ext4: allow readdir()'s of large empty directories to be interrupted

If a directory has a large number of empty blocks, iterating over all
of them can take a long time, leading to scheduler warnings and users
getting irritated when they can't kill a process in the middle of one
of these long-running readdir operations.  Fix this by adding checks to
ext4_readdir() and ext4_htree_fill_tree().

This was reverted earlier due to a typo in the original commit where I
experimented with using signal_pending() instead of
fatal_signal_pending().  The test was in the wrong place if we were
going to return signal_pending() since we would end up returning
duplicant entries.  See 9f2394c9be47 for a more detailed explanation.

Added fix as suggested by Linus to check for signal_pending() in
in the filldir() functions.

Reported-by: Benjamin LaHaise <bcrl@kvack.org>
Google-Bug-Id: 27880676
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

4 files changed, 18 insertions, 0 deletions

diff --git a/fs/compat.c b/fs/compat.c
index a71936a3f4cb..f940cb20562f 100644
--- a/fs/compat.c
+++ b/fs/compat.c
@@ -936,6 +936,8 @@ static int compat_filldir(struct dir_context *ctx, const char *name, int namlen,
         }
         dirent = buf->previous;
         if (dirent) {
+                if (signal_pending(current))
+                        return -EINTR;
                 if (__put_user(offset, &dirent->d_off))
                         goto efault;
         }
@@ -1020,6 +1022,8 @@ static int compat_filldir64(struct dir_context *ctx, const char *name,
         dirent = buf->previous;
 
         if (dirent) {
+                if (signal_pending(current))
+                        return -EINTR;
                 if (__put_user_unaligned(offset, &dirent->d_off))
                         goto efault;
         }
diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
index 561d7308b393..4173bfe21114 100644
--- a/fs/ext4/dir.c
+++ b/fs/ext4/dir.c
@@ -150,6 +150,11 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
         while (ctx->pos < inode->i_size) {
                 struct ext4_map_blocks map;
 
+                if (fatal_signal_pending(current)) {
+                        err = -ERESTARTSYS;
+                        goto errout;
+                }
+                cond_resched();
                 map.m_lblk = ctx->pos >> EXT4_BLOCK_SIZE_BITS(sb);
                 map.m_len = 1;
                 err = ext4_map_blocks(NULL, inode, &map, 0);
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 48e4b8907826..c07422d254b6 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1107,6 +1107,11 @@ int ext4_htree_fill_tree(struct file *dir_file, __u32 start_hash,
         }
 
         while (1) {
+                if (fatal_signal_pending(current)) {
+                        err = -ERESTARTSYS;
+                        goto errout;
+                }
+                cond_resched();
                 block = dx_get_block(frame->at);
                 ret = htree_dirblock_to_tree(dir_file, dir, block, &hinfo,
                                              start_hash, start_minor_hash);
diff --git a/fs/readdir.c b/fs/readdir.c
index e69ef3b79787..5f2d4bee5a73 100644
--- a/fs/readdir.c
+++ b/fs/readdir.c
@@ -169,6 +169,8 @@ static int filldir(struct dir_context *ctx, const char *name, int namlen,
         }
         dirent = buf->previous;
         if (dirent) {
+                if (signal_pending(current))
+                        return -EINTR;
                 if (__put_user(offset, &dirent->d_off))
                         goto efault;
         }
@@ -248,6 +250,8 @@ static int filldir64(struct dir_context *ctx, const char *name, int namlen,
                 return -EINVAL;
         dirent = buf->previous;
         if (dirent) {
+                if (signal_pending(current))
+                        return -EINTR;
                 if (__put_user(offset, &dirent->d_off))
                         goto efault;
         }