From: John Hubbard <jhubbard@nvidia.com>
To: Pasha Tatashin <pasha.tatashin@soleen.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
linux-mm <linux-mm@kvack.org>,
linux-m68k@lists.linux-m68k.org,
Anshuman Khandual <anshuman.khandual@arm.com>,
Matthew Wilcox <willy@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
william.kucharski@oracle.com,
Mike Kravetz <mike.kravetz@oracle.com>,
Vlastimil Babka <vbabka@suse.cz>,
Geert Uytterhoeven <geert@linux-m68k.org>,
schmitzmic@gmail.com, Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@redhat.com>,
Johannes Weiner <hannes@cmpxchg.org>,
Roman Gushchin <guro@fb.com>,
Muchun Song <songmuchun@bytedance.com>,
weixugc@google.com, Greg Thelen <gthelen@google.com>
Subject: Re: [RFC 3/8] mm: Avoid using set_page_count() in set_page_recounted()
Date: Wed, 27 Oct 2021 18:35:44 -0700 [thread overview]
Message-ID: <27b7177c-71be-9ff2-716e-caaa5035d451@nvidia.com> (raw)
In-Reply-To: <19d16b40-355f-3f79-dcba-e1d8d2216d33@nvidia.com>
On 10/27/21 18:20, John Hubbard wrote:
>>> But it's still not good to have this function name doing something completely
>>> different than its name indicates.
>>
>> I see, I can rename it to: 'set_page_recounted/get_page_recounted' ?
>>
>
> What? No, that's not where I was going at all. The function is already
> named set_page_refcounted(), and one of the problems I see is that your
> changes turn it into something that most certainly does not
> set_page_refounted(). Instead, this patch *increments* the refcount.
> That is not the same thing.
>
> And then it uses a .config-sensitive assertion to "prevent" problems.
> And by that I mean, the wording throughout this series seems to equate
> VM_BUG_ON_PAGE() assertions with real assertions. They are only active,
> however, in CONFIG_DEBUG_VM configurations, and provide no protection at
> all for normal (most distros) users. That's something that the wording,
> comments, and even design should be tweaked to account for.
...and to clarify a bit more, maybe this also helps:
These patches are attempting to improve debugging, and that is fine, as
far as debugging goes. However, a point that seems to be slightly
misunderstood is: incrementing a bad refcount value is not actually any
better than overwriting it, from a recovery point of view. Maybe (?)
it's better from a debugging point of view.
That's because the problem occurred before this code, and its debug-only
assertions, ran. Once here, the code cannot actually recover: there is
no automatic way to recover from a refcount that it 1, -1, 2, or 706,
when it was supposed to be zero. Incrementing it is, again, not really
necessarily better than setting: setting it might actually make the
broken system appear to run--and in some cases, even avoid symptoms.
Whereas incrementing doesn't cover anything up. The only thing you can
really does is just panic() or BUG(), really.
Don't get me wrong, I don't want bugs covered up. But the claim that
incrementing is somehow better deserves some actual thinking about it.
Overall, I'm inclined to *not* switch anything over to incrementing the
refcounts. Instead, go ahead and:
a) Add assertions up to a "reasonable" point (some others have pointed
out that you don't need quite all of the assertions you've added).
b) Remove set_page_count() calls where possible--maybe everywhere.
c) Fix any bugs found along the way.
d) ...but, leave the basic logic as-is: no changing over to
page_ref_inc_return().
Anyway, that's my take on it.
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2021-10-28 1:35 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-26 17:38 [RFC 0/8] Hardening page _refcount Pasha Tatashin
2021-10-26 17:38 ` [RFC 1/8] mm: add overflow and underflow checks for page->_refcount Pasha Tatashin
2021-10-26 19:48 ` Matthew Wilcox
2021-10-26 21:34 ` Pasha Tatashin
2021-10-27 1:21 ` Pasha Tatashin
2021-10-27 3:04 ` Matthew Wilcox
2021-10-27 18:22 ` Pasha Tatashin
2021-10-27 7:46 ` Muchun Song
2021-10-27 18:22 ` Pasha Tatashin
2021-10-28 4:08 ` Muchun Song
2021-10-26 17:38 ` [RFC 2/8] mm/hugetlb: remove useless set_page_count() Pasha Tatashin
2021-10-26 18:44 ` Mike Kravetz
2021-10-26 18:50 ` Pasha Tatashin
2021-10-26 21:19 ` Mike Kravetz
2021-10-26 17:38 ` [RFC 3/8] mm: Avoid using set_page_count() in set_page_recounted() Pasha Tatashin
2021-10-26 17:53 ` John Hubbard
2021-10-26 18:01 ` John Hubbard
2021-10-26 18:14 ` Pasha Tatashin
2021-10-26 18:21 ` Pasha Tatashin
2021-10-27 5:12 ` John Hubbard
2021-10-27 18:27 ` Pasha Tatashin
2021-10-28 1:20 ` John Hubbard
2021-10-28 1:35 ` John Hubbard [this message]
2021-11-01 14:30 ` Pasha Tatashin
2021-11-01 19:35 ` John Hubbard
2021-11-01 14:22 ` Pasha Tatashin
2021-11-01 19:31 ` John Hubbard
2021-11-01 19:42 ` John Hubbard
2021-10-26 17:38 ` [RFC 4/8] mm: remove set_page_count() from page_frag_alloc_align Pasha Tatashin
2021-10-26 17:38 ` [RFC 5/8] mm: avoid using set_page_count() when pages are freed into allocator Pasha Tatashin
2021-10-26 17:38 ` [RFC 6/8] mm: rename init_page_count() -> page_ref_init() Pasha Tatashin
2021-10-27 6:46 ` Geert Uytterhoeven
2021-10-26 17:38 ` [RFC 7/8] mm: remove set_page_count() Pasha Tatashin
2021-10-26 17:38 ` [RFC 8/8] mm: simplify page_ref_* functions Pasha Tatashin
2021-10-26 18:23 ` [RFC 0/8] Hardening page _refcount Matthew Wilcox
2021-10-26 18:30 ` Pasha Tatashin
2021-10-26 20:13 ` Matthew Wilcox
2021-10-26 21:24 ` Pasha Tatashin
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