shmem: fix faulting into a hole while it's punched

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>

1 files changed, 52 insertions, 4 deletions

diff --git a/mm/shmem.c b/mm/shmem.c
index 91b912b37b8c..8f419cff9e34 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -80,11 +80,12 @@ static struct vfsmount *shm_mnt;
 #define SHORT_SYMLINK_LEN 128
 
 /*
- * shmem_fallocate and shmem_writepage communicate 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.
+ * shmem_fallocate communicates with shmem_fault or shmem_writepage 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 {
+        int     mode;           /* FALLOC_FL mode currently operating */
         pgoff_t start;          /* start of range currently being fallocated */
         pgoff_t next;           /* the next page offset to be fallocated */
         pgoff_t nr_falloced;    /* how many new pages have been fallocated */
@@ -759,6 +760,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
                         spin_lock(&inode->i_lock);
                         shmem_falloc = inode->i_private;
                         if (shmem_falloc &&
+                            !shmem_falloc->mode &&
                             index >= shmem_falloc->start &&
                             index < shmem_falloc->next)
                                 shmem_falloc->nr_unswapped++;
@@ -1233,6 +1235,44 @@ static int shmem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
         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 ||
+                    shmem_falloc->mode != FALLOC_FL_PUNCH_HOLE ||
+                    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 shmem_fallocate() 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);
@@ -1729,18 +1769,26 @@ static long shmem_fallocate(struct file *file, int mode, loff_t offset,
 
         mutex_lock(&inode->i_mutex);
 
+        shmem_falloc.mode = mode & ~FALLOC_FL_KEEP_SIZE;
+
         if (mode & FALLOC_FL_PUNCH_HOLE) {
                 struct address_space *mapping = file->f_mapping;
                 loff_t unmap_start = round_up(offset, PAGE_SIZE);
                 loff_t unmap_end = round_down(offset + len, 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, offset, offset + len - 1);
                 /* No need to unmap again: hole-punching leaves COWed pages */
                 error = 0;
-                goto out;
+                goto undone;
         }
 
         /* We need to check rlimit even when FALLOC_FL_KEEP_SIZE */