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

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Message-Id: <20231006105810.17fcb352e33cbcab1645099b@linux-foundation.org>
Date:   Fri, 6 Oct 2023 10:58:10 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Zach O'Keefe" <zokeefe@...gle.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Saurabh Singh Sengar <ssengar@...rosoft.com>,
        Yang Shi <shy828301@...il.com>,
        Matthew Wilcox <willy@...radead.org>,
        David Hildenbrand <david@...hat.com>,
        Ryan Roberts <ryan.roberts@....com>
Subject: Re: [PATCH v4] mm/thp: fix
 "mm: thp: kill __transhuge_page_enabled()"

On Mon, 25 Sep 2023 13:01:10 -0700 "Zach O'Keefe" <zokeefe@...gle.com> 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.
>
> ...
>
> @@ -100,11 +100,11 @@ bool hugepage_vma_check(struct vm_area_struct *vma, unsigned long vm_flags,
>  		return in_pf;
>  

Ryan's "mm: thp: introduce anon_orders and anon_always_mask sysfs
files" changes hugepage_vma_check() to return an unsigned int, so this
patch will need some rework to fit in after that.

However Ryan's overall series "variable-order, large folios for
anonymous memory" is in early days and might not make it.

And as I don't know what is the urgency of this patch ("mm/thp: fix
"mm: thp: kill __transhuge_page_enabled()"), I'm unable to decide which
patch needs to come first (thus requiring rework of the other patch).

Please discuss!