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

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Message-ID: <f5a37f8d-d888-9085-2f2b-1e350a267396@redhat.com>
Date:   Fri, 26 May 2023 18:46:15 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Khalid Aziz <khalid.aziz@...cle.com>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        akpm@...ux-foundation.org, ying.huang@...el.com,
        mgorman@...hsingularity.net, baolin.wang@...ux.alibaba.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Khalid Aziz <khalid@...nel.org>
Subject: Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during
 scan

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