Hi Mike,
On Thu, Feb 21, 2019 at 11:11:06AM -0800, Mike Kravetz wrote:
On 2/20/19 10:09 PM, Andrew Morton wrote:
On Tue, 12 Feb 2019 14:14:00 -0800 Mike Kravetz mike.kravetz@oracle.com wrote:
diff --git a/mm/hugetlb.c b/mm/hugetlb.c index a80832487981..f859e319e3eb 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c
...
@@ -3863,6 +3862,10 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm, } spin_unlock(ptl);
- /* May already be set if not newly allocated page */
- set_page_huge_active(page);
This is wrong. We need to only set_page_huge_active() for newly allocated pages. Why? We could have got the page from the pagecache, and it could be that the page is !page_huge_active() because it has been isolated for migration. Therefore, we do not want to set it active here.
I have also found another race with migration when removing a page from a file. When a huge page is removed from the pagecache, the page_mapping() field is cleared yet page_private continues to point to the subpool until the page is actually freed by free_huge_page(). free_huge_page is what adjusts the counts for the subpool. 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, check for this condition before migrating a huge page. If the condition is detected, return EBUSY for the page.
Both issues are addressed in the updated patch below.
Sorry for the churn. As I find and fix one issue I seem to discover another. There is still at least one more issue with private pages when COW comes into play. I continue to work that. I wanted to send this patch earlier as it is pretty easy to hit the bugs if you try. If you would prefer another approach, let me know.
From: Mike Kravetz mike.kravetz@oracle.com Date: Thu, 21 Feb 2019 11:01:04 -0800 Subject: [PATCH] huegtlbfs: fix races and page leaks during migration
Subject still contains a typo.
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 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, check for this condition before migrating a huge page. If the condition is detected, return EBUSY for the page.
Cc: stable@vger.kernel.org Fixes: bcc54222309c ("mm: hugetlb: introduce page_huge_active") Signed-off-by: Mike Kravetz mike.kravetz@oracle.com
fs/hugetlbfs/inode.c | 12 ++++++++++++ mm/hugetlb.c | 12 +++++++++--- mm/migrate.c | 11 +++++++++++ 3 files changed, 32 insertions(+), 3 deletions(-)
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c index 32920a10100e..a7fa037b876b 100644 --- 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 index a80832487981..e9c92e925b7e 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c
...
@@ -3863,6 +3864,11 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm, }
spin_unlock(ptl);
- /* Make newly allocated pages active */
You already have a perfect explanation about why we need this "if",
... We could have got the page from the pagecache, and it could be that the page is !page_huge_active() because it has been isolated for migration.
so you could improve this comment with it.
Anyway, I agree to what/how you try to fix.
Reviewed-by: Naoya Horiguchi n-horiguchi@ah.jp.nec.com
Thanks, Naoya Horiguchi