linux-kernel - [PATCH 5.3 172/197] fs/dax: Fix pmd vs pte conflict detection

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Message-Id: <20191027203404.171724171@linuxfoundation.org>
Date:   Sun, 27 Oct 2019 22:01:30 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Jeff Smits <jeff.smits@...el.com>,
        Doug Nelson <doug.nelson@...el.com>, Jan Kara <jack@...e.cz>,
        Jeff Moyer <jmoyer@...hat.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Dan Williams <dan.j.williams@...el.com>
Subject: [PATCH 5.3 172/197] fs/dax: Fix pmd vs pte conflict detection

From: Dan Williams <dan.j.williams@...el.com>

commit 6370740e5f8ef12de7f9a9bf48a0393d202cd827 upstream.

Users reported a v5.3 performance regression and inability to establish
huge page mappings. A revised version of the ndctl "dax.sh" huge page
unit test identifies commit 23c84eb78375 "dax: Fix missed wakeup with
PMD faults" as the source.

Update get_unlocked_entry() to check for NULL entries before checking
the entry order, otherwise NULL is misinterpreted as a present pte
conflict. The 'order' check needs to happen before the locked check as
an unlocked entry at the wrong order must fallback to lookup the correct
order.

Reported-by: Jeff Smits <jeff.smits@...el.com>
Reported-by: Doug Nelson <doug.nelson@...el.com>
Cc: <stable@...r.kernel.org>
Fixes: 23c84eb78375 ("dax: Fix missed wakeup with PMD faults")
Reviewed-by: Jan Kara <jack@...e.cz>
Cc: Jeff Moyer <jmoyer@...hat.com>
Cc: Matthew Wilcox (Oracle) <willy@...radead.org>
Reviewed-by: Johannes Thumshirn <jthumshirn@...e.de>
Link: https://lore.kernel.org/r/157167532455.3945484.11971474077040503994.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@...el.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 fs/dax.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/fs/dax.c
+++ b/fs/dax.c
@@ -220,10 +220,11 @@ static void *get_unlocked_entry(struct x
 
 	for (;;) {
 		entry = xas_find_conflict(xas);
+		if (!entry || WARN_ON_ONCE(!xa_is_value(entry)))
+			return entry;
 		if (dax_entry_order(entry) < order)
 			return XA_RETRY_ENTRY;
-		if (!entry || WARN_ON_ONCE(!xa_is_value(entry)) ||
-				!dax_is_locked(entry))
+		if (!dax_is_locked(entry))
 			return entry;
 
 		wq = dax_entry_waitqueue(xas, entry, &ewait.key);