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Date:   Tue, 15 Dec 2020 00:20:23 +0000
From:   Pavel Begunkov <asml.silence@...il.com>
To:     linux-block@...r.kernel.org
Cc:     Jens Axboe <axboe@...nel.dk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Ming Lei <ming.lei@...hat.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J . Wong" <darrick.wong@...cle.com>,
        "Martin K . Petersen" <martin.petersen@...cle.com>,
        Jonathan Corbet <corbet@....net>, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, io-uring@...r.kernel.org,
        linux-kernel@...r.kernel.org, target-devel@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-doc@...r.kernel.org
Subject: [PATCH v1 4/6] block/psi: remove PSI annotations from direct IO

As reported, we must not do pressure stall information accounting for
direct IO, because otherwise it tells that it's thrashing a page when
actually doing IO on hot data.

Apparently, bio_iov_iter_get_pages() is used only by paths doing direct
IO, so just make it avoid setting BIO_WORKINGSET, it also saves us CPU
cycles on doing that. For fs/direct-io.c just clear the flag before
submit_bio(), it's not of much concern performance-wise.

Reported-by: Christoph Hellwig <hch@...radead.org>
Suggested-by: Christoph Hellwig <hch@...radead.org>
Suggested-by: Johannes Weiner <hannes@...xchg.org>
Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
---
 block/bio.c    | 25 ++++++++++++++++---------
 fs/direct-io.c |  2 ++
 2 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/block/bio.c b/block/bio.c
index 4a8f77bb3956..3192358c411f 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -963,18 +963,22 @@ EXPORT_SYMBOL_GPL(bio_release_pages);
 static int __bio_iov_bvec_add_pages(struct bio *bio, struct iov_iter *iter)
 {
 	const struct bio_vec *bv = iter->bvec;
-	unsigned int len;
-	size_t size;
+	struct page *page = bv->bv_page;
+	bool same_page = false;
+	unsigned int off, len;
 
 	if (WARN_ON_ONCE(iter->iov_offset > bv->bv_len))
 		return -EINVAL;
 
 	len = min_t(size_t, bv->bv_len - iter->iov_offset, iter->count);
-	size = bio_add_page(bio, bv->bv_page, len,
-				bv->bv_offset + iter->iov_offset);
-	if (unlikely(size != len))
-		return -EINVAL;
-	iov_iter_advance(iter, size);
+	off = bv->bv_offset + iter->iov_offset;
+
+	if (!__bio_try_merge_page(bio, page, len, off, &same_page)) {
+		if (bio_full(bio, len))
+			return -EINVAL;
+		bio_add_page_noaccount(bio, page, len, off);
+	}
+	iov_iter_advance(iter, len);
 	return 0;
 }
 
@@ -1023,8 +1027,8 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 				put_page(page);
 		} else {
 			if (WARN_ON_ONCE(bio_full(bio, len)))
-                                return -EINVAL;
-			__bio_add_page(bio, page, len, offset);
+				return -EINVAL;
+			bio_add_page_noaccount(bio, page, len, offset);
 		}
 		offset = 0;
 	}
@@ -1099,6 +1103,9 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
  * fit into the bio, or are requested in @iter, whatever is smaller. If
  * MM encounters an error pinning the requested pages, it stops. Error
  * is returned only if 0 pages could be pinned.
+ *
+ * It also doesn't set BIO_WORKINGSET, so is intended for direct IO. If used
+ * otherwise the caller is responsible to do that to keep PSI happy.
  */
 int bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
 {
diff --git a/fs/direct-io.c b/fs/direct-io.c
index d53fa92a1ab6..914a7f600ecd 100644
--- a/fs/direct-io.c
+++ b/fs/direct-io.c
@@ -426,6 +426,8 @@ static inline void dio_bio_submit(struct dio *dio, struct dio_submit *sdio)
 	unsigned long flags;
 
 	bio->bi_private = dio;
+	/* PSI is only for paging IO */
+	bio_clear_flag(bio, BIO_WORKINGSET);
 
 	spin_lock_irqsave(&dio->bio_lock, flags);
 	dio->refcount++;
-- 
2.24.0

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