Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during scan

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