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

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Message-ID: <a6e84fce-31e8-bdd2-8ed9-de099f5e2230@oracle.com>
Date:   Fri, 9 Jun 2023 17:28:37 -0600
From:   Khalid Aziz <khalid.aziz@...cle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
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 6/9/23 16:11, Andrew Morton wrote:
> 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.

Hi Andrew,

I would still recommend that we use this patch and fix the existing problem, unless there are objections from other 
people. This is not an optimal solution but it works and is similar to what kernel does elsewhere. When a better 
function to check pinned pages is available, we start using that function instead.

Thanks,
Khalid