btrfs: fix RWF_NOWAIT writes blocking on extent locks and waiting for IO

A RWF_NOWAIT write is not supposed to wait on filesystem locks that can be
held for a long time or for ongoing IO to complete.

However when calling check_can_nocow(), if the inode has prealloc extents
or has the NOCOW flag set, we can block on extent (file range) locks
through the call to btrfs_lock_and_flush_ordered_range(). Such lock can
take a significant amount of time to be available. For example, a fiemap
task may be running, and iterating through the entire file range checking
all extents and doing backref walking to determine if they are shared,
or a readpage operation may be in progress.

Also at btrfs_lock_and_flush_ordered_range(), called by check_can_nocow(),
after locking the file range we wait for any existing ordered extent that
is in progress to complete. Another operation that can take a significant
amount of time and defeat the purpose of RWF_NOWAIT.

So fix this by trying to lock the file range and if it's currently locked
return -EAGAIN to user space. If we are able to lock the file range without
waiting and there is an ordered extent in the range, return -EAGAIN as
well, instead of waiting for it to complete. Finally, don't bother trying
to lock the snapshot lock of the root when attempting a RWF_NOWAIT write,
as that is only important for buffered writes.

Fixes: edf064e7c6fec3 ("btrfs: nowait aio support")
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 6d5d905..2520605 100644 (file)
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -1533,7 +1533,7 @@ lock_and_cleanup_extent_if_need(struct btrfs_inode *inode, struct page **pages,
 }
 
 static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
-                                   size_t *write_bytes)
+                                   size_t *write_bytes, bool nowait)
 {
        struct btrfs_fs_info *fs_info = inode->root->fs_info;
        struct btrfs_root *root = inode->root;
@@ -1541,27 +1541,43 @@ static noinline int check_can_nocow(struct btrfs_inode *inode, loff_t pos,
        u64 num_bytes;
        int ret;
 
-       if (!btrfs_drew_try_write_lock(&root->snapshot_lock))
+       if (!nowait && !btrfs_drew_try_write_lock(&root->snapshot_lock))
                return -EAGAIN;
 
        lockstart = round_down(pos, fs_info->sectorsize);
        lockend = round_up(pos + *write_bytes,
                           fs_info->sectorsize) - 1;
+       num_bytes = lockend - lockstart + 1;
 
-       btrfs_lock_and_flush_ordered_range(inode, lockstart,
-                                          lockend, NULL);
+       if (nowait) {
+               struct btrfs_ordered_extent *ordered;
+
+               if (!try_lock_extent(&inode->io_tree, lockstart, lockend))
+                       return -EAGAIN;
+
+               ordered = btrfs_lookup_ordered_range(inode, lockstart,
+                                                    num_bytes);
+               if (ordered) {
+                       btrfs_put_ordered_extent(ordered);
+                       ret = -EAGAIN;
+                       goto out_unlock;
+               }
+       } else {
+               btrfs_lock_and_flush_ordered_range(inode, lockstart,
+                                                  lockend, NULL);
+       }
 
-       num_bytes = lockend - lockstart + 1;
        ret = can_nocow_extent(&inode->vfs_inode, lockstart, &num_bytes,
                        NULL, NULL, NULL);
        if (ret <= 0) {
                ret = 0;
-               btrfs_drew_write_unlock(&root->snapshot_lock);
+               if (!nowait)
+                       btrfs_drew_write_unlock(&root->snapshot_lock);
        } else {
                *write_bytes = min_t(size_t, *write_bytes ,
                                     num_bytes - pos + lockstart);
        }
-
+out_unlock:
        unlock_extent(&inode->io_tree, lockstart, lockend);
 
        return ret;
@@ -1633,7 +1649,7 @@ static noinline ssize_t btrfs_buffered_write(struct kiocb *iocb,
                        if ((BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
                                                      BTRFS_INODE_PREALLOC)) &&
                            check_can_nocow(BTRFS_I(inode), pos,
-                                       &write_bytes) > 0) {
+                                           &write_bytes, false) > 0) {
                                /*
                                 * For nodata cow case, no need to reserve
                                 * data space.
@@ -1912,12 +1928,11 @@ static ssize_t btrfs_file_write_iter(struct kiocb *iocb,
                 */
                if (!(BTRFS_I(inode)->flags & (BTRFS_INODE_NODATACOW |
                                              BTRFS_INODE_PREALLOC)) ||
-                   check_can_nocow(BTRFS_I(inode), pos, &nocow_bytes) <= 0) {
+                   check_can_nocow(BTRFS_I(inode), pos, &nocow_bytes,
+                                   true) <= 0) {
                        inode_unlock(inode);
                        return -EAGAIN;
                }
-               /* check_can_nocow() locks the snapshot lock on success */
-               btrfs_drew_write_unlock(&root->snapshot_lock);
                /*
                 * There are holes in the range or parts of the range that must
                 * be COWed (shared extents, RO block groups, etc), so just bail