From: John Hubbard <jhubbard@nvidia.com>
To: Matthew Wilcox <willy@infradead.org>
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: Sun, 28 May 2023 16:49:52 -0700 [thread overview]
Message-ID: <9821bd9c-7c30-8f0c-68e4-6b1d312bc032@nvidia.com> (raw)
In-Reply-To: <ZHF2byaRlaX3W6Md@casper.infradead.org>
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.
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.
thanks,
--
John Hubbard
NVIDIA
next prev parent reply other threads:[~2023-05-28 23:50 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 [this message]
2023-05-29 0:31 ` Matthew Wilcox
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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