linux-kernel - Re: [PULL] vhost: cleanups and fixes

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Date:   Tue, 12 Jun 2018 04:36:54 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     KVM list <kvm@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        Network Development <netdev@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        wei.w.wang@...el.com
Subject: Re: [PULL] vhost: cleanups and fixes

On Mon, Jun 11, 2018 at 11:44:05AM -0700, Linus Torvalds wrote:
> On Mon, Jun 11, 2018 at 11:32 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > So now we apparently do a GFP_KERNEL allocation insider the mm zone
> > lock, which is broken on just _so_ many levels.
> 
> Oh, I see the comment about how it doesn't actually do an allocation
> at all because it's a single-entry.
> 
> Still too damn ugly to live, and much too fragile. No way in hell do
> we even _hint_ at a GFP_KERNEL when we're inside a context that can't
> do any memory allocation at all.
> 
> Plus I'm not convinced it's a "no allocation" path even despite that
> comment, because it also does a "dma_map_page()" etc, which can cause
> allocations to do the dma mapping thing afaik. No?

Well no because DMA is triggered by the IOMMU flag and
that is always off for the balloon. But I hear what you
are saying about it being fragile.

> Maybe there's some reason why that doesn't happen either, but
> basically this whole callchain looks *way* to complicated to be used
> under a core VM spinlock.
> 
>                 Linus

Maybe it will help to have GFP_NONE which will make any allocation
fail if attempted. Linus, would this address your comment?
-- 
MST