linux-kernel - Re: [RFC PATCH] usb: host: xhci: allow __GFP

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Date:   Wed, 22 May 2019 10:56:01 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Oliver Neukum <oneukum@...e.com>
cc:     Jaewon Kim <jaewon31.kim@...il.com>,
        Christoph Hellwig <hch@...radead.org>, <linux-mm@...ck.org>,
        <gregkh@...uxfoundation.org>,
        Jaewon Kim <jaewon31.kim@...sung.com>,
        <m.szyprowski@...sung.com>, <ytk.lee@...sung.com>,
        <linux-kernel@...r.kernel.org>, <linux-usb@...r.kernel.org>
Subject: Re: [RFC PATCH] usb: host: xhci: allow __GFP_FS in dma allocation

On Wed, 22 May 2019, Oliver Neukum wrote:

> On Di, 2019-05-21 at 10:00 -0400, Alan Stern wrote:
> > 
> > Changing configurations amounts to much the same as disconnecting,
> > because both operations destroy all the existing interfaces.
> > 
> > Disconnect can arise in two different ways.
> > 
> >         Physical hot-unplug: All I/O operations will fail.
> > 
> >         Rmmod or unbind: I/O operations will succeed.
> > 
> > The second case is probably okay.  The first we can do nothing about.  
> > However, in either case we do need to make sure that memory allocations
> > do not require any writebacks.  This suggests that we need to call
> > memalloc_noio_save() from within usb_unbind_interface().
> 
> I agree with the problem, but I fail to see why this issue would be
> specific to USB. Shouldn't this be done in the device core layer?

Only for drivers that are on the block-device writeback path.  The 
device core doesn't know which drivers these are.

Alan Stern