linux-kernel - Re: [PATCH v8 mm-new 02/12] mm: thp: remove vm_flags parameter from khugepaged_enter

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Message-ID: <CALOAHbAGPZX+V4CBRyGhHtQ2mVFJWD4CUX+9Fujp-JAiK426Xg@mail.gmail.com>
Date: Sun, 28 Sep 2025 10:35:33 +0800
From: Yafang Shao <laoar.shao@...il.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: akpm@...ux-foundation.org, david@...hat.com, ziy@...dia.com, 
	baolin.wang@...ux.alibaba.com, lorenzo.stoakes@...cle.com, 
	Liam.Howlett@...cle.com, npache@...hat.com, ryan.roberts@....com, 
	dev.jain@....com, hannes@...xchg.org, gutierrez.asier@...wei-partners.com, 
	willy@...radead.org, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org, 
	ameryhung@...il.com, rientjes@...gle.com, corbet@....net, 21cnbao@...il.com, 
	shakeel.butt@...ux.dev, tj@...nel.org, lance.yang@...ux.dev, 
	bpf@...r.kernel.org, linux-mm@...ck.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Yang Shi <shy828301@...il.com>
Subject: Re: [PATCH v8 mm-new 02/12] mm: thp: remove vm_flags parameter from khugepaged_enter_vma()

On Fri, Sep 26, 2025 at 10:50 PM Usama Arif <usamaarif642@...il.com> wrote:
>
>
>
> On 26/09/2025 10:33, Yafang Shao wrote:
> > The khugepaged_enter_vma() function requires handling in two specific
> > scenarios:
> > 1. New VMA creation
> >   When a new VMA is created, if vma->vm_mm is not present in
> >   khugepaged_mm_slot, it must be added. In this case,
> >   khugepaged_enter_vma() is called after vma->vm_flags have been set,
> >   allowing direct use of the VMA's flags.
> > 2. VMA flag modification
> >   When vma->vm_flags are modified (particularly when VM_HUGEPAGE is set),
> >   the system must recheck whether to add vma->vm_mm to khugepaged_mm_slot.
> >   Currently, khugepaged_enter_vma() is called before the flag update, so
> >   the call must be relocated to occur after vma->vm_flags have been set.
> >
> > Additionally, khugepaged_enter_vma() is invoked in other contexts, such as
> > during VMA merging. However, these calls are unnecessary because the
> > existing VMA already ensures that vma->vm_mm is registered in
> > khugepaged_mm_slot. While removing these redundant calls represents a
> > potential optimization, that change should be addressed separately.
> > Because VMA merging only occurs when the vm_flags of both VMAs are
> > identical (excluding special flags like VM_SOFTDIRTY), we can safely use
> > target->vm_flags instead.
> >
>
> The patch looks good to me, but if we are sure that khugepaged_enter_vma
> is not needed in VMA merging case,

Calling khugepaged_enter_vma() is unnecessary during VMA merging
because it's already handled: for non-anonymous VMAs, it's called upon
creation, and for anonymous VMAs, it's handled at page fault.

> we should remove it in this patch itself.

I'd prefer to handle this cleanup separately. The goal is to keep the
THP changes minimal, even though I've already made significant
modifications ;-)

> If the reason we are removing what flags are being considered when calling
> khugepaged_enter_vma in VMA merging case is because the calls are unnecessary,

Actually, the rationale is that the flags can be removed because:

 Because VMA merging only occurs when the vm_flags of both VMAs are
 identical (excluding special flags like VM_SOFTDIRTY), we can safely use
 target->vm_flags instead.

I will update the commit log to clarify this point.

> then we should just remove the calls and not modify them
> (if its safe and functionally correct :))


-- 
Regards
Yafang