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Subject[PATCH 2/2] hugetlbfs: Revert "use i_mmap_rwsem for more pmd sharing synchronization"
Date
This reverts commit b43a9990055958e70347c56f90ea2ae32c67334c

The reverted commit caused issues with migration and poisoning of anon
huge pages. The LTP move_pages12 test will cause an "unable to handle
kernel NULL pointer" BUG would occur with stack similar to:

RIP: 0010:down_write+0x1b/0x40
Call Trace:
migrate_pages+0x81f/0xb90
__ia32_compat_sys_migrate_pages+0x190/0x190
do_move_pages_to_node.isra.53.part.54+0x2a/0x50
kernel_move_pages+0x566/0x7b0
__x64_sys_move_pages+0x24/0x30
do_syscall_64+0x5b/0x180
entry_SYSCALL_64_after_hwframe+0x44/0xa9

The purpose of the reverted patch was to fix some long existing races
with huge pmd sharing. It used i_mmap_rwsem for this purpose with the
idea that this could also be used to address truncate/page fault races
with another patch. Further analysis has determined that i_mmap_rwsem
can not be used to address all these hugetlbfs synchronization issues.
Therefore, revert this patch while working an another approach to the
underlying issues.

Reported-by: Jan Stancek <jstancek@redhat.com>
Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
mm/hugetlb.c | 64 +++++++++++----------------------------------
mm/memory-failure.c | 16 ++----------
mm/migrate.c | 13 +--------
mm/rmap.c | 4 ---
mm/userfaultfd.c | 11 ++------
5 files changed, 20 insertions(+), 88 deletions(-)

diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 5671ac9d13bb..06643af2905f 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -3238,7 +3238,6 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
struct page *ptepage;
unsigned long addr;
int cow;
- struct address_space *mapping = vma->vm_file->f_mapping;
struct hstate *h = hstate_vma(vma);
unsigned long sz = huge_page_size(h);
struct mmu_notifier_range range;
@@ -3250,23 +3249,13 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,
mmu_notifier_range_init(&range, src, vma->vm_start,
vma->vm_end, MMU_NOTIFY_CLEAR);
mmu_notifier_invalidate_range_start(&range);
- } else {
- /*
- * For shared mappings i_mmap_rwsem must be held to call
- * huge_pte_alloc, otherwise the returned ptep could go
- * away if part of a shared pmd and another thread calls
- * huge_pmd_unshare.
- */
- i_mmap_lock_read(mapping);
}

for (addr = vma->vm_start; addr < vma->vm_end; addr += sz) {
spinlock_t *src_ptl, *dst_ptl;
-
src_pte = huge_pte_offset(src, addr, sz);
if (!src_pte)
continue;
-
dst_pte = huge_pte_alloc(dst, addr, sz);
if (!dst_pte) {
ret = -ENOMEM;
@@ -3337,8 +3326,6 @@ int copy_hugetlb_page_range(struct mm_struct *dst, struct mm_struct *src,

if (cow)
mmu_notifier_invalidate_range_end(&range);
- else
- i_mmap_unlock_read(mapping);

return ret;
}
@@ -3785,18 +3772,14 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
};

/*
- * hugetlb_fault_mutex and i_mmap_rwsem must be
- * dropped before handling userfault. Reacquire
- * after handling fault to make calling code simpler.
+ * hugetlb_fault_mutex must be dropped before
+ * handling userfault. Reacquire after handling
+ * fault to make calling code simpler.
*/
hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping,
idx, haddr);
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
- i_mmap_unlock_read(mapping);
-
ret = handle_userfault(&vmf, VM_UFFD_MISSING);
-
- i_mmap_lock_read(mapping);
mutex_lock(&hugetlb_fault_mutex_table[hash]);
goto out;
}
@@ -3944,11 +3927,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,

ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
if (ptep) {
- /*
- * Since we hold no locks, ptep could be stale. That is
- * OK as we are only making decisions based on content and
- * not actually modifying content here.
- */
entry = huge_ptep_get(ptep);
if (unlikely(is_hugetlb_entry_migration(entry))) {
migration_entry_wait_huge(vma, mm, ptep);
@@ -3956,31 +3934,20 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
} else if (unlikely(is_hugetlb_entry_hwpoisoned(entry)))
return VM_FAULT_HWPOISON_LARGE |
VM_FAULT_SET_HINDEX(hstate_index(h));
+ } else {
+ ptep = huge_pte_alloc(mm, haddr, huge_page_size(h));
+ if (!ptep)
+ return VM_FAULT_OOM;
}

- /*
- * Acquire i_mmap_rwsem before calling huge_pte_alloc and hold
- * 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
- * something changed.
- */
mapping = vma->vm_file->f_mapping;
- i_mmap_lock_read(mapping);
- ptep = huge_pte_alloc(mm, haddr, huge_page_size(h));
- if (!ptep) {
- i_mmap_unlock_read(mapping);
- return VM_FAULT_OOM;
- }
+ idx = vma_hugecache_offset(h, vma, haddr);

/*
* Serialize hugepage allocation and instantiation, so that we don't
* get spurious allocation failures if two CPUs race to instantiate
* the same page in the page cache.
*/
- idx = vma_hugecache_offset(h, vma, haddr);
hash = hugetlb_fault_mutex_hash(h, mm, vma, mapping, idx, haddr);
mutex_lock(&hugetlb_fault_mutex_table[hash]);

@@ -4068,7 +4035,6 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
}
out_mutex:
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
- i_mmap_unlock_read(mapping);
/*
* Generally it's safe to hold refcount during waiting page lock. But
* here we just wait to defer the next page fault to avoid busy loop and
@@ -4674,12 +4640,10 @@ void adjust_range_if_pmd_sharing_possible(struct vm_area_struct *vma,
* Search for a shareable pmd page for hugetlb. In any case calls pmd_alloc()
* and returns the corresponding pte. While this is not necessary for the
* !shared pmd case because we can allocate the pmd later as well, it makes the
- * code much cleaner.
- *
- * This routine must be called with i_mmap_rwsem held in at least read mode.
- * For hugetlbfs, this prevents removal of any page table entries associated
- * with the address space. This is important as we are setting up sharing
- * based on existing page table entries (mappings).
+ * code much cleaner. pmd allocation is essential for the shared case because
+ * pud has to be populated inside the same i_mmap_rwsem section - otherwise
+ * racing tasks could either miss the sharing (see huge_pte_offset) or select a
+ * bad pmd for sharing.
*/
pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
{
@@ -4696,6 +4660,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
if (!vma_shareable(vma, addr))
return (pte_t *)pmd_alloc(mm, pud, addr);

+ i_mmap_lock_write(mapping);
vma_interval_tree_foreach(svma, &mapping->i_mmap, idx, idx) {
if (svma == vma)
continue;
@@ -4725,6 +4690,7 @@ pte_t *huge_pmd_share(struct mm_struct *mm, unsigned long addr, pud_t *pud)
spin_unlock(ptl);
out:
pte = (pte_t *)pmd_alloc(mm, pud, addr);
+ i_mmap_unlock_write(mapping);
return pte;
}

@@ -4735,7 +4701,7 @@ 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 and i_mmap_rwsem held in write mode.
+ * called with page table lock held.
*
* 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 6379fff1a5ff..7c72f2a95785 100644
--- a/mm/memory-failure.c
+++ b/mm/memory-failure.c
@@ -966,7 +966,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
enum ttu_flags ttu = TTU_IGNORE_MLOCK | TTU_IGNORE_ACCESS;
struct address_space *mapping;
LIST_HEAD(tokill);
- bool unmap_success = true;
+ bool unmap_success;
int kill = 1, forcekill;
struct page *hpage = *hpagep;
bool mlocked = PageMlocked(hpage);
@@ -1028,19 +1028,7 @@ static bool hwpoison_user_mappings(struct page *p, unsigned long pfn,
if (kill)
collect_procs(hpage, &tokill, flags & MF_ACTION_REQUIRED);

- if (!PageHuge(hpage)) {
- unmap_success = try_to_unmap(hpage, ttu);
- } else 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 higer level.
- */
- i_mmap_lock_write(mapping);
- unmap_success = try_to_unmap(hpage, ttu|TTU_RMAP_LOCKED);
- i_mmap_unlock_write(mapping);
- }
+ 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 d8730bd8d878..b4f0557b83fb 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1324,19 +1324,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
goto put_anon;

if (page_mapped(hpage)) {
- struct address_space *mapping = page_mapping(hpage);
-
- /*
- * try_to_unmap could potentially call huge_pmd_unshare.
- * Because of this, take semaphore in write mode here and
- * set TTU_RMAP_LOCKED to let lower levels know we have
- * taken the lock.
- */
- 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);
+ TTU_MIGRATION|TTU_IGNORE_MLOCK|TTU_IGNORE_ACCESS);
page_was_mapped = 1;
}

diff --git a/mm/rmap.c b/mm/rmap.c
index 000de5d02468..62e47f3462cf 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -25,7 +25,6 @@
* page->flags PG_locked (lock_page)
* hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share)
* mapping->i_mmap_rwsem
- * hugetlb_fault_mutex (hugetlbfs specific page fault mutex)
* anon_vma->rwsem
* mm->page_table_lock or pte_lock
* zone_lru_lock (in mark_page_accessed, isolate_lru_page)
@@ -1381,9 +1380,6 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma,
/*
* If sharing is possible, start and end will be adjusted
* accordingly.
- *
- * If called for a huge page, caller must hold i_mmap_rwsem
- * in write mode as it is possible to call huge_pmd_unshare.
*/
adjust_range_if_pmd_sharing_possible(vma, &range.start,
&range.end);
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 065c1ce191c4..d59b5a73dfb3 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -267,14 +267,10 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
VM_BUG_ON(dst_addr & ~huge_page_mask(h));

/*
- * Serialize via i_mmap_rwsem and hugetlb_fault_mutex.
- * i_mmap_rwsem ensures the dst_pte remains valid even
- * in the case of shared pmds. fault mutex prevents
- * races with other faulting threads.
+ * Serialize via hugetlb_fault_mutex
*/
- mapping = dst_vma->vm_file->f_mapping;
- i_mmap_lock_read(mapping);
idx = linear_page_index(dst_vma, dst_addr);
+ mapping = dst_vma->vm_file->f_mapping;
hash = hugetlb_fault_mutex_hash(h, dst_mm, dst_vma, mapping,
idx, dst_addr);
mutex_lock(&hugetlb_fault_mutex_table[hash]);
@@ -283,7 +279,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
dst_pte = huge_pte_alloc(dst_mm, dst_addr, huge_page_size(h));
if (!dst_pte) {
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
- i_mmap_unlock_read(mapping);
goto out_unlock;
}

@@ -291,7 +286,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
dst_pteval = huge_ptep_get(dst_pte);
if (!huge_pte_none(dst_pteval)) {
mutex_unlock(&hugetlb_fault_mutex_table[hash]);
- i_mmap_unlock_read(mapping);
goto out_unlock;
}

@@ -299,7 +293,6 @@ static __always_inline ssize_t __mcopy_atomic_hugetlb(struct mm_struct *dst_mm,
dst_addr, src_addr, &page);

mutex_unlock(&hugetlb_fault_mutex_table[hash]);
- i_mmap_unlock_read(mapping);
vm_alloc_shared = vm_shared;

cond_resched();
--
2.17.2
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