Re: [PATCH] hugetlbfs: Take read_lock on i_mmap for PMD sharing

[Mike Kravetz <mike.kravetz@oracle.com>]

On 11/8/19 11:10 AM, Mike Kravetz wrote:
> On 11/7/19 6:04 PM, Davidlohr Bueso wrote:
>> On Thu, 07 Nov 2019, Mike Kravetz wrote:
>>
>>> Note that huge_pmd_share now increments the page count with the semaphore
>>> held just in read mode.  It is OK to do increments in parallel without
>>> synchronization.  However, we don't want anyone else changing the count
>>> while that check in huge_pmd_unshare is happening.  Hence, the need for
>>> taking the semaphore in write mode.
>>
>> This would be a nice addition to the changelog methinks.
> 
> Last night I remembered there is one place where we currently take
> i_mmap_rwsem in read mode and potentially call huge_pmd_unshare.  That
> is in try_to_unmap_one.  Yes, there is a potential race here today.

Actually there is no race there today.  Callers to huge_pmd_unshare
hold the page table lock.  So, this synchronizes those unshare calls
from  page migration and page poisoning.

> But that race is somewhat contained as you need two threads doing some
> combination of page migration and page poisoning to race.  This change
> now allows migration or poisoning to race with page fault.  I would
> really prefer if we do not open up the race window in this manner.

But, we do open a race window by changing huge_pmd_share to take the
i_mmap_rwsem in read mode as in the original patch.  

Here is the additional code needed to take the semaphore in write mode
for the huge_pmd_unshare calls via try_to_unmap_one.  We would need to
combine this with Longman's patch.  Please take a look and provide feedback.
Some of the changes are subtle, especially the exception for MAP_PRIVATE
mappings, but I tried to add sufficient comments.

From 21735818a520705c8573b8d543b8f91aa187bd5d Mon Sep 17 00:00:00 2001
From: Mike Kravetz <mike.kravetz@oracle.com>
Date: Fri, 8 Nov 2019 17:25:37 -0800
Subject: [PATCH] Changes needed for taking i_mmap_rwsem in write mode before
 call to huge_pmd_unshare in try_to_unmap_one.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
 mm/hugetlb.c        |  9 ++++++++-
 mm/memory-failure.c | 28 +++++++++++++++++++++++++++-
 mm/migrate.c        | 27 +++++++++++++++++++++++++--
 3 files changed, 60 insertions(+), 4 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index f78891f92765..73d9136549a5 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -4883,7 +4883,14 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
  * indicated by page_count > 1, unmap is achieved by clearing pud and
  * decrementing the ref count. If count == 1, the pte page is not shared.
  *
- * called with page table lock held.
+ * Must be called while holding page table lock.
+ * In general, the caller should also hold the i_mmap_rwsem in write mode.
+ * This is to prevent races with page faults calling huge_pmd_share which
+ * will not be holding the page table lock, but will be holding i_mmap_rwsem
+ * in read mode.  It is possible to call without holding i_mmap_rwsem in
+ * write mode if the caller KNOWS the page table is associated with a private
+ * mapping.  This is because private mappings can not share PMDs and can
+ * not race with huge_pmd_share calls during page faults.
  *
  * returns: 1 successfully unmapped a shared pte page
  *	    0 the underlying pte page is not shared, or it is the last user
diff --git a/mm/memory-failure.c b/mm/memory-failure.c
index 3151c87dff73..8f52b22cf71b 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -1030,7 +1030,33 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
 	if (kill)
 		collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);
 
-	unmap_success = try_to_unmap(hpage, ttu);
+	if (!PageHuge(hpage)) {
+		unmap_success = try_to_unmap(hpage, ttu);
+	} else {
+		mapping = page_mapping(hpage);
+		if (mapping) {
+			/*
+			 * For hugetlb pages, try_to_unmap could potentially
+			 * call huge_pmd_unshare.  Because of this, take
+			 * semaphore in write mode here and set TTU_RMAP_LOCKED
+			 * to indicate we have taken the lock at this higher
+			 * level.
+			 */
+			i_mmap_lock_write(mapping);
+			unmap_success = try_to_unmap(hpage,
+							ttu|TTU_RMAP_LOCKED);
+			i_mmap_unlock_write(mapping);
+		} else {
+			/*
+			 * !mapping implies a MAP_PRIVATE huge page mapping.
+			 * Since PMDs will never be shared in a private
+			 * mapping, it is safe to let huge_pmd_unshare be
+			 * called with the semaphore in read mode.
+			 */
+			unmap_success = try_to_unmap(hpage, ttu);
+		}
+	}
+
 	if (!unmap_success)
 		pr_err("Memory failure: %#lx: failed to unmap page (mapcount=%d)\n",
 		       pfn, page_mapcount(hpage));
diff --git a/mm/migrate.c b/mm/migrate.c
index 4fe45d1428c8..9cae5a4f1e48 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1333,8 +1333,31 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 		goto put_anon;
 
 	if (page_mapped(hpage)) {
-		try_to_unmap(hpage,
-			TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
+		struct address_space *mapping = page_mapping(hpage);
+
+		if (mapping) {
+			/*
+			 * try_to_unmap could potentially call huge_pmd_unshare.
+			 * Because of this, take semaphore in write mode here
+			 * and set TTU_RMAP_LOCKED to indicate we have taken
+			 * the lock at this higher level.
+			 */
+			i_mmap_lock_write(mapping);
+			try_to_unmap(hpage,
+				TTU_MIGRATION|TTU_IGNORE_MLOCK|
+				TTU_IGNORE_ACCESS|TTU_RMAP_LOCKED);
+			i_mmap_unlock_write(mapping);
+		} else {
+			/*
+			 * !mapping implies a MAP_PRIVATE huge page mapping.
+			 * Since PMDs will never be shared in a private
+			 * mapping, it is safe to let huge_pmd_unshare be
+			 * called with the semaphore in read mode.
+			 */
+			try_to_unmap(hpage,
+				TTU_MIGRATION|TTU_IGNORE_MLOCK|
+				TTU_IGNORE_ACCESS);
+		}
 		page_was_mapped = 1;
 	}
 
-- 
2.23.0