Re: [PATCH] fs/dcache.c: re-add cond_resched() in shrink_dcache_parent()

Andrew Morton
2018-04-13T21:14:33Z

On Fri, 13 Apr 2018 13:28:23 -0700 Khazhismel Kumykov <khazhy@google.com> wrote:

> shrink_dcache_parent may spin waiting for a parallel shrink_dentry_list.
> In this case we may have 0 dentries to dispose, so we will never
> schedule out while waiting for the parallel shrink_dentry_list to
> complete.
>
> Tested that this fixes syzbot reports of stalls in shrink_dcache_parent()

Well I guess the patch is OK as a stopgap, but things seem fairly
messed up in there. shrink_dcache_parent() shouldn't be doing a
busywait, waiting for the concurrent shrink_dentry_list().

Either we should be waiting (sleeping) for the concurrent operation to
complete or we should just bail out of shrink_dcache_parent(), perhaps
with

if (list_empty(&data.dispose))
break;

or similar. Dunno.

That block comment over `struct select_data' is not a good one. "It
returns zero iff...". *What* returns zero? select_collect()? No it
doesn't, it returns an `enum d_walk_ret'. Perhaps the comment is
trying to refer to select_data.found. And the real interpretation of
select_data.found is, umm, hard to describe. "Counts the number of
dentries which are on a shrink list or which were moved to the dispose
list". Why? What's that all about?

This code needs a bit of thought, documentation and perhaps a redo,
I suspect.