From: Christoph Hellwig <hch@lst.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>,
iommu <iommu@lists.linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>, Eric Dumazet <edumazet@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Christoph Hellwig <hch@lst.de>,
Dan Williams <dan.j.williams@intel.com>
Subject: Re: [PATCH] dma-debug: fix debug_dma_assert_idle(), use rcu_read_lock()
Date: Fri, 14 Aug 2020 07:42:41 +0200 [thread overview]
Message-ID: <20200814054241.GA719@lst.de> (raw)
In-Reply-To: <CAHk-=whYLHtbeF6BFmoiik9PTjP2+pnpWxXLE9f0ccnT0LAd5A@mail.gmail.com>
On Thu, Aug 13, 2020 at 12:02:41PM -0700, Linus Torvalds wrote:
> Yeah, that's ridiculously expensive, and serializes things for no good reason.
>
> Your patch looks obviously correct to me (Christoph?),
It also looks correct to me.
> but it also
> makes me go "why are we doing this in the first place"?
>
> Because it looks to me like
> (a) the debug check is wrong
> (b) this is left-over from early debugging
>
> In particular, I don't see why we couldn't do a COW on a page that is
> under writeback at the same time. We're not changing the page that is
> doing DMA.
Yes. We don't need to check for a DMA to the device, but a DMA from
the device while under DMA obviously is bogus. But then again you'd
need to try really hard to do that.
> In fact, the whole "COW with DMA" makes me feel like the real bug may
> have been due that whole "ambiguous COW" thing, which was fixed in
> 17839856fd58 ("gup: document and work around "COW can break either
> way" issue")
>
> That debug thing goes back almost 7 years, and I don't think it has
> caught anything in those seven years, but I could be wrong.
>
> The commit that adds it does talk about a bug, but that code was
> removed entirely eventually. And google shows no hits for
> debug_dma_assert_idle() since - until your email.
>
> So my gut feel is that we should remove the check entirely, although
> your patch does seem like a big improvement.
>
> Christoph?
The whole thing predates my involvement with the code, but I defintively
think the patch from Hugh is a major improvement. But I would also
have no problem with just removing it entirely.
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WARNING: multiple messages have this Message-ID (diff)
From: Christoph Hellwig <hch@lst.de>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Hugh Dickins <hughd@google.com>, Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>,
Dan Williams <dan.j.williams@intel.com>,
Eric Dumazet <edumazet@google.com>,
iommu <iommu@lists.linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH] dma-debug: fix debug_dma_assert_idle(), use rcu_read_lock()
Date: Fri, 14 Aug 2020 07:42:41 +0200 [thread overview]
Message-ID: <20200814054241.GA719@lst.de> (raw)
In-Reply-To: <CAHk-=whYLHtbeF6BFmoiik9PTjP2+pnpWxXLE9f0ccnT0LAd5A@mail.gmail.com>
On Thu, Aug 13, 2020 at 12:02:41PM -0700, Linus Torvalds wrote:
> Yeah, that's ridiculously expensive, and serializes things for no good reason.
>
> Your patch looks obviously correct to me (Christoph?),
It also looks correct to me.
> but it also
> makes me go "why are we doing this in the first place"?
>
> Because it looks to me like
> (a) the debug check is wrong
> (b) this is left-over from early debugging
>
> In particular, I don't see why we couldn't do a COW on a page that is
> under writeback at the same time. We're not changing the page that is
> doing DMA.
Yes. We don't need to check for a DMA to the device, but a DMA from
the device while under DMA obviously is bogus. But then again you'd
need to try really hard to do that.
> In fact, the whole "COW with DMA" makes me feel like the real bug may
> have been due that whole "ambiguous COW" thing, which was fixed in
> 17839856fd58 ("gup: document and work around "COW can break either
> way" issue")
>
> That debug thing goes back almost 7 years, and I don't think it has
> caught anything in those seven years, but I could be wrong.
>
> The commit that adds it does talk about a bug, but that code was
> removed entirely eventually. And google shows no hits for
> debug_dma_assert_idle() since - until your email.
>
> So my gut feel is that we should remove the check entirely, although
> your patch does seem like a big improvement.
>
> Christoph?
The whole thing predates my involvement with the code, but I defintively
think the patch from Hugh is a major improvement. But I would also
have no problem with just removing it entirely.
next prev parent reply other threads:[~2020-08-14 5:42 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-13 3:17 [PATCH] dma-debug: fix debug_dma_assert_idle(), use rcu_read_lock() Hugh Dickins via iommu
2020-08-13 3:17 ` Hugh Dickins
2020-08-13 19:02 ` Linus Torvalds
2020-08-13 19:02 ` Linus Torvalds
2020-08-13 23:37 ` Dan Williams
2020-08-13 23:37 ` Dan Williams
2020-08-14 5:42 ` Christoph Hellwig [this message]
2020-08-14 5:42 ` Christoph Hellwig
2020-08-14 22:40 ` Linus Torvalds
2020-08-14 22:40 ` Linus Torvalds
2020-08-15 0:26 ` Hugh Dickins via iommu
2020-08-15 0:26 ` Hugh Dickins
2020-08-15 0:59 ` Linus Torvalds
2020-08-15 0:59 ` Linus Torvalds
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