linux-kernel - Re: [PATCH v1 6/9] mm: Change dup

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Message-ID: <fkfi4zldsecyio6xnd6blou27me4nno3v3tnsaxz6xlt63yy2z@nxl7izptm6yx>
Date: Wed, 10 Sep 2025 14:23:48 +0100
From: Pedro Falcato <pfalcato@...e.de>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	maple-tree@...ts.infradead.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	David Hildenbrand <david@...hat.com>, Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, 
	Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>, 
	Michal Hocko <mhocko@...e.com>, Jann Horn <jannh@...gle.com>, 
	Charan Teja Kalla <quic_charante@...cinc.com>, shikemeng@...weicloud.com, kasong@...cent.com, nphamcs@...il.com, 
	bhe@...hat.com, baohua@...nel.org, chrisl@...nel.org, 
	Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH v1 6/9] mm: Change dup_mmap() recovery

On Tue, Sep 09, 2025 at 03:09:42PM -0400, Liam R. Howlett wrote:
> When the dup_mmap() fails during the vma duplication or setup, don't
> write the XA_ZERO entry in the vma tree.  Instead, destroy the tree and
> free the new resources, leaving an empty vma tree.
> 
> Using XA_ZERO introduced races where the vma could be found between
> dup_mmap() dropping all locks and exit_mmap() taking the locks.  The
> race can occur because the mm can be reached through the other trees
> via successfully copied vmas and other methods such as the swapoff code.
> 
> XA_ZERO was marking the location to stop vma removal and pagetable
> freeing.  The newly created arguments to the unmap_vmas() and
> free_pgtables() serve this function.
> 
> Replacing the XA_ZERO entry use with the new argument list also means
> the checks for xa_is_zero() are no longer necessary so these are also
> removed.
> 
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>

Reviewed-by: Pedro Falcato <pfalcato@...e.de>

-- 
Pedro