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

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Message-ID: <ZHDh4Jeb/vKY+nGU@casper.infradead.org>
Date:   Fri, 26 May 2023 17:44:16 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Khalid Aziz <khalid.aziz@...cle.com>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        akpm@...ux-foundation.org, david@...hat.com, 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 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.