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

From: Matthew Wilcox <willy@infradead.org>
To: Khalid Aziz <khalid.aziz@oracle.com>
Cc: Steven Sistare <steven.sistare@oracle.com>,
	akpm@linux-foundation.org, david@redhat.com,
	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: Fri, 26 May 2023 17:44:16 +0100	[thread overview]
Message-ID: <ZHDh4Jeb/vKY+nGU@casper.infradead.org> (raw)
In-Reply-To: <60367660-f4a3-06dc-4d17-4dbdc733ef74@oracle.com>

On Fri, May 26, 2023 at 09:44:34AM -0600, Khalid Aziz wrote:
> > Oh, I think I found it!  pin_user_pages_remote() is called by
> > vaddr_get_pfns().  If these are the pages you're concerned about,
> > then the efficient way to do what you want is simply to call
> > folio_maybe_dma_pinned().  Far more efficient than the current mess
> > of total_mapcount().
> 
> vfio pinned pages triggered this change. Wouldn't checking refcounts against
> mapcount provide a more generalized way of detecting non-migratable pages?

Well, you changed the comment to say that we were concerned about
long-term pins.  If we are, than folio_maybe_dma_pinned() is how to test
for long-term pins.  If we want to skip pages which are short-term pinned,
then we need to not change the comment, and keep using mapcount/refcount
differences.