linux-kernel - Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page

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Message-ID: <CAAa6QmRrnRHEEQMMYe20GLXj7g+LVVHVRAKUdSLy=jUW=khb2A@mail.gmail.com>
Date:   Wed, 16 Aug 2023 14:47:45 -0700
From:   "Zach O'Keefe" <zokeefe@...gle.com>
To:     Saurabh Singh Sengar <ssengar@...rosoft.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Yang Shi <shy828301@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [EXTERNAL] [PATCH] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

> We have a out of tree driver that maps huge pages through a file handle and
> relies on -> huge_fault. It used to work in 5.19 kernels but 6.1 changed this
> behaviour.
>
> I don’t think reverting the earlier behaviour of fault_path for huge pages should
> impact kernel negatively.
>
> Do you think we can restore this earlier behaviour of kernel to allow page fault
> for huge pages via ->huge_fault.

That seems reasonable to me. I think using the existence of a
->huge_fault() handler as a predicate to return "true" makes sense to
me. The "normal" flow for file-backed memory along fault path still
needs to return "false", so that we correctly fallback to ->fault()
handler. Unless there are objections, I can do that in a v2.