From: Matthew Wilcox <willy@infradead.org>
To: John Hubbard <jhubbard@nvidia.com>
Cc: David Hildenbrand <david@redhat.com>,
Khalid Aziz <khalid.aziz@oracle.com>,
Steven Sistare <steven.sistare@oracle.com>,
akpm@linux-foundation.org, ying.huang@intel.com,
mgorman@techsingularity.net, baolin.wang@linux.alibaba.com,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Khalid Aziz <khalid@kernel.org>
Subject: Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during scan
Date: Mon, 29 May 2023 01:31:49 +0100 [thread overview]
Message-ID: <ZHPydXSAfRq8sh0u@casper.infradead.org> (raw)
In-Reply-To: <9821bd9c-7c30-8f0c-68e4-6b1d312bc032@nvidia.com>
On Sun, May 28, 2023 at 04:49:52PM -0700, John Hubbard wrote:
> On 5/26/23 20:18, Matthew Wilcox wrote:
> > On Fri, May 26, 2023 at 07:11:05PM -0700, John Hubbard wrote:
> > > > So any user with 1024 processes can fragment physical memory? :/
> > > >
> > > > Sorry, I'd like to minimize the usage of folio_maybe_dma_pinned().
> > >
> > > I was actually thinking that we should minimize any more cases of
> > > fragile mapcount and refcount comparison, which then leads to
> > > Matthew's approach here!
> >
> > I was wondering if we shouldn't make folio_maybe_dma_pinned() a little
> > more accurate. eg:
> >
> > if (folio_test_large(folio))
> > return atomic_read(&folio->_pincount) > 0;
> > return (unsigned)(folio_ref_count(folio) - folio_mapcount(folio)) >=
> > GUP_PIN_COUNTING_BIAS;
>
> I'm trying to figure out what might be wrong with that, but it seems
> OK. We must have talked about this earlier, but I recall vaguely that
> there was not a lot of concern about the case of a page being mapped
> > 1024 times. Because pinned or not, it's likely to be effectively
> locked into memory due to LRU effects. As mentioned here, too.
That was my point of view, but David convinced me that a hostile process
can effectively lock its own memory into place.
> Anyway, sure.
>
> A detail:
>
> The unsigned cast, I'm not sure that helps or solves anything, right?
> That is, other than bugs, is it possible to get refcount < mapcount?
>
> And if it's only due to bugs, then the casting, again, isn't likely to
> going to mitigate the fallout from whatever mess the bug caused.
I wasn't thinking too hard about the cast. If the caller has the folio
lock, I don't think it's possible for refcount < mapcount. This caller
has a refcount, but doesn't hold the lock, so it is possible for them
to read mapcount first, then have both mapcount and refcount decremented
and see refcount < mapcount.
I don't think it matters too much. We don't hold the folio lock, so
it might transition from pinned to unpinned as much as a refcount might
be decremented or a mapcount incremented. What's important is that a
hostile process can't prevent memory from being moved indefinitely.
David, have I missed something else?
next prev parent reply other threads:[~2023-05-29 0:32 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-05-25 19:15 [PATCH v4] mm, compaction: Skip all non-migratable pages during scan Khalid Aziz
2023-05-25 19:58 ` Matthew Wilcox
2023-05-25 20:15 ` Steven Sistare
2023-05-25 20:45 ` Matthew Wilcox
2023-05-25 21:31 ` Matthew Wilcox
2023-05-26 15:44 ` Khalid Aziz
2023-05-26 16:44 ` Matthew Wilcox
2023-05-26 16:46 ` David Hildenbrand
2023-05-26 18:46 ` Matthew Wilcox
2023-05-26 18:50 ` David Hildenbrand
2023-05-27 2:11 ` John Hubbard
2023-05-27 3:18 ` Matthew Wilcox
2023-05-28 23:49 ` John Hubbard
2023-05-29 0:31 ` Matthew Wilcox [this message]
2023-05-29 9:25 ` David Hildenbrand
2023-05-30 15:42 ` Khalid Aziz
2023-06-09 22:11 ` Andrew Morton
2023-06-09 23:28 ` Khalid Aziz
2023-05-25 20:41 ` Khalid Aziz
2023-05-29 3:01 ` Huang, Ying
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