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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Alexander Viro <viro@zeniv.linux.org.uk>,
        Benjamin LaHaise <bcrl@kvack.org>,
        linux-fsdevel@vger.kernel.org, linux-aio@kvack.org,
        linux-kernel@vger.kernel.org
Cc:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>,
        Ira Weiny <ira.weiny@intel.com>
Subject: [RESEND PATCH] fs/aio: Replace kmap{,_atomic}() with kmap_local_page()
Date:   Sun, 16 Oct 2022 17:06:56 +0200
Message-Id: <20221016150656.5803-1-fmdefrancesco@gmail.com>
MIME-Version: 1.0
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Precedence: bulk

The use of kmap() and kmap_atomic() are being deprecated in favor of
kmap_local_page().

There are two main problems with kmap(): (1) It comes with an overhead as
the mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap’s pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.

With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored and still valid.

Since its use in fs/aio.c is safe everywhere, it should be preferred.

Therefore, replace kmap() and kmap_atomic() with kmap_local_page() in
fs/aio.c.

Tested with xfstests on a QEMU/KVM x86_32 VM, 6GB RAM, booting a kernel
with HIGHMEM64GB enabled.

Cc: "Venkataramanan, Anirudh" <anirudh.venkataramanan@intel.com>
Suggested-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
---

I've tested with "./check -g aio". The tests in this group fail 3/26
times, with and without my patch. Therefore, these changes don't introduce
further errors. I'm not aware of any further tests I may run, so that
any suggestions would be precious and much appreciated :-)

I'm resending this patch because some recipients were missing in the 
previous submissions. In the meantime I'm also adding some more information 
in the commit message. There are no changes in the code.

 fs/aio.c | 32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

diff --git a/fs/aio.c b/fs/aio.c
index 3c249b938632..343fea0c6d1a 100644
--- a/fs/aio.c
+++ b/fs/aio.c
@@ -567,7 +567,7 @@ static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
 	ctx->user_id = ctx->mmap_base;
 	ctx->nr_events = nr_events; /* trusted copy */
 
-	ring = kmap_atomic(ctx->ring_pages[0]);
+	ring = kmap_local_page(ctx->ring_pages[0]);
 	ring->nr = nr_events;	/* user copy */
 	ring->id = ~0U;
 	ring->head = ring->tail = 0;
@@ -575,7 +575,7 @@ static int aio_setup_ring(struct kioctx *ctx, unsigned int nr_events)
 	ring->compat_features = AIO_RING_COMPAT_FEATURES;
 	ring->incompat_features = AIO_RING_INCOMPAT_FEATURES;
 	ring->header_length = sizeof(struct aio_ring);
-	kunmap_atomic(ring);
+	kunmap_local(ring);
 	flush_dcache_page(ctx->ring_pages[0]);
 
 	return 0;
@@ -678,9 +678,9 @@ static int ioctx_add_table(struct kioctx *ctx, struct mm_struct *mm)
 					 * we are protected from page migration
 					 * changes ring_pages by ->ring_lock.
 					 */
-					ring = kmap_atomic(ctx->ring_pages[0]);
+					ring = kmap_local_page(ctx->ring_pages[0]);
 					ring->id = ctx->id;
-					kunmap_atomic(ring);
+					kunmap_local(ring);
 					return 0;
 				}
 
@@ -1024,9 +1024,9 @@ static void user_refill_reqs_available(struct kioctx *ctx)
 		 * against ctx->completed_events below will make sure we do the
 		 * safe/right thing.
 		 */
-		ring = kmap_atomic(ctx->ring_pages[0]);
+		ring = kmap_local_page(ctx->ring_pages[0]);
 		head = ring->head;
-		kunmap_atomic(ring);
+		kunmap_local(ring);
 
 		refill_reqs_available(ctx, head, ctx->tail);
 	}
@@ -1132,12 +1132,12 @@ static void aio_complete(struct aio_kiocb *iocb)
 	if (++tail >= ctx->nr_events)
 		tail = 0;
 
-	ev_page = kmap_atomic(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
+	ev_page = kmap_local_page(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
 	event = ev_page + pos % AIO_EVENTS_PER_PAGE;
 
 	*event = iocb->ki_res;
 
-	kunmap_atomic(ev_page);
+	kunmap_local(ev_page);
 	flush_dcache_page(ctx->ring_pages[pos / AIO_EVENTS_PER_PAGE]);
 
 	pr_debug("%p[%u]: %p: %p %Lx %Lx %Lx\n", ctx, tail, iocb,
@@ -1151,10 +1151,10 @@ static void aio_complete(struct aio_kiocb *iocb)
 
 	ctx->tail = tail;
 
-	ring = kmap_atomic(ctx->ring_pages[0]);
+	ring = kmap_local_page(ctx->ring_pages[0]);
 	head = ring->head;
 	ring->tail = tail;
-	kunmap_atomic(ring);
+	kunmap_local(ring);
 	flush_dcache_page(ctx->ring_pages[0]);
 
 	ctx->completed_events++;
@@ -1214,10 +1214,10 @@ static long aio_read_events_ring(struct kioctx *ctx,
 	mutex_lock(&ctx->ring_lock);
 
 	/* Access to ->ring_pages here is protected by ctx->ring_lock. */
-	ring = kmap_atomic(ctx->ring_pages[0]);
+	ring = kmap_local_page(ctx->ring_pages[0]);
 	head = ring->head;
 	tail = ring->tail;
-	kunmap_atomic(ring);
+	kunmap_local(ring);
 
 	/*
 	 * Ensure that once we've read the current tail pointer, that
@@ -1249,10 +1249,10 @@ static long aio_read_events_ring(struct kioctx *ctx,
 		avail = min(avail, nr - ret);
 		avail = min_t(long, avail, AIO_EVENTS_PER_PAGE - pos);
 
-		ev = kmap(page);
+		ev = kmap_local_page(page);
 		copy_ret = copy_to_user(event + ret, ev + pos,
 					sizeof(*ev) * avail);
-		kunmap(page);
+		kunmap_local(ev);
 
 		if (unlikely(copy_ret)) {
 			ret = -EFAULT;
@@ -1264,9 +1264,9 @@ static long aio_read_events_ring(struct kioctx *ctx,
 		head %= ctx->nr_events;
 	}
 
-	ring = kmap_atomic(ctx->ring_pages[0]);
+	ring = kmap_local_page(ctx->ring_pages[0]);
 	ring->head = head;
-	kunmap_atomic(ring);
+	kunmap_local(ring);
 	flush_dcache_page(ctx->ring_pages[0]);
 
 	pr_debug("%li  h%u t%u\n", ret, head, tail);
-- 
2.36.1