From: David Howells dhowells@redhat.com
[ Upstream commit ec0fa0b659144d9c68204d23f627b6a65fa53e50 ]
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@redhat.com Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- 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 1d13d2e882ada..0fe8844b4bee2 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 792ac711985eb..e1ebead2e505a 100644 --- a/fs/afs/internal.h +++ b/fs/afs/internal.h @@ -810,6 +810,7 @@ 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 a121c247d95a3..0a98cf36e78a3 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; }