[PATCH 3.4 02/23] shmem: fix faulting into a hole while its punched

Greg Kroah-Hartman
2014-07-26T20:21:29Z

From: Hugh Dickins <hughd@google.com>

commit f00cdc6df7d7cfcabb5b740911e6788cb0802bdb upstream.

Trinity finds that mmap access to a hole while it's punched from shmem
can prevent the madvise(MADV_REMOVE) or fallocate(FALLOC_FL_PUNCH_HOLE)
from completing, until the reader chooses to stop; with the puncher's
hold on i_mutex locking out all other writers until it can complete.

It appears that the tmpfs fault path is too light in comparison with its
hole-punching path, lacking an i_data_sem to obstruct it; but we don't
want to slow down the common case.

Extend shmem_fallocate()'s existing range notification mechanism, so
shmem_fault() can refrain from faulting pages into the hole while it's
punched, waiting instead on i_mutex (when safe to sleep; or repeatedly
faulting when not).

[akpm@linux-foundation.org: coding-style fixes]
Signed-off-by: Hugh Dickins <hughd@google.com>
Reported-by: Sasha Levin <sasha.levin@oracle.com>
Tested-by: Sasha Levin <sasha.levin@oracle.com>
Cc: Dave Jones <davej@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
mm/shmem.c | 91 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
mm/truncate.c | 25 ---------------
2 files changed, 91 insertions(+), 25 deletions(-)

--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -76,6 +76,16 @@ static struct vfsmount *shm_mnt;
/* Symlink up to this size is kmalloc'ed instead of using a swappable page */
#define SHORT_SYMLINK_LEN 128

+/*
+ * vmtruncate_range() communicates with shmem_fault via
+ * inode->i_private (with i_mutex making sure that it has only one user at
+ * a time): we would prefer not to enlarge the shmem inode just for that.
+ */
+struct shmem_falloc {
+ pgoff_t start; /* start of range currently being fallocated */
+ pgoff_t next; /* the next page offset to be fallocated */
+};
+
struct shmem_xattr {
struct list_head list; /* anchored by shmem_inode_info->xattr_list */
char *name; /* xattr name */
@@ -1060,6 +1070,43 @@ static int shmem_fault(struct vm_area_st
int error;
int ret = VM_FAULT_LOCKED;

+ /*
+ * Trinity finds that probing a hole which tmpfs is punching can
+ * prevent the hole-punch from ever completing: which in turn
+ * locks writers out with its hold on i_mutex. So refrain from
+ * faulting pages into the hole while it's being punched, and
+ * wait on i_mutex to be released if vmf->flags permits.
+ */
+ if (unlikely(inode->i_private)) {
+ struct shmem_falloc *shmem_falloc;
+
+ spin_lock(&inode->i_lock);
+ shmem_falloc = inode->i_private;
+ if (!shmem_falloc ||
+ vmf->pgoff < shmem_falloc->start ||
+ vmf->pgoff >= shmem_falloc->next)
+ shmem_falloc = NULL;
+ spin_unlock(&inode->i_lock);
+ /*
+ * i_lock has protected us from taking shmem_falloc seriously
+ * once return from vmtruncate_range() went back up that stack.
+ * i_lock does not serialize with i_mutex at all, but it does
+ * not matter if sometimes we wait unnecessarily, or sometimes
+ * miss out on waiting: we just need to make those cases rare.
+ */
+ if (shmem_falloc) {
+ if ((vmf->flags & FAULT_FLAG_ALLOW_RETRY) &&
+ !(vmf->flags & FAULT_FLAG_RETRY_NOWAIT)) {
+ up_read(&vma->vm_mm->mmap_sem);
+ mutex_lock(&inode->i_mutex);
+ mutex_unlock(&inode->i_mutex);
+ return VM_FAULT_RETRY;
+ }
+ /* cond_resched? Leave that to GUP or return to user */
+ return VM_FAULT_NOPAGE;
+ }
+ }
+
error = shmem_getpage(inode, vmf->pgoff, &vmf->page, SGP_CACHE, &ret);
if (error)
return ((error == -ENOMEM) ? VM_FAULT_OOM : VM_FAULT_SIGBUS);
@@ -1071,6 +1118,44 @@ static int shmem_fault(struct vm_area_st
return ret;
}

+int vmtruncate_range(struct inode *inode, loff_t lstart, loff_t lend)
+{
+ /*
+ * If the underlying filesystem is not going to provide
+ * a way to truncate a range of blocks (punch a hole) -
+ * we should return failure right now.
+ * Only CONFIG_SHMEM shmem.c ever supported i_op->truncate_range().
+ */
+ if (inode->i_op->truncate_range != shmem_truncate_range)
+ return -ENOSYS;
+
+ mutex_lock(&inode->i_mutex);
+ {
+ struct shmem_falloc shmem_falloc;
+ struct address_space *mapping = inode->i_mapping;
+ loff_t unmap_start = round_up(lstart, PAGE_SIZE);
+ loff_t unmap_end = round_down(1 + lend, PAGE_SIZE) - 1;
+
+ shmem_falloc.start = unmap_start >> PAGE_SHIFT;
+ shmem_falloc.next = (unmap_end + 1) >> PAGE_SHIFT;
+ spin_lock(&inode->i_lock);
+ inode->i_private = &shmem_falloc;
+ spin_unlock(&inode->i_lock);
+
+ if ((u64)unmap_end > (u64)unmap_start)
+ unmap_mapping_range(mapping, unmap_start,
+ 1 + unmap_end - unmap_start, 0);
+ shmem_truncate_range(inode, lstart, lend);
+ /* No need to unmap again: hole-punching leaves COWed pages */
+
+ spin_lock(&inode->i_lock);
+ inode->i_private = NULL;
+ spin_unlock(&inode->i_lock);
+ }
+ mutex_unlock(&inode->i_mutex);
+ return 0;
+}
+
#ifdef CONFIG_NUMA
static int shmem_set_policy(struct vm_area_struct *vma, struct mempolicy *mpol)
{
@@ -2547,6 +2632,12 @@ void shmem_truncate_range(struct inode *
}
EXPORT_SYMBOL_GPL(shmem_truncate_range);

+int vmtruncate_range(struct inode *inode, loff_t lstart, loff_t lend)
+{
+ /* Only CONFIG_SHMEM shmem.c ever supported i_op->truncate_range(). */
+ return -ENOSYS;
+}
+
#define shmem_vm_ops generic_file_vm_ops
#define shmem_file_operations ramfs_file_operations
#define shmem_get_inode(sb, dir, mode, dev, flags) ramfs_get_inode(sb, dir, mode, dev)
--- a/mm/truncate.c
+++ b/mm/truncate.c
@@ -603,31 +603,6 @@ int vmtruncate(struct inode *inode, loff
}
EXPORT_SYMBOL(vmtruncate);

-int vmtruncate_range(struct inode *inode, loff_t lstart, loff_t lend)
-{
- struct address_space *mapping = inode->i_mapping;
- loff_t holebegin = round_up(lstart, PAGE_SIZE);
- loff_t holelen = 1 + lend - holebegin;
-
- /*
- * If the underlying filesystem is not going to provide
- * a way to truncate a range of blocks (punch a hole) -
- * we should return failure right now.
- */
- if (!inode->i_op->truncate_range)
- return -ENOSYS;
-
- mutex_lock(&inode->i_mutex);
- inode_dio_wait(inode);
- unmap_mapping_range(mapping, holebegin, holelen, 1);
- inode->i_op->truncate_range(inode, lstart, lend);
- /* unmap again to remove racily COWed private pages */
- unmap_mapping_range(mapping, holebegin, holelen, 1);
- mutex_unlock(&inode->i_mutex);
-
- return 0;
-}
-
/**
* truncate_pagecache_range - unmap and remove pagecache that is hole-punched
* @inode: inode