Re: Kernel oops with 6.14 when enabling TLS

[Hannes Reinecke <hare@suse.de>]

On 3/4/25 19:05, Matthew Wilcox wrote:
> On Tue, Mar 04, 2025 at 04:53:09PM +0000, Matthew Wilcox wrote:
>> Right, that's what happened in the block layer.  We mark the bio with
>> BIO_PAGE_PINNED if the pincount needs to be dropped.  As a transitional
>> period, we had BIO_PAGE_REFFED which indicated that the page refcount
>> needed to be dropped.  Perhaps there's something similar that network
>> could be doing.
> 
> Until that time ... how does this look as a quick hack to avoid
> reverting the slab change?
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index d6fed25243c3..ca08a923ac6d 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1520,7 +1520,10 @@ 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)
> @@ -1614,6 +1617,8 @@ 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;
> 

Good news and bad news ...
Good news: TLS works again!
Bad news: no errors.

Question to the wise: this is not the only place in iov_iter.c where we 
do a 'get_page()'. Do we leave them and wait for others to report 
regressions, knowing fully well that the current code _has_ issues?
Or shouldn't we rather clean them up?

I guess the real fix would be to fiddle with the 'bio_add_page()' logic;
we are always adding a 'page' reference to the bio, completely ignoring
whether this page is a slab page or a normal one.

Discussion at LSF, maybe?

Cheers,

Hannes
-- 
Dr. Hannes Reinecke                  Kernel Storage Architect
hare@suse.de                                +49 911 74053 688
SUSE Software Solutions GmbH, Frankenstr. 146, 90461 Nürnberg
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