOLL_GET, FOLL_PIN or 0, depending on the path
you go through.

David