linux-kernel - Re: [PATCH v4] mm/thp: fix "mm: thp: kill __transhuge_page

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Message-ID: <225e8018-ef79-5514-a6ce-f5994206efe9@redhat.com>
Date:   Fri, 6 Oct 2023 20:53:33 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Zach O'Keefe <zokeefe@...gle.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v4] mm/thp: fix "mm: thp: kill __transhuge_page_enabled()"

On 25.09.23 22:01, 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, this was a bad
> assumption to make as it assumes the only reason to support ->huge_fault
> was for DAX (which is not true in general).
> 
> Remove the VM_NO_KHUGEPAGED check when not in collapse path and give
> any ->huge_fault handler a chance to handle the fault.  Note that we
> don't validate the file mode or mapping alignment, which is consistent
> with the behavior before the aforementioned commits.
> 
> Fixes: 7da4e2cb8b1f ("mm: thp: kill __transhuge_page_enabled()")
> Reported-by: Saurabh Singh Sengar <ssengar@...rosoft.com>
> Signed-off-by: Zach O'Keefe <zokeefe@...gle.com>
> Cc: Yang Shi <shy828301@...il.com>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: David Hildenbrand <david@...hat.com>
> ---
> I've updated the changelog to reflect discussions in [1] -- leaving
> ack to David / Matthew on whether to take the patch.

Works for me.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb