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

[Vlastimil Babka <vbabka@suse.cz>]

On 3/10/25 15:35, 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@suse.de>

Closes:
https://lore.kernel.org/all/08c29e4b-2f71-4b6d-8046-27e407214d8c@suse.com/

> Fixes: 9aec2fb0fd5e (slab: allocate frozen pages)
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Note it's a 6.14 hotfix for kernel oopses due to page refcount overflow.

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  include/linux/mm.h | 7 ++++++-
>  lib/iov_iter.c     | 8 ++++++--
>  2 files changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 61de65c4e430..4e118cbe0556 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1539,7 +1539,10 @@ static inline void folio_get(struct folio *folio)
>  
>  static inline void get_page(struct page *page)
>  {
> -	folio_get(page_folio(page));
> +	struct folio *folio = page_folio(page);
> +	if (WARN_ON_ONCE(folio_test_slab(folio)))
> +		return;
> +	folio_get(folio);
>  }
>  
>  static inline __must_check bool try_get_page(struct page *page)
> @@ -1633,6 +1636,8 @@ static inline void put_page(struct page *page)
>  {
>  	struct folio *folio = page_folio(page);
>  
> +	if (folio_test_slab(folio))
> +		return;
>  	folio_put(folio);
>  }
>  
> diff --git a/lib/iov_iter.c b/lib/iov_iter.c
> index 65f550cb5081..8c7fdb7d8c8f 100644
> --- a/lib/iov_iter.c
> +++ b/lib/iov_iter.c
> @@ -1190,8 +1190,12 @@ static ssize_t __iov_iter_get_pages_alloc(struct iov_iter *i,
>  		if (!n)
>  			return -ENOMEM;
>  		p = *pages;
> -		for (int k = 0; k < n; k++)
> -			get_page(p[k] = page + k);
> +		for (int k = 0; k < n; k++) {
> +			struct folio *folio = page_folio(page);
> +			p[k] = page + k;
> +			if (!folio_test_slab(folio))
> +				folio_get(folio);
> +		}
>  		maxsize = min_t(size_t, maxsize, n * PAGE_SIZE - *start);
>  		i->count -= maxsize;
>  		i->iov_offset += maxsize;