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

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Message-Id: <20180413141430.2788e2562e3e24bd273fe78b@linux-foundation.org>
Date:   Fri, 13 Apr 2018 14:14:30 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Khazhismel Kumykov <khazhy@...gle.com>
Cc:     nborisov@...e.com, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Jeff Mahoney <jeffm@...e.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] fs/dcache.c: re-add cond_resched() in
 shrink_dcache_parent()

On Fri, 13 Apr 2018 13:28:23 -0700 Khazhismel Kumykov <khazhy@...gle.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.