Re: "Bad page state" errors when calling BULKSTAT under memory pressure?
From: Dave Chinner <david@fromorbit.com>
Date: 2016-08-25 00:35:54
On Wed, Aug 24, 2016 at 04:42:37PM -0700, Darrick J. Wong wrote:
Hi everyone, [cc'ing Brian because he was the last one to touch xfs_buf.c] I've been stress-testing xfs_scrub against a 900GB filesystem with 2M inodes using a VM with 512M of RAM. I've noticed that I get BUG messages about pages with negative refcount, but only if the system is under memory pressure. No errors are seen if the VM memory is increased to, say, 20GB. : BUG: Bad page state in process xfs_scrub pfn:00426 : page:ffffea0000010980 count:-1 mapcount:0 mapping: (null) index:0x0 : flags: 0x0() : page dumped because: nonzero _count
Unless we are double-freeing a buffer, that's not an XFS problem. Have you tried with memory posioning and allocation debug turned on?
: Modules linked in: xfs libcrc32c sch_fq_codel af_packet : CPU: 1 PID: 2058 Comm: xfs_scrub Not tainted 4.8.0-rc3-mcsum #18
the mm architecture was significantly modified in 4.8.0-rc1 - it went from per-zone to per-node infrastructure, so it's entirely possible this is a memory reclaim regression. can you reproduce it on an older kernel (e.g. 4.7.0)?
quoted hunk ↗ jump to hunk
Obviously, a page refcount of -1 is not a good sign. I had a hunch that the page in question was (hopefully) a page backing an xfs_buf, so I applied the following debug patch:diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c index 607cc29..144b976 100644 --- a/fs/xfs/xfs_buf.c +++ b/fs/xfs/xfs_buf.c@@ -317,7 +317,7 @@ xfs_buf_free( for (i = 0; i < bp->b_page_count; i++) { struct page *page = bp->b_pages[i]; - +if (page_ref_count(page) != 1) {xfs_err(NULL, "%s: OHNO! daddr=%llu page=%p ref=%d", __func__, bp->b_bn, page, page_ref_count(page)); dump_stack();} __free_page(page); } } else if (bp->b_flags & _XBF_KMEM)I then saw this: : SGI XFS with ACLs, security attributes, realtime, debug enabled : XFS (sda): Mounting V4 Filesystem : XFS (sda): Ending clean mount : XFS: xfs_buf_free: OHNO! daddr=113849120 page=ffffea0000010980 ref=0
Which implies something else has dropped the page reference count on us while we hold a reference to it. What you might like to check what the page reference counts are on /allocation/ to see if we're being handed a page from the freelist with a bad ref count.... If the ref counts are good at allocation, but bad on free, then I very much doubt it's an XFS problem. We don't actually touch the page reference count anywhere, so let's make sure that it's not a double free or something like that in XFS first. Cheers, Dave. -- Dave Chinner david@fromorbit.com _______________________________________________ xfs mailing list xfs@oss.sgi.com http://oss.sgi.com/mailman/listinfo/xfs