linux-kernel - Re: [RFC PATCH] mm/huge_memory: Do not clobber swp_entry

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Message-Id: <20221019161810.7510df1f37658a2b71c5e3a7@linux-foundation.org>
Date:   Wed, 19 Oct 2022 16:18:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Yang Shi <shy828301@...il.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Matthew Wilcox <willy@...radead.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] mm/huge_memory: Do not clobber swp_entry_t during
 THP split

On Wed, 19 Oct 2022 11:17:14 -0700 Yang Shi <shy828301@...il.com> wrote:

> > The intent of commit b653db77350c patch was to avoid the case where
> > PG_private is clear but folio->private is not-NULL. However, THP tail
> > pages uses page->private for "swp_entry_t if folio_test_swapcache()" as
> > stated in the documentation for struct folio. This patch only clobbers
> > page->private for tail pages if the head page was not in swapcache and
> > warns once if page->private had an unexpected value.
> 
> It looks like the same issue fixed by
> https://lore.kernel.org/linux-mm/20220906190602.1626037-1-bfoster@redhat.com/

It is.

As I asked earlier this week, what about reverting b653db77350c?  Why
do we care about the value of ->private for non-PG_private pages?