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Message-Id: <1425575884-2574-17-git-send-email-aarcange@redhat.com>
Date: Thu, 5 Mar 2015 18:17:59 +0100
From: Andrea Arcangeli <aarcange@...hat.com>
To: qemu-devel@...gnu.org, kvm@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-mm@...ck.org,
linux-api@...r.kernel.org,
Android Kernel Team <kernel-team@...roid.com>
Cc: "Kirill A. Shutemov" <kirill@...temov.name>,
Pavel Emelyanov <xemul@...allels.com>,
Sanidhya Kashyap <sanidhya.gatech@...il.com>,
zhang.zhanghailiang@...wei.com,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andres Lagar-Cavilla <andreslc@...gle.com>,
Dave Hansen <dave@...1.net>,
Paolo Bonzini <pbonzini@...hat.com>,
Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
Andy Lutomirski <luto@...capital.net>,
Andrew Morton <akpm@...ux-foundation.org>,
Sasha Levin <sasha.levin@...cle.com>,
Hugh Dickins <hughd@...gle.com>,
Peter Feiner <pfeiner@...gle.com>,
"Dr. David Alan Gilbert" <dgilbert@...hat.com>,
Christopher Covington <cov@...eaurora.org>,
Johannes Weiner <hannes@...xchg.org>,
Robert Love <rlove@...gle.com>,
Dmitry Adamushko <dmitry.adamushko@...il.com>,
Neil Brown <neilb@...e.de>, Mike Hommey <mh@...ndium.org>,
Taras Glek <tglek@...illa.com>, Jan Kara <jack@...e.cz>,
KOSAKI Motohiro <kosaki.motohiro@...il.com>,
Michel Lespinasse <walken@...gle.com>,
Minchan Kim <minchan@...nel.org>,
Keith Packard <keithp@...thp.com>,
"Huangpeng (Peter)" <peter.huangpeng@...wei.com>,
Anthony Liguori <anthony@...emonkey.ws>,
Stefan Hajnoczi <stefanha@...il.com>,
Wenchao Xia <wenchaoqemu@...il.com>,
Andrew Jones <drjones@...hat.com>,
Juan Quintela <quintela@...hat.com>
Subject: [PATCH 16/21] userfaultfd: remap_pages: rmap preparation
As far as the rmap code is concerned, rmap_pages only alters the
page->mapping and page->index. It does it while holding the page
lock. However there are a few places that in presence of anon pages
are allowed to do rmap walks without the page lock (split_huge_page
and page_referenced_anon). Those places that are doing rmap walks
without taking the page lock first, must be updated to re-check that
the page->mapping didn't change after they obtained the anon_vma
lock. remap_pages takes the anon_vma lock for writing before altering
the page->mapping, so if the page->mapping is still the same after
obtaining the anon_vma lock (without the page lock), the rmap walks
can go ahead safely (and remap_pages will wait them to complete before
proceeding).
remap_pages serializes against itself with the page lock.
All other places taking the anon_vma lock while holding the mmap_sem
for writing, don't need to check if the page->mapping has changed
after taking the anon_vma lock, regardless of the page lock, because
remap_pages holds the mmap_sem for reading.
There's one constraint enforced to allow this simplification: the
source pages passed to remap_pages must be mapped only in one vma, but
this is not a limitation when used to handle userland page faults. The
source addresses passed to remap_pages should be set as VM_DONTCOPY
with MADV_DONTFORK to avoid any risk of the mapcount of the pages
increasing, if fork runs in parallel in another thread, before or
while remap_pages runs.
Signed-off-by: Andrea Arcangeli <aarcange@...hat.com>
---
mm/huge_memory.c | 23 +++++++++++++++++++----
mm/rmap.c | 9 +++++++++
2 files changed, 28 insertions(+), 4 deletions(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 8f1b6a5..1e25cb3 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1902,6 +1902,7 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
{
struct anon_vma *anon_vma;
int ret = 1;
+ struct address_space *mapping;
BUG_ON(is_huge_zero_page(page));
BUG_ON(!PageAnon(page));
@@ -1913,10 +1914,24 @@ int split_huge_page_to_list(struct page *page, struct list_head *list)
* page_lock_anon_vma_read except the write lock is taken to serialise
* against parallel split or collapse operations.
*/
- anon_vma = page_get_anon_vma(page);
- if (!anon_vma)
- goto out;
- anon_vma_lock_write(anon_vma);
+ for (;;) {
+ mapping = ACCESS_ONCE(page->mapping);
+ anon_vma = page_get_anon_vma(page);
+ if (!anon_vma)
+ goto out;
+ anon_vma_lock_write(anon_vma);
+ /*
+ * We don't hold the page lock here so
+ * remap_pages_huge_pmd can change the anon_vma from
+ * under us until we obtain the anon_vma lock. Verify
+ * that we obtained the anon_vma lock before
+ * remap_pages did.
+ */
+ if (likely(mapping == ACCESS_ONCE(page->mapping)))
+ break;
+ anon_vma_unlock_write(anon_vma);
+ put_anon_vma(anon_vma);
+ }
ret = 0;
if (!PageCompound(page))
diff --git a/mm/rmap.c b/mm/rmap.c
index 5e3e090..5ab2df1 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -492,6 +492,7 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
struct anon_vma *root_anon_vma;
unsigned long anon_mapping;
+repeat:
rcu_read_lock();
anon_mapping = (unsigned long) ACCESS_ONCE(page->mapping);
if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
@@ -530,6 +531,14 @@ struct anon_vma *page_lock_anon_vma_read(struct page *page)
rcu_read_unlock();
anon_vma_lock_read(anon_vma);
+ /* check if remap_anon_pages changed the anon_vma */
+ if (unlikely((unsigned long) ACCESS_ONCE(page->mapping) != anon_mapping)) {
+ anon_vma_unlock_read(anon_vma);
+ put_anon_vma(anon_vma);
+ anon_vma = NULL;
+ goto repeat;
+ }
+
if (atomic_dec_and_test(&anon_vma->refcount)) {
/*
* Oops, we held the last refcount, release the lock
--
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