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: <CAMuHMdW4-fKLiymBeKtMFM7g4VqEKOfugYzUe=6dP_XKegFWhw@mail.gmail.com>
Date:   Fri, 8 Dec 2017 08:43:37 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Alan Stern <stern@...land.harvard.edu>
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()

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?

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds