linux-kernel - Re: [PATCH 2/5] kernel.h: Add non_block

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Date:   Fri, 16 Aug 2019 11:05:01 -0400
From:   Jerome Glisse <jglisse@...hat.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     Michal Hocko <mhocko@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org DRI Development" 
        <dri-devel@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Christian König <christian.koenig@....com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Wei Wang <wvw@...gle.com>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jann Horn <jannh@...gle.com>, Feng Tang <feng.tang@...el.com>,
        Kees Cook <keescook@...omium.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Daniel Vetter <daniel.vetter@...el.com>
Subject: Re: [PATCH 2/5] kernel.h: Add non_block_start/end()

On Fri, Aug 16, 2019 at 11:31:45AM -0300, Jason Gunthorpe wrote:
> On Fri, Aug 16, 2019 at 02:26:25PM +0200, Michal Hocko wrote:
> > On Fri 16-08-19 09:19:06, Jason Gunthorpe wrote:
> > > On Fri, Aug 16, 2019 at 10:10:29AM +0200, Michal Hocko wrote:
> > > > On Thu 15-08-19 17:13:23, Jason Gunthorpe wrote:
> > > > > On Thu, Aug 15, 2019 at 09:35:26PM +0200, Michal Hocko wrote:

[...]

> > > I would like to inject it into the notifier path as this is very
> > > difficult for driver authors to discover and know about, but I'm
> > > worried about your false positive remark.
> > > 
> > > I think I understand we can use only GFP_ATOMIC in the notifiers, but
> > > we need a strategy to handle OOM to guarentee forward progress.
> > 
> > Your example is from the notifier registration IIUC. 
> 
> Yes, that is where this commit hit it.. Triggering this under an
> actual notifier to get a lockdep report is hard.
> 
> > Can you pre-allocate before taking locks? Could you point me to some
> > examples when the allocation is necessary in the range notifier
> > callback?
> 
> Hmm. I took a careful look, I only found mlx5 as obviously allocating
> memory:
> 
>  mlx5_ib_invalidate_range()
>   mlx5_ib_update_xlt()
>    __get_free_pages(gfp, get_order(size));
> 
> However, I think this could be changed to fall back to some small
> buffer if allocation fails. The existing scheme looks sketchy
> 
> nouveau does:
> 
>  nouveau_svmm_invalidate
>   nvif_object_mthd
>    kmalloc(GFP_KERNEL)
> 
> But I think it reliably uses a stack buffer here
> 
> i915 I think Daniel said he audited.
> 
> amd_mn.. The actual invalidate_range_start does not allocate memory,
> but it is entangled with so many locks it would need careful analysis
> to be sure.
> 
> The others look generally OK, which is good, better than I hoped :)

It is on my TODO list to get rid of allocation in notifier callback
(iirc nouveau already use the stack unless it was lost in all the
revision it wants through). Anyway i do not think we need allocation
in notifier.

Cheers,
Jérôme