On Thu, Feb 07, 2019 at 10:50:55AM -0800, Mike Kravetz wrote:
On 1/30/19 1:14 PM, Mike Kravetz wrote:
Files can be created and mapped in an explicitly mounted hugetlbfs filesystem. If pages in such files are migrated, the filesystem usage will not be decremented for the associated pages. This can result in mmap or page allocation failures as it appears there are fewer pages in the filesystem than there should be.
Does anyone have a little time to take a look at this?
While migration of hugetlb pages 'should' not be a common issue, we have seen it happen via soft memory errors/page poisoning in production environments. Didn't see a leak in that case as it was with pages in a Sys V shared mem segment. However, our DB code is starting to make use of files in explicitly mounted hugetlbfs filesystems. Therefore, we are more likely to hit this bug in the field.
Hi Mike,
Thank you for finding/reporting the problem. # sorry for my late response.
For example, a test program which hole punches, faults and migrates pages in such a file (1G in size) will eventually fail because it can not allocate a page. Reported counts and usage at time of failure:
node0 537 free_hugepages 1024 nr_hugepages 0 surplus_hugepages node1 1000 free_hugepages 1024 nr_hugepages 0 surplus_hugepages
Filesystem Size Used Avail Use% Mounted on nodev 4.0G 4.0G 0 100% /var/opt/hugepool
Note that the filesystem shows 4G of pages used, while actual usage is 511 pages (just under 1G). Failed trying to allocate page 512.
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. Also, migrate_page_states() unconditionally clears page_private and PagePrivate of the old page. It is unlikely, but possible that these fields could be non-NULL and are needed at hugetlb free page time. So, do not touch these fields for hugetlb pages.
Cc: stable@vger.kernel.org Fixes: 290408d4a250 ("hugetlb: hugepage migration core") Signed-off-by: Mike Kravetz mike.kravetz@oracle.com
fs/hugetlbfs/inode.c | 10 ++++++++++ mm/migrate.c | 10 ++++++++-- 2 files changed, 18 insertions(+), 2 deletions(-)
diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c index 32920a10100e..fb6de1db8806 100644 --- a/fs/hugetlbfs/inode.c +++ b/fs/hugetlbfs/inode.c @@ -859,6 +859,16 @@ 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* if needed.*/- if (page_private(page) && !page_private(newpage)) {
set_page_private(newpage, page_private(page));set_page_private(page, 0);
You don't have to copy PagePrivate flag?
- }
- if (mode != MIGRATE_SYNC_NO_COPY) migrate_page_copy(newpage, page); else
diff --git a/mm/migrate.c b/mm/migrate.c index f7e4bfdc13b7..0d9708803553 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -703,8 +703,14 @@ void migrate_page_states(struct page *newpage, struct page *page) */ if (PageSwapCache(page)) ClearPageSwapCache(page);
- ClearPagePrivate(page);
- set_page_private(page, 0);
- /*
* Unlikely, but PagePrivate and page_private could potentially* contain information needed at hugetlb free page time.*/- if (!PageHuge(page)) {
ClearPagePrivate(page);set_page_private(page, 0);- }
# This argument is mainly for existing code...
According to the comment on migrate_page():
/* * Common logic to directly migrate a single LRU page suitable for * pages that do not use PagePrivate/PagePrivate2. * * Pages are locked upon entry and exit. */ int migrate_page(struct address_space *mapping, ...
So this common logic assumes that page_private is not used, so why do we explicitly clear page_private in migrate_page_states()? buffer_migrate_page(), which is commonly used for the case when page_private is used, does that clearing outside migrate_page_states(). So I thought that hugetlbfs_migrate_page() could do in the similar manner. IOW, migrate_page_states() should not do anything on PagePrivate. But there're a few other .migratepage callbacks, and I'm not sure all of them are safe for the change, so this approach might not fit for a small fix.
# BTW, there seems a typo in $SUBJECT.
Thanks, Naoya Horiguchi