If writeback_cache is enabled, then the size, mtime and ctime attributes of
regular files are always valid in the kernel's cache.  They are retrieved
from userspace only when the inode is freshly looked up.

Add a more generic "cache_mask", that indicates which attributes are
currently valid in cache.

This patch doesn't change behavior.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

In case of writeback_cache fuse_fillattr() would revert the queried
attributes to the cached version.

Move this to fuse_change_attributes() in order to manage the writeback
logic in a central helper.  This will be necessary for patches that follow.

Only fuse_do_getattr() -> fuse_fillattr() uses the attributes after calling
fuse_change_attributes(), so this should not change behavior.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

There are two instances of "bool is_wb = fc->writeback_cache" where the
actual use mostly involves checking "is_wb && S_ISREG(inode->i_mode)".

Clean up these cases by storing the second condition in the local variable.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

In writeback cache mode mtime/ctime updates are cached, and flushed to the
server using the ->write_inode() callback.

Closing the file will result in a dirty inode being immediately written,
but in other cases the inode can remain dirty after all references are
dropped.  This result in the inode being written back from reclaim, which
can deadlock on a regular allocation while the request is being served.

The usual mechanisms (GFP_NOFS/PF_MEMALLOC*) don't work for FUSE, because
serving a request involves unrelated userspace process(es).

Instead do the same as for dirty pages: make sure the inode is written
before the last reference is gone.

 - fallocate(2)/copy_file_range(2): these call file_update_time() or
   file_modified(), so flush the inode before returning from the call

 - unlink(2), link(2) and rename(2): these call fuse_update_ctime(), so
   flush the ctime directly from this helper

Reported-by: chenguanyou <chenguanyou@xiaomi.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Instead of "goto err", return error directly, since there's no error
cleanup to do now.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Syzkaller reports a null pointer dereference in fuse_test_super() that is
caused by sb->s_fs_info being NULL.

This is due to the fact that fuse_fill_super() is initializing s_fs_info,
which is too late, it's already on the fs_supers list.  The initialization
needs to be done in sget_fc() with the sb_lock held.

Move allocation of fuse_mount and fuse_conn from fuse_fill_super() into
fuse_get_tree().

After this ->kill_sb() will always be called with non-NULL ->s_fs_info,
hence fuse_mount_destroy() can drop the test for non-NULL "fm".

Reported-by: syzbot+74a15f02ccb51f398601@syzkaller.appspotmail.com
Fixes: 5d5b74aa9c76 ("fuse: allow sharing existing sb")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

1. call fuse_mount_destroy() for open coded variants

2. before deactivate_locked_super() don't need fuse_mount destruction since
that will now be done (if ->s_fs_info is not cleared)

3. rearrange fuse_mount setup in fuse_get_tree_submount() so that the
regular pattern can be used

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

The ->put_super callback is called from generic_shutdown_super() in case of
a fully initialized sb.  This is called from kill_***_super(), which is
called from ->kill_sb instances.

Fuse uses ->put_super to destroy the fs specific fuse_mount and drop the
reference to the fuse_conn, while it does the same on each error case
during sb setup.

This patch moves the destruction from fuse_put_super() to
fuse_mount_destroy(), called at the end of all ->kill_sb instances.  A
follup patch will clean up the error paths.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Checking "fm" works because currently sb->s_fs_info is cleared on error
paths; however, sb->s_root is what generic_shutdown_super() checks to
determine whether the sb was fully initialized or not.

This change will allow cleanup of sb setup error paths.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

Pull fuse updates from Miklos Szeredi:

 - Allow mounting an active fuse device. Previously the fuse device
   would always be mounted during initialization, and sharing a fuse
   superblock was only possible through mount or namespace cloning

 - Fix data flushing in syncfs (virtiofs only)

 - Fix data flushing in copy_file_range()

 - Fix a possible deadlock in atomic O_TRUNC

 - Misc fixes and cleanups

* tag 'fuse-update-5.15' of git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse:
  fuse: remove unused arg in fuse_write_file_get()
  fuse: wait for writepages in syncfs
  fuse: flush extending writes
  fuse: truncate pagecache on atomic_o_trunc
  fuse: allow sharing existing sb
  fuse: move fget() to fuse_get_tree()
  fuse: move option checking into fuse_fill_super()
  fuse: name fs_context consistently
  fuse: fix use after free in fuse_read_interrupt()