linux-kernel - Re: USB: hub: Delete an error message for a failed memory allocation in usb_hub_clear_tt

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Message-ID: <Pine.LNX.4.44L0.1712081235230.1371-100000@iolanthe.rowland.org>
Date:   Fri, 8 Dec 2017 12:42:05 -0500 (EST)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
cc:     Dan Carpenter <dan.carpenter@...cle.com>,
        SF Markus Elfring <elfring@...rs.sourceforge.net>,
        USB list <linux-usb@...r.kernel.org>,
        Joe Perches <joe@...ches.com>,
        Daniel Drake <drake@...lessm.com>,
        Dmitry Fleytman <dmitry@...nix.com>,
        Eugene Korenevsky <ekorenevsky@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Günter Röck <linux@...ck-us.net>,
        Johan Hovold <johan@...nel.org>,
        Mathias Nyman <mathias.nyman@...ux.intel.com>,
        Peter Chen <peter.chen@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: Re: USB: hub: Delete an error message for a failed memory allocation
 in usb_hub_clear_tt_buffer()

On Fri, 8 Dec 2017, Geert Uytterhoeven wrote:

> Hi Alan,
> 
> On Thu, Dec 7, 2017 at 10:26 PM, Alan Stern <stern@...land.harvard.edu> wrote:
> > On Thu, 7 Dec 2017, Dan Carpenter wrote:
> >> The standard is to treat them like errors and try press forward in a
> >> degraded mode but don't print a message.  Checkpatch.pl complains if you
> >> print a warning for allocation failures.  A lot of low level functions
> >> handle their own messages pretty well but especially kmalloc() does.
> >
> > Which brings us back to my original objection.  If an allocation
> > failure has localized effects, but the low-level warning is unable to
> > specify what will be affected, how is the user supposed to connect the
> > effect to the cause?
> 
> The backtrace would include usb_hub_clear_tt_buffer, so the user will
> know USB is affected.
> Note that the cause of the memory exhaustion is usually not the caller.
> Unless it requests a real big block of memory, in which case that will be
> mentioned in the backtrace, too.
> 
> In this particular case, the driver uses GFP_ATOMIC, so allocation failures
> aren't that rare, and I think the driver should be prepared for that, and try
> to recover gracefully.
> 
> The comment
> 
>                 /* FIXME recover somehow ... RESET_TT? */
> 
> makes me think it isn't.
> 
> As this is a pretty small allocation, perhaps it can be done beforehand, without
> GFP_ATOMIC?

I can't see how to make that work.  We don't know beforehand how many
structures will be needed at any time.

Alan Stern