netdev - Re: [PATCH] mm: Decline to manipulate the refcount on a slab page

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Message-ID: <77fa8d7e-4752-4979-affe-aa45c8d7795a@suse.de>
Date: Mon, 10 Mar 2025 17:57:51 +0100
From: Hannes Reinecke <hare@...e.de>
To: "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc: netdev@...r.kernel.org, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH] mm: Decline to manipulate the refcount on a slab page

On 3/10/25 15:27, Matthew Wilcox (Oracle) wrote:
> Slab pages now have a refcount of 0, so nobody should be trying to
> manipulate the refcount on them.  Doing so has little effect; the object
> could be freed and reallocated to a different purpose, although the slab
> itself would not be until the refcount was put making it behave rather
> like TYPESAFE_BY_RCU.
> 
> Unfortunately, __iov_iter_get_pages_alloc() does take a refcount.
> Fix that to not change the refcount, and make put_page() silently not
> change the refcount.  get_page() warns so that we can fix any other
> callers that need to be changed.
> 
> Long-term, networking needs to stop taking a refcount on the pages that
> it uses and rely on the caller to hold whatever references are necessary
> to make the memory stable.  In the medium term, more page types are going
> to hav a zero refcount, so we'll want to move get_page() and put_page()
> out of line.
> 
> Reported-by: Hannes Reinecke <hare@...e.de>
> Fixes: 9aec2fb0fd5e (slab: allocate frozen pages)
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>   include/linux/mm.h | 7 ++++++-
>   lib/iov_iter.c     | 8 ++++++--
>   2 files changed, 12 insertions(+), 3 deletions(-)
> 

I assume we will have a discussion at LSF around frozen pages/slab
behaviour?
It's not just networking, also every driver using iov_alloc_pages() and
friends is potentially affected.
And it would be good to clarify rules how these iterators should be
used.

But that doesn't affect this patch, so:

Reviewed-by: Hannes Reinecke <hare@...e.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@...e.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
HRB 36809 (AG Nürnberg), GF: I. Totev, A. McDonald, W. Knoblich