linux-kernel - Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

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Date:   Mon, 7 Aug 2023 09:43:42 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <stable@...r.kernel.org>, <willy@...radead.org>,
        <vishal.moola@...il.com>, <wangkefeng.wang@...wei.com>,
        <minchan@...nel.org>, <yuzhao@...gle.com>, <david@...hat.com>,
        <ryan.roberts@....com>, <shy828301@...il.com>
Subject: Re: [PATCH 0/2] don't use mapcount() to check large folio sharing

On Fri, 4 Aug 2023 15:14:53 +0800 "Yin, Fengwei" <fengwei.yin@...el.com> wrote:

> Hi Andrew,
> 
> On 8/2/2023 8:39 PM, Yin, Fengwei wrote:
> > Hi Andrew,
> > 
> > On 7/29/2023 1:24 AM, Andrew Morton wrote:
> >> On Sat, 29 Jul 2023 00:13:54 +0800 Yin Fengwei <fengwei.yin@...el.com> wrote:
> >>
> >>> In madvise_cold_or_pageout_pte_range() and madvise_free_pte_range(),
> >>> folio_mapcount() is used to check whether the folio is shared. But it's
> >>> not correct as folio_mapcount() returns total mapcount of large folio.
> >>>
> >>> Use folio_estimated_sharers() here as the estimated number is enough.
> >>
> >> What are the user-visible runtime effects of these changes?
> >>
> >> (and please try to avoid using the same Subject: for different patches)
> >>
> > 
> > Can you hold on these patches to mm-unstable? I think we need to wait for
> > David's work on folio_maybe_mapped_shared() and redo the fix base on that.
> > Thanks and sorry for the noise.
> Sorry for bothering you again for this patchset.
> 
> Let me explain the situation here:
>   - The reason to hold on the patches to mm-unstable is that I don't want to
>     promote the fix in this patch (using folio_estimated_sharers()). The
>     correct way is waiting for folio_maybe_mapped_shared() from David.
> 
>     Merging these patches motivate using folio_estimated_sharers() in other
>     places. So once folio_maybe_mapped_shared() is ready, we need to replace
>     folio_estimated_sharers() with folio_maybe_mapped_shared().
> 
>   - For this specific patches, if they are suitable for stable, we may want to
>     merge it (special for stable branch. I assume folio_maybe_mapped_shared()
>     may not be back ported to stable branch).
> 
> So how do we deal with this situation? Thanks in advance.
> 

I think I'll stage them for 6.5, with a cc:stable.

I'll drop the current three patches.  Please resend with

a) different Subject:s for all patches and

b) changelogs which fully describe the user-visible effects of the change.

Thanks.