linux-kernel - Re: [PATCH 0/4] mm: ksm: prevent KSM from entirely breaking VMA merging

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Message-ID: <135556a3-9b72-4ec0-acc3-21ee0d15ebe3@redhat.com>
Date: Mon, 19 May 2025 13:53:43 +0200
From: David Hildenbrand <david@...hat.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>,
 Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
 Pedro Falcato <pfalcato@...e.de>, Xu Xin <xu.xin16@....com.cn>,
 Chengming Zhou <chengming.zhou@...ux.dev>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/4] mm: ksm: prevent KSM from entirely breaking VMA
 merging

On 19.05.25 10:51, Lorenzo Stoakes wrote:
> When KSM-by-default is established using prctl(PR_SET_MEMORY_MERGE), this
> defaults all newly mapped VMAs to having VM_MERGEABLE set, and thus makes
> them available to KSM for samepage merging. It also sets VM_MERGEABLE in
> all existing VMAs.
> 
> However this causes an issue upon mapping of new VMAs - the initial flags
> will never have VM_MERGEABLE set when attempting a merge with adjacent VMAs
> (this is set later in the mmap() logic), and adjacent VMAs will ALWAYS have
> VM_MERGEABLE set.

Just to clarify, you mean that VM_MERGEABLE is set later, during 
__mmap_new_vma()->ksm_add_vma()->__ksm_add_vma(), and we are already 
past vma_merge_new_range(), correct?

-- 
Cheers,

David / dhildenb