linux-kernel - Re: [PATCH] /dev/zero: make private mapping full anonymous mapping

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Message-ID: <08e7f845-6b86-4fdc-830d-23f15ad90475@redhat.com>
Date: Tue, 14 Jan 2025 14:01:32 +0100
From: David Hildenbrand <david@...hat.com>
To: Yang Shi <yang@...amperecomputing.com>, arnd@...db.de,
 gregkh@...uxfoundation.org, Liam.Howlett@...cle.com,
 lorenzo.stoakes@...cle.com, vbabka@...e.cz, jannh@...gle.com,
 willy@...radead.org, liushixin2@...wei.com, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] /dev/zero: make private mapping full anonymous mapping

On 13.01.25 23:30, Yang Shi wrote:
> When creating private mapping for /dev/zero, the driver makes it an
> anonymous mapping by calling set_vma_anonymous().  But it just sets
> vm_ops to NULL, vm_file is still valid and vm_pgoff is also file offset.
> 
> This is a special case and the VMA doesn't look like either anonymous VMA
> or file VMA.  It confused other kernel subsystem, for example, khugepaged [1].
> 
 > It seems pointless to keep such special case.  Making private 
/dev/zero> mapping a full anonymous mapping doesn't change the semantic of
> /dev/zero either.
> 
> The user visible effect is the mapping entry shown in /proc/<PID>/smaps
> and /proc/<PID>/maps.
> 
> Before the change:
> ffffb7190000-ffffb7590000 rw-p 00001000 00:06 8                          /dev/zero
> 
> After the change:
> ffffb6130000-ffffb6530000 rw-p 00000000 00:00 0
> 

Hm, not sure about this. It's actually quite consistent to have that 
output in smaps the way it is. You mapped a file at an offset, and it 
behaves like an anonymous mapping apart from that.

Not sure if the buggy khugepaged thing is a good indicator to warrant 
this change.

-- 
Cheers,

David / dhildenb