linux-kernel - Re: [GIT PULL] MM updates for 6.5-rc1

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Date:   Wed, 28 Jun 2023 10:27:44 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        David Howells <dhowells@...hat.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>
Cc:     linux-mm@...ck.org, mm-commits@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] MM updates for 6.5-rc1

On Mon, 26 Jun 2023 at 08:50, Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> Linus, please merge the MM updates for the 6.5-rc cycle.
>  [...]
> merge conflict in mm/gup.c, vs block tree:
> https://lkml.kernel.org/r/20230616115856.3ce7682c@canb.auug.org.au

Hmm. I think this merge resolution in -next is wrong.

It now does a common

        folio = try_get_folio(page, refs);
        if (flags & FOLL_GET)
                return folio;

for both FOLL_GET and FOLL_PIN, and then *after* that for the FOLL_PIN
case it does that

        /*
         * Don't take a pin on the zero page - it's not going anywhere
         * and it is used in a *lot* of places.
         */
        if (is_zero_page(page))
                return page_folio(page);

but by then it has already done the try_get_folio().

End result: it has already updated refcounts, despite the comment
saying not to do that.

So I think it needs to match the comment (and the try_grab_page()
logic), and just basically

        if (flags & FOLL_GET)
                return try_get_folio(page, refs);

        if (is_zero_page(page))
                return page_folio(page);

        folio = try_get_folio(page, refs);
        if (!folio)
                return NULL;

instead.

That's what my resolution is going to do, but let's add others to the
participants list just in case somebody goes "No, Linus, the reason
-next does that is XYZ"...

               Linus