[LU-15413] WBC: endless loop in balance_dirty_pages

• Type: Technical task
• Resolution: Unresolved
• Priority: Minor
• Assignee: Qian Yingjin
• Reporter: Qian Yingjin

Description

When write a larger file into Lustre with WBC enabled (aging_keep flush mode), it trapped into a endless loop:

dd if=/dev/zero of=/mnt/lustre/tdir/tfile bs=1M count=4096

cat /proc/22735/stack
[<0>] balance_dirty_pages+0x426/0xcd0
[<0>] balance_dirty_pages_ratelimited+0x2af/0x3b0
[<0>] generic_perform_write+0x16a/0x1b0
[<0>] __generic_file_write_iter+0xfa/0x1c0
[<0>] generic_file_write_iter+0xab/0x150
[<0>] memfs_file_write_iter+0xd7/0x180 [lustre]
[<0>] new_sync_write+0x124/0x170
[<0>] vfs_write+0xa5/0x1a0
[<0>] ksys_write+0x4f/0xb0
[<0>] do_syscall_64+0x5b/0x1b0
[<0>] entry_SYSCALL_64_after_hwframe+0x65/0xca
[<0>] 0xffffffffffffffff

The reason is because that the kernel found the current writing process tries to write out some dirty pages in @balance_dirty_pages() due to the rate limit mechanism in Linux kernel, but the pages are pinned in MemFS, and are not reclaimable.
We found that for a client with 96G memory, it will trap into the endless loop when the write size is larger than 8G.

Here, there are two solution:
one solution is to disable dirty account for the DBI:

sb->s_bdi->capabilities |= BDI_CAP_NO_ACCT_DIRTY;

void balance_dirty_pages_ratelimited(struct address_space *mapping)
{
	struct inode *inode = mapping->host;
	struct backing_dev_info *bdi = inode_to_bdi(inode);
	struct bdi_writeback *wb = NULL;
	int ratelimit;
	int *p;

	if (!bdi_cap_account_dirty(bdi))
		return;
      ...

By this way, it will not trigger to call balance_dirty_pages. It can write as many cache pages as possible before reaching the page cache limits in MemFS.

Another solution is that:
when write-out inode in @balance_dirty_pages->wb_start_background_writeback(), the client assimilates the cache pages from MemFS into Lustre, after that the assimilated pages in Lustre are reclaimable, the dirty pages can be written out to Lustre backend.