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

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: Sat, 27 May 2023 04:18:07 +0100	[thread overview]
Message-ID: <ZHF2byaRlaX3W6Md@casper.infradead.org> (raw)
In-Reply-To: <846b770c-9f63-90a2-0435-ec82484e3f74@nvidia.com>

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;