hugetlbfs: fix races and page leaks during migration

mjkravetz · torvalds · commit cb6acd01e2e4 · 2019-03-01T09:02:33.000-08:00
hugetlb pages should only be migrated if they are 'active'. The routines set/clear_page_huge_active() modify the active state of hugetlb pages. When a new hugetlb page is allocated at fault time, set_page_huge_active is called before the page is locked. Therefore, another thread could race and migrate the page while it is being added to page table by the fault code. This race is somewhat hard to trigger, but can be seen by strategically adding udelay to simulate worst case scheduling behavior. Depending on 'how' the code races, various BUG()s could be triggered. To address this issue, simply delay the set_page_huge_active call until after the page is successfully added to the page table. Hugetlb pages can also be leaked at migration time if the pages are associated with a file in an explicitly mounted hugetlbfs filesystem. For example, consider a two node system with 4GB worth of huge pages available. A program mmaps a 2G file in a hugetlbfs filesystem. It then migrates the pages associated with the file from one node to another. When the program exits, huge page counts are as follows: node0 1024 free_hugepages 1024 nr_hugepages node1 0 free_hugepages 1024 nr_hugepages Filesystem Size Used Avail Use% Mounted on nodev 4.0G 2.0G 2.0G 50% /var/opt/hugepool That is as expected. 2G of huge pages are taken from the free_hugepages counts, and 2G is the size of the file in the explicitly mounted filesystem. If the file is then removed, the counts become: node0 1024 free_hugepages 1024 nr_hugepages node1 1024 free_hugepages 1024 nr_hugepages Filesystem Size Used Avail Use% Mounted on nodev 4.0G 2.0G 2.0G 50% /var/opt/hugepool Note that the filesystem still shows 2G of pages used, while there actually are no huge pages in use. The only way to 'fix' the filesystem accounting is to unmount the filesystem If a hugetlb page is associated with an explicitly mounted filesystem, this information in contained in the page_private field. At migration time, this information is not preserved. To fix, simply transfer page_private from old to new page at migration time if necessary. There is a related race with removing a huge page from a file and migration. When a huge page is removed from the pagecache, the page_mapping() field is cleared, yet page_private remains set until the page is actually freed by free_huge_page(). A page could be migrated while in this state. However, since page_mapping() is not set the hugetlbfs specific routine to transfer page_private is not called and we leak the page count in the filesystem. To fix that, check for this condition before migrating a huge page. If the condition is detected, return EBUSY for the page. Link: http://lkml.kernel.org/r/74510272-7319-7372-9ea6-ec914734c179@oracle.com Link: http://lkml.kernel.org/r/20190212221400.3512-1-mike.kravetz@oracle.com Fixes: bcc5422 ("mm: hugetlb: introduce page_huge_active") Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com> Reviewed-by: Naoya Horiguchi <n-horiguchi@ah.jp.nec.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Andrea Arcangeli <aarcange@redhat.com> Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com> Cc: Mel Gorman <mgorman@techsingularity.net> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: <stable@vger.kernel.org> [mike.kravetz@oracle.com: v2] Link: http://lkml.kernel.org/r/7534d322-d782-8ac6-1c8d-a8dc380eb3ab@oracle.com [mike.kravetz@oracle.com: update comment and changelog] Link: http://lkml.kernel.org/r/420bcfd6-158b-38e4-98da-26d0cd85bd01@oracle.com Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
@@ -859,6 +859,18 @@ static int hugetlbfs_migrate_page(struct address_space *mapping,
 	rc = migrate_huge_page_move_mapping(mapping, newpage, page);
 	if (rc != MIGRATEPAGE_SUCCESS)
 		return rc;
+
+	/*
+	 * page_private is subpool pointer in hugetlb pages.  Transfer to
+	 * new page.  PagePrivate is not associated with page_private for
+	 * hugetlb pages and can not be set here as only page_huge_active
+	 * pages can be migrated.
+	 */
+	if (page_private(page)) {
+		set_page_private(newpage, page_private(page));
+		set_page_private(page, 0);
+	}
+
 	if (mode != MIGRATE_SYNC_NO_COPY)
 		migrate_page_copy(newpage, page);
 	else
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
@@ -3624,7 +3624,6 @@ static vm_fault_t hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
 	copy_user_huge_page(new_page, old_page, address, vma,
 			    pages_per_huge_page(h));
 	__SetPageUptodate(new_page);
-	set_page_huge_active(new_page);
 
 	mmu_notifier_range_init(&range, mm, haddr, haddr + huge_page_size(h));
 	mmu_notifier_invalidate_range_start(&range);
@@ -3645,6 +3644,7 @@ static vm_fault_t hugetlb_cow(struct mm_struct *mm, struct vm_area_struct *vma,
 				make_huge_pte(vma, new_page, 1));
 		page_remove_rmap(old_page, true);
 		hugepage_add_new_anon_rmap(new_page, vma, haddr);
+		set_page_huge_active(new_page);
 		/* Make the old page be freed below */
 		new_page = old_page;
 	}
@@ -3729,6 +3729,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 	pte_t new_pte;
 	spinlock_t *ptl;
 	unsigned long haddr = address & huge_page_mask(h);
+	bool new_page = false;
 
 	/*
 	 * Currently, we are forced to kill the process in the event the
@@ -3790,7 +3791,7 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 		}
 		clear_huge_page(page, address, pages_per_huge_page(h));
 		__SetPageUptodate(page);
-		set_page_huge_active(page);
+		new_page = true;
 
 		if (vma->vm_flags & VM_MAYSHARE) {
 			int err = huge_add_to_page_cache(page, mapping, idx);
@@ -3861,6 +3862,15 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
 	}
 
 	spin_unlock(ptl);
+
+	/*
+	 * Only make newly allocated pages active.  Existing pages found
+	 * in the pagecache could be !page_huge_active() if they have been
+	 * isolated for migration.
+	 */
+	if (new_page)
+		set_page_huge_active(page);
+
 	unlock_page(page);
 out:
 	return ret;
@@ -4095,7 +4105,6 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	 * the set_pte_at() write.
 	 */
 	__SetPageUptodate(page);
-	set_page_huge_active(page);
 
 	mapping = dst_vma->vm_file->f_mapping;
 	idx = vma_hugecache_offset(h, dst_vma, dst_addr);
@@ -4163,6 +4172,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm,
 	update_mmu_cache(dst_vma, dst_addr, dst_pte);
 
 	spin_unlock(ptl);
+	set_page_huge_active(page);
 	if (vm_shared)
 		unlock_page(page);
 	ret = 0;
diff --git a/mm/migrate.c b/mm/migrate.c
@@ -1315,6 +1315,16 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 		lock_page(hpage);
 	}
 
+	/*
+	 * Check for pages which are in the process of being freed.  Without
+	 * page_mapping() set, hugetlbfs specific move page routine will not
+	 * be called and we could leak usage counts for subpools.
+	 */
+	if (page_private(hpage) && !page_mapping(hpage)) {
+		rc = -EBUSY;
+		goto out_unlock;
+	}
+
 	if (PageAnon(hpage))
 		anon_vma = page_get_anon_vma(hpage);
 
@@ -1345,6 +1355,7 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 		put_new_page = NULL;
 	}
 
+out_unlock:
 	unlock_page(hpage);
 out:
 	if (rc != -EAGAIN)
