From: Hugh Dickins <hughd@google.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Hugh Dickins <hughd@google.com>,
linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
linux-nvdimm@lists.01.org, linux-kernel@vger.kernel.org
Subject: BUG_ON(!mapping_empty(&inode->i_data))
Date: Tue, 30 Mar 2021 18:30:22 -0700 (PDT) [thread overview]
Message-ID: <alpine.LSU.2.11.2103301654520.2648@eggly.anvils> (raw)
Running my usual tmpfs kernel builds swapping load, on Sunday's rc4-mm1
mmotm (I never got to try rc3-mm1 but presume it behaved the same way),
I hit clear_inode()'s BUG_ON(!mapping_empty(&inode->i_data)); on two
machines, within an hour or few, repeatably though not to order.
The stack backtrace has always been clear_inode < ext4_clear_inode <
ext4_evict_inode < evict < dispose_list < prune_icache_sb <
super_cache_scan < do_shrink_slab < shrink_slab_memcg < shrink_slab <
shrink_node_memgs < shrink_node < balance_pgdat < kswapd.
ext4 is the disk filesystem I read the source to build from, and also
the filesystem I use on a loop device on a tmpfs file: I have not tried
with other filesystems, nor checked whether perhaps it happens always on
the loop one or always on the disk one. I have not seen it happen with
tmpfs - probably because its inodes cannot be evicted by the shrinker
anyway; I have not seen it happen when "rm -rf" evicts ext4 or tmpfs
inodes (but suspect that may be down to timing, or less pressure).
I doubt it's a matter of filesystem: think it's an XArray thing.
Whenever I've looked at the XArray nodes involved, the root node
(shift 6) contained one or three (adjacent) pointers to empty shift
0 nodes, which each had offset and parent and array correctly set.
Is there some way in which empty nodes can get left behind, and so
fail eviction's mapping_empty() check?
I did wonder whether some might get left behind if xas_alloc() fails
(though probably the tree here is too shallow to show that). Printks
showed that occasionally xas_alloc() did fail while testing (maybe at
memcg limit), but there was no correlation with the BUG_ONs.
I did wonder whether this is a long-standing issue, which your new
BUG_ON is the first to detect: so tried 5.12-rc5 clear_inode() with
a BUG_ON(!xa_empty(&inode->i_data.i_pages)) after its nrpages and
nrexceptional BUG_ONs. The result there surprised me: I expected
it to behave the same way, but it hits that BUG_ON in a minute or
so, instead of an hour or so. Was there a fix you made somewhere,
to avoid the BUG_ON(!mapping_empty) most of the time? but needs
more work. I looked around a little, but didn't find any.
I had hoped to work this out myself, and save us both some writing:
but better hand over to you, in the hope that you'll quickly guess
what's up, then I can try patches. I do like the no-nrexceptionals
series, but there's something still to be fixed.
Hugh
next reply other threads:[~2021-03-31 1:30 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-31 1:30 Hugh Dickins [this message]
2021-03-31 2:49 ` BUG_ON(!mapping_empty(&inode->i_data)) Matthew Wilcox
2021-03-31 21:58 ` BUG_ON(!mapping_empty(&inode->i_data)) Hugh Dickins
2021-04-01 17:06 ` BUG_ON(!mapping_empty(&inode->i_data)) Matthew Wilcox
2021-04-02 3:13 ` BUG_ON(!mapping_empty(&inode->i_data)) Matthew Wilcox
2021-04-02 13:27 ` BUG_ON(!mapping_empty(&inode->i_data)) Matthew Wilcox
2021-04-02 17:04 ` BUG_ON(!mapping_empty(&inode->i_data)) Matthew Wilcox
2021-04-02 20:24 ` BUG_ON(!mapping_empty(&inode->i_data)) Hugh Dickins
2021-04-02 21:16 ` BUG_ON(!mapping_empty(&inode->i_data)) Hugh Dickins
2021-04-30 4:16 ` BUG_ON(!mapping_empty(&inode->i_data)) Andrew Morton
2021-04-30 5:41 ` BUG_ON(!mapping_empty(&inode->i_data)) Hugh Dickins
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=alpine.LSU.2.11.2103301654520.2648@eggly.anvils \
--to=hughd@google.com \
--cc=akpm@linux-foundation.org \
--cc=linux-fsdevel@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mm@kvack.org \
--cc=linux-nvdimm@lists.01.org \
--cc=willy@infradead.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).