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Date:   Fri, 26 May 2023 18:46:15 +0200
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Subject: Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during
 scan
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To:     Matthew Wilcox <willy@infradead.org>,
        Khalid Aziz <khalid.aziz@oracle.com>
Cc:     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>
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From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <ZHDh4Jeb/vKY+nGU@casper.infradead.org>
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On 26.05.23 18:44, Matthew Wilcox wrote:
> 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.
> 

folio_maybe_dma_pinned() is all about FOLL_PIN, not FOLL_LONGTERM.

folio_maybe_dma_pinned() would skip migrating any page that has more 
than 1024 references. (shared libraries?)

-- 
Thanks,

David / dhildenb