The afs filesystem has a lock[*] that it uses to serialise I/O operations
going to the server (vnode->io_lock), as the server will only perform one
modification operation at a time on any given file or directory. This
prevents the the filesystem from filling up all the call slots to a server
with calls that aren't going to be executed in parallel anyway, thereby
allowing operations on other files to obtain slots.
[*] Note that is probably redundant for directories at least since
i_rwsem is used to serialise directory modifications and
lookup/reading vs modification. The server does allow parallel
non-modification ops, however.
When a file truncation op completes, we truncate the in-memory copy of the
file to match - but we do it whilst still holding the io_lock, the idea
being to prevent races with other operations.
However, if writeback starts in a worker thread simultaneously with
truncation (whilst notify_change() is called with i_rwsem locked, writeback
pays it no heed), it may manage to set PG_writeback bits on the pages that
will get truncated before afs_setattr_success() manages to call
truncate_pagecache(). Truncate will then wait for those pages - whilst
still inside io_lock:
# cat /proc/8837/stack
[<0>] wait_on_page_bit_common+0x184/0x1e7
[<0>] truncate_inode_pages_range+0x37f/0x3eb
[<0>] truncate_pagecache+0x3c/0x53
[<0>] afs_setattr_success+0x4d/0x6e
[<0>] afs_wait_for_operation+0xd8/0x169
[<0>] afs_do_sync_operation+0x16/0x1f
[<0>] afs_setattr+0x1fb/0x25d
[<0>] notify_change+0x2cf/0x3c4
[<0>] do_truncate+0x7f/0xb2
[<0>] do_sys_ftruncate+0xd1/0x104
[<0>] do_syscall_64+0x2d/0x3a
[<0>] entry_SYSCALL_64_after_hwframe+0x44/0xa9
The writeback operation, however, stalls indefinitely because it needs to
get the io_lock to proceed:
# cat /proc/5940/stack
[<0>] afs_get_io_locks+0x58/0x1ae
[<0>] afs_begin_vnode_operation+0xc7/0xd1
[<0>] afs_store_data+0x1b2/0x2a3
[<0>] afs_write_back_from_locked_page+0x418/0x57c
[<0>] afs_writepages_region+0x196/0x224
[<0>] afs_writepages+0x74/0x156
[<0>] do_writepages+0x2d/0x56
[<0>] __writeback_single_inode+0x84/0x207
[<0>] writeback_sb_inodes+0x238/0x3cf
[<0>] __writeback_inodes_wb+0x68/0x9f
[<0>] wb_writeback+0x145/0x26c
[<0>] wb_do_writeback+0x16a/0x194
[<0>] wb_workfn+0x74/0x177
[<0>] process_one_work+0x174/0x264
[<0>] worker_thread+0x117/0x1b9
[<0>] kthread+0xec/0xf1
[<0>] ret_from_fork+0x1f/0x30
and thus deadlock has occurred.
Note that whilst afs_setattr() calls filemap_write_and_wait(), the fact
that the caller is holding i_rwsem doesn't preclude more pages being
dirtied through an mmap'd region.
Fix this by:
(1) Use the vnode validate_lock to mediate access between afs_setattr()
and afs_writepages():
(a) Exclusively lock validate_lock in afs_setattr() around the whole
RPC operation.
(b) If WB_SYNC_ALL isn't set on entry to afs_writepages(), trying to
shared-lock validate_lock and returning immediately if we couldn't
get it.
(c) If WB_SYNC_ALL is set, wait for the lock.
The validate_lock is also used to validate a file and to zap its cache
if the file was altered by a third party, so it's probably a good fit
for this.
(2) Move the truncation outside of the io_lock in setattr, using the same
hook as is used for local directory editing.
This requires the old i_size to be retained in the operation record as
we commit the revised status to the inode members inside the io_lock
still, but we still need to know if we reduced the file size.
Fixes: d2ddc776a458 ("afs: Overhaul volume and server record caching and fileserver rotation")
Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---
fs/afs/inode.c | 47 ++++++++++++++++++++++++++++++++++++++---------
fs/afs/internal.h | 1 +
fs/afs/write.c | 11 +++++++++++
3 files changed, 50 insertions(+), 9 deletions(-)
diff --git a/fs/afs/inode.c b/fs/afs/inode.c
index 1d13d2e882ad..0fe8844b4bee 100644
--- a/fs/afs/inode.c
+++ b/fs/afs/inode.c
@@ -810,14 +810,32 @@ void afs_evict_inode(struct inode *inode)
static void afs_setattr_success(struct afs_operation *op)
{
- struct inode *inode = &op->file[0].vnode->vfs_inode;
+ struct afs_vnode_param *vp = &op->file[0];
+ struct inode *inode = &vp->vnode->vfs_inode;
+ loff_t old_i_size = i_size_read(inode);
+
+ op->setattr.old_i_size = old_i_size;
+ afs_vnode_commit_status(op, vp);
+ /* inode->i_size has now been changed. */
+
+ if (op->setattr.attr->ia_valid & ATTR_SIZE) {
+ loff_t size = op->setattr.attr->ia_size;
+ if (size > old_i_size)
+ pagecache_isize_extended(inode, old_i_size, size);
+ }
+}
+
+static void afs_setattr_edit_file(struct afs_operation *op)
+{
+ struct afs_vnode_param *vp = &op->file[0];
+ struct inode *inode = &vp->vnode->vfs_inode;
- afs_vnode_commit_status(op, &op->file[0]);
if (op->setattr.attr->ia_valid & ATTR_SIZE) {
- loff_t i_size = inode->i_size, size = op->setattr.attr->ia_size;
- if (size > i_size)
- pagecache_isize_extended(inode, i_size, size);
- truncate_pagecache(inode, size);
+ loff_t size = op->setattr.attr->ia_size;
+ loff_t i_size = op->setattr.old_i_size;
+
+ if (size < i_size)
+ truncate_pagecache(inode, size);
}
}
@@ -825,6 +843,7 @@ static const struct afs_operation_ops afs_setattr_operation = {
.issue_afs_rpc = afs_fs_setattr,
.issue_yfs_rpc = yfs_fs_setattr,
.success = afs_setattr_success,
+ .edit_dir = afs_setattr_edit_file,
};
/*
@@ -863,11 +882,16 @@ int afs_setattr(struct dentry *dentry, struct iattr *attr)
if (S_ISREG(vnode->vfs_inode.i_mode))
filemap_write_and_wait(vnode->vfs_inode.i_mapping);
+ /* Prevent any new writebacks from starting whilst we do this. */
+ down_write(&vnode->validate_lock);
+
op = afs_alloc_operation(((attr->ia_valid & ATTR_FILE) ?
afs_file_key(attr->ia_file) : NULL),
vnode->volume);
- if (IS_ERR(op))
- return PTR_ERR(op);
+ if (IS_ERR(op)) {
+ ret = PTR_ERR(op);
+ goto out_unlock;
+ }
afs_op_set_vnode(op, 0, vnode);
op->setattr.attr = attr;
@@ -880,5 +904,10 @@ int afs_setattr(struct dentry *dentry, struct iattr *attr)
op->file[0].update_ctime = 1;
op->ops = &afs_setattr_operation;
- return afs_do_sync_operation(op);
+ ret = afs_do_sync_operation(op);
+
+out_unlock:
+ up_write(&vnode->validate_lock);
+ _leave(" = %d", ret);
+ return ret;
}
diff --git a/fs/afs/internal.h b/fs/afs/internal.h
index 18042b7dab6a..e5f0446f27e5 100644
--- a/fs/afs/internal.h
+++ b/fs/afs/internal.h
@@ -812,6 +812,7 @@ struct afs_operation {
} store;
struct {
struct iattr *attr;
+ loff_t old_i_size;
} setattr;
struct afs_acl *acl;
struct yfs_acl *yacl;
diff --git a/fs/afs/write.c b/fs/afs/write.c
index 4b2265cb1891..da12abd6db21 100644
--- a/fs/afs/write.c
+++ b/fs/afs/write.c
@@ -738,11 +738,21 @@ static int afs_writepages_region(struct address_space *mapping,
int afs_writepages(struct address_space *mapping,
struct writeback_control *wbc)
{
+ struct afs_vnode *vnode = AFS_FS_I(mapping->host);
pgoff_t start, end, next;
int ret;
_enter("");
+ /* We have to be careful as we can end up racing with setattr()
+ * truncating the pagecache since the caller doesn't take a lock here
+ * to prevent it.
+ */
+ if (wbc->sync_mode == WB_SYNC_ALL)
+ down_read(&vnode->validate_lock);
+ else if (!down_read_trylock(&vnode->validate_lock))
+ return 0;
+
if (wbc->range_cyclic) {
start = mapping->writeback_index;
end = -1;
@@ -762,6 +772,7 @@ int afs_writepages(struct address_space *mapping,
ret = afs_writepages_region(mapping, wbc, start, end, &next);
}
+ up_read(&vnode->validate_lock);
_leave(" = %d", ret);
return ret;
}
