linux-kernel - Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page

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Date:   Thu, 24 Aug 2023 09:39:38 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Zach O'Keefe <zokeefe@...gle.com>, linux-mm@...ck.org,
        Yang Shi <shy828301@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH v3] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

On 22.08.23 01:48, Zach O'Keefe wrote:
> The 6.0 commits:
> 
> commit 9fec51689ff6 ("mm: thp: kill transparent_hugepage_active()")
> commit 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> 
> merged "can we have THPs in this VMA?" logic that was previously done
> separately by fault-path, khugepaged, and smaps "THPeligible" checks.
> 
> During the process, the semantics of the fault path check changed in two
> ways:
> 
> 1) A VM_NO_KHUGEPAGED check was introduced (also added to smaps path).
> 2) We no longer checked if non-anonymous memory had a vm_ops->huge_fault
>     handler that could satisfy the fault.  Previously, this check had been
>     done in create_huge_pud() and create_huge_pmd() routines, but after
>     the changes, we never reach those routines.
> 
> During the review of the above commits, it was determined that in-tree
> users weren't affected by the change; most notably, since the only relevant
> user (in terms of THP) of VM_MIXEDMAP or ->huge_fault is DAX, which is
> explicitly approved early in approval logic.  However, there is at least
> one occurrence where an out-of-tree driver that used
> VM_HUGEPAGE|VM_MIXEDMAP with a vm_ops->huge_fault handler, was broken.

... so all we did is break an arbitrary out-of-tree driver? Sorry to 
say, but why should we care?

Is there any in-tree code affected and needs a "Fixes:" ?

-- 
Cheers,

David / dhildenb