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

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Date:   Thu, 25 May 2023 22:31:15 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Steven Sistare <steven.sistare@...cle.com>
Cc:     Khalid Aziz <khalid.aziz@...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 Thu, May 25, 2023 at 09:45:34PM +0100, Matthew Wilcox wrote:
> > > Isn't "long term pinning" the wrong description of the problem?  Long term
> > > pins suggest to me FOLL_LONGTERM.  I think this is simple short term
> > > pins that we care about here.
> > 
> > vfio pins are held for a long time - Steve
> 
> So this is a third sense of "pinned pages" that is neither what
> filesystems nor the mm means by pinned pages, but whatever it is that
> vfio means by pinned pages?  If only "pin" weren't such a desirable
> word.  Can somebody explain to me in small words what a vfio pin looks
> like because I've tried reading vfio_iommu_type1_pin_pages() and I
> don't recognise anything there that looks like pinning in either of
> the other two senses.

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().