lkml.org 
[lkml]   [2021]   [Apr]   [1]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: BUG_ON(!mapping_empty(&inode->i_data))
On Wed, Mar 31, 2021 at 02:58:12PM -0700, Hugh Dickins wrote:
> I suspect there's a bug in the XArray handling in collapse_file(),
> which sometimes leaves empty nodes behind.

Urp, yes, that can easily happen.

/* This will be less messy when we use multi-index entries */
do {
xas_lock_irq(&xas);
xas_create_range(&xas);
if (!xas_error(&xas))
break;
if (!xas_nomem(&xas, GFP_KERNEL)) {
result = SCAN_FAIL;
goto out;
}

xas_create_range() can absolutely create nodes with zero entries.
So if we create m/n nodes and then it runs out of memory (or cgroup
denies it), we can leave nodes in the tree with zero entries.

There are three options for fixing it ...
- Switch to using multi-index entries. We need to do this anyway, but
I don't yet have a handle on the bugs that you found last time I
pushed this into linux-next. At -rc5 seems like a late stage to be
trying this solution.
- Add an xas_prune_range() that gets called on failure. Should be
straightforward to write, but will be obsolete as soon as we do the
above and it's a pain for the callers.
- Change how xas_create_range() works to merely preallocate the xa_nodes
and not insert them into the tree until we're trying to insert data into
them. I favour this option, and this scenario is amenable to writing
a test that will simulate failure halfway through.

I'm going to start on option 3 now.

\
 
 \ /
  Last update: 2021-04-01 20:40    [W:0.069 / U:0.284 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site