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

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Message-Id: <20230609151111.bb7908254853a0879db07746@linux-foundation.org>
Date:   Fri, 9 Jun 2023 15:11:11 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Khalid Aziz <khalid.aziz@...cle.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        John Hubbard <jhubbard@...dia.com>,
        Steven Sistare <steven.sistare@...cle.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 Tue, 30 May 2023 09:42:33 -0600 Khalid Aziz <khalid.aziz@...cle.com> wrote:

> > I completely agree that we should be careful with such mapcount vs. pagecount checks, and if we can use something 
> > better, let's use something *better*.
> 
> When we have a reliable folio_maybe_dma_longterm_pinned() function, it will be better to call that instead of doing 
> refcount vs mapcount check. Until that better function to check for pinned pages is in place, may I propose that the 
> current patch fixes a customer problem though not optimally and is a good enough working solution. When a better 
> function is in place, page_has_extra_refs() function can be updated to rely on this other function instead of refcount 
> vs mapcount.

We seem rather stuck with this patch.  I think I'll drop it while we
ponder a way forward.