[PATCH 1/8] hugetlbfs: revert use i_mmap_rwsem to address page fault/truncate race

Mike Kravetz
2022-08-24T17:58:58Z

Commit c0d0381ade79 ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
synchronization") added code to take i_mmap_rwsem in read mode for the
duration of fault processing. The use of i_mmap_rwsem to prevent
fault/truncate races depends on this. However, this has been shown to
cause performance/scaling issues. As a result, that code will be
reverted. Since the use i_mmap_rwsem to address page fault/truncate races
depends on this, it must also be reverted.

In a subsequent patch, code will be added to detect the fault/truncate
race and back out operations as required.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
fs/hugetlbfs/inode.c | 30 +++++++++---------------------
mm/hugetlb.c | 23 ++++++++++++-----------
2 files changed, 21 insertions(+), 32 deletions(-)

diff --git a/fs/hugetlbfs/inode.c b/fs/hugetlbfs/inode.c
index f7a5b5124d8a..a32031e751d1 100644
--- a/fs/hugetlbfs/inode.c
+++ b/fs/hugetlbfs/inode.c
@@ -419,9 +419,10 @@ hugetlb_vmdelete_list(struct rb_root_cached *root, pgoff_t start, pgoff_t end,
* In this case, we first scan the range and release found pages.
* After releasing pages, hugetlb_unreserve_pages cleans up region/reserve
* maps and global counts. Page faults can not race with truncation
- * in this routine. hugetlb_no_page() holds i_mmap_rwsem and prevents
- * page faults in the truncated range by checking i_size. i_size is
- * modified while holding i_mmap_rwsem.
+ * in this routine. hugetlb_no_page() prevents page faults in the
+ * truncated range. It checks i_size before allocation, and again after
+ * with the page table lock for the page held. The same lock must be
+ * acquired to unmap a page.
* hole punch is indicated if end is not LLONG_MAX
* In the hole punch case we scan the range and release found pages.
* Only when releasing a page is the associated region/reserve map
@@ -451,16 +452,8 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
u32 hash = 0;

index = folio->index;
- if (!truncate_op) {
- /*
- * Only need to hold the fault mutex in the
- * hole punch case. This prevents races with
- * page faults. Races are not possible in the
- * case of truncation.
- */
- hash = hugetlb_fault_mutex_hash(mapping, index);
- mutex_lock(&hugetlb_fault_mutex_table[hash]);
- }
+ hash = hugetlb_fault_mutex_hash(mapping, index);
+ mutex_lock(&hugetlb_fault_mutex_table[hash]);

/*
* If folio is mapped, it was faulted in after being
@@ -504,8 +497,7 @@ static void remove_inode_hugepages(struct inode *inode, loff_t lstart,
}

folio_unlock(folio);
- if (!truncate_op)
- mutex_unlock(&hugetlb_fault_mutex_table[hash]);
+ mutex_unlock(&hugetlb_fault_mutex_table[hash]);
}
folio_batch_release(&fbatch);
cond_resched();
@@ -543,8 +535,8 @@ static void hugetlb_vmtruncate(struct inode *inode, loff_t offset)
BUG_ON(offset & ~huge_page_mask(h));
pgoff = offset >> PAGE_SHIFT;

- i_mmap_lock_write(mapping);
i_size_write(inode, offset);
+ i_mmap_lock_write(mapping);
if (!RB_EMPTY_ROOT(&mapping->i_mmap.rb_root))
hugetlb_vmdelete_list(&mapping->i_mmap, pgoff, 0,
ZAP_FLAG_DROP_MARKER);
@@ -703,11 +695,7 @@ static long hugetlbfs_fallocate(struct file *file, int mode, loff_t offset,
/* addr is the offset within the file (zero based) */
addr = index * hpage_size;

- /*
- * fault mutex taken here, protects against fault path
- * and hole punch. inode_lock previously taken protects
- * against truncation.
- */
+ /* mutex taken here, fault path and hole punch */
hash = hugetlb_fault_mutex_hash(mapping, index);
mutex_lock(&hugetlb_fault_mutex_table[hash]);

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 9a72499486c1..70bc7f867bc0 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5575,18 +5575,17 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
}

/*
- * We can not race with truncation due to holding i_mmap_rwsem.
- * i_size is modified when holding i_mmap_rwsem, so check here
- * once for faults beyond end of file.
+ * Use page lock to guard against racing truncation
+ * before we get page_table_lock.
*/
- size = i_size_read(mapping->host) >> huge_page_shift(h);
- if (idx >= size)
- goto out;
-
retry:
new_page = false;
page = find_lock_page(mapping, idx);
if (!page) {
+ size = i_size_read(mapping->host) >> huge_page_shift(h);
+ if (idx >= size)
+ goto out;
+
/* Check for page in userfault range */
if (userfaultfd_missing(vma)) {
ret = hugetlb_handle_userfault(vma, mapping, idx,
@@ -5677,6 +5676,10 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
}

ptl = huge_pte_lock(h, mm, ptep);
+ size = i_size_read(mapping->host) >> huge_page_shift(h);
+ if (idx >= size)
+ goto backout;
+
ret = 0;
/* If pte changed from under us, retry */
if (!pte_same(huge_ptep_get(ptep), old_pte))
@@ -5785,10 +5788,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,

/*
* Acquire i_mmap_rwsem before calling huge_pte_alloc and hold
- * until finished with ptep. This serves two purposes:
- * 1) It prevents huge_pmd_unshare from being called elsewhere
- * and making the ptep no longer valid.
- * 2) It synchronizes us with i_size modifications during truncation.
+ * until finished with ptep. This prevents huge_pmd_unshare from
+ * being called elsewhere and making the ptep no longer valid.
*
* ptep could have already be assigned via huge_pte_offset. That
* is OK, as huge_pte_alloc will return the same value unless
--
2.37.1