linux-kernel - Re: [PATCH 1/2] fs: ext4: use BUG_ON if writepage call comes from direct reclaim

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Message-ID: <20180703234331.GA5104@thunk.org>
Date:   Tue, 3 Jul 2018 19:43:31 -0400
From:   "Theodore Y. Ts'o" <tytso@....edu>
To:     Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     mgorman@...hsingularity.net, adilger.kernel@...ger.ca,
        darrick.wong@...cle.com, dchinner@...hat.com,
        akpm@...ux-foundation.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] fs: ext4: use BUG_ON if writepage call comes from
 direct reclaim

On Tue, Jul 03, 2018 at 10:05:04AM -0700, Yang Shi wrote:
> I'm not sure if it is a good choice to let filesystem handle such vital VM
> regression. IMHO, writing out filesystem page from direct reclaim context is
> a vital VM bug. It means something is definitely wrong in VM. It should
> never happen.

If it does happen, it should happen reliably; this isn't the sort of
thing where some linked list had gotten corrupted.  This would be a
structural problem in the VM code.

So presumably, if the WARN_ON triggered, it should be be noticed by VM
developers, and they should fix it.

In general, though, BUG_ON's should be avoided unless there really is
no way to recover.

> It sounds ok to have filesystem throw out warning and handle it, but I'm not
> sure if someone will just ignore the warning, but it should *never* be
> ignored.

If a kernel develper (a VM developer in this case) ignores a warning,
that's just simply professional malpractice.  In general WARN_ON's
should only be used as a sign of a kernel bug.  So they should never
be ignored.

						- Ted