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

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Message-ID: <20180415223439.GC30522@ZenIV.linux.org.uk>
Date:   Sun, 15 Apr 2018 23:34:39 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nikolay Borisov <nborisov@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Khazhismel Kumykov <khazhy@...gle.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        David Rientjes <rientjes@...gle.com>,
        Goldwyn Rodrigues <rgoldwyn@...e.de>,
        Jeff Mahoney <jeffm@...e.com>,
        Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH] fs/dcache.c: re-add cond_resched() in
 shrink_dcache_parent()

On Sun, Apr 15, 2018 at 10:54:55PM +0100, Al Viro wrote:
> On Sun, Apr 15, 2018 at 09:40:54PM +0100, Al Viro wrote:
> 
> > BTW, the current placement of cond_resched() looks bogus; suppose we
> > have collected a lot of victims and ran into need_resched().  We leave
> > d_walk() and call shrink_dentry_list().  At that point there's a lot
> > of stuff on our shrink list and anybody else running into them will
> > have to keep scanning.  Giving up the timeslice before we take care
> > of any of those looks like a bad idea, to put it mildly, and that's
> > precisely what will happen.
> > 
> > What about doing that in the end of __dentry_kill() instead?  And to
> > hell with both existing call sites - dput() one (before going to
> > the parent) is obviously covered by that (dentry_kill() only returns
> > non-NULL after having called __dentry_kill()) and in shrink_dentry_list()
> > we'll get to it as soon as we go through all dentries that can be
> > immediately kicked off the shrink list.  Which, AFAICS, improves the
> > situation, now that shrink_lock_dentry() contains no trylock loops...
> > 
> > Comments?
> 
> What I mean is something like this (cumulative diff, it'll obviously need
> to be carved up into 3--4 commits):

... and carved-up version is in vfs.git#work.dcache.  Could syzbot folks
hit it with their reproducers?