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.
