Re: [PATCH] mm: delete unsafe BUG from page_cache_add_speculative()

[Matthew Wilcox]

On Wed, Dec 08, 2021 at 11:19:18PM -0800, Hugh Dickins wrote:
> It is not easily reproducible, but on 5.16-rc I have several times hit
> the VM_BUG_ON_PAGE(PageTail(page), page) in page_cache_add_speculative():
> usually from filemap_get_read_batch() for an ext4 read, yesterday from
> next_uptodate_page() from filemap_map_pages() for a shmem fault.
> 
> That BUG used to be placed where page_ref_add_unless() had succeeded,
> but now it is placed before folio_ref_add_unless() is attempted: that
> is not safe, since it is only the acquired reference which makes the
> page safe from racing THP collapse or split.
> 
> We could keep the BUG, checking PageTail only when folio_ref_try_add_rcu()
> has succeeded; but I don't think it adds much value - just delete it.

Whoops, that was careless of me.  I agree with your reasoning and patch.

Reviewed-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>

> Fixes: 020853b6f5ea ("mm: Add folio_try_get_rcu()")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> ---
> 
>  include/linux/pagemap.h |    1 -
>  1 file changed, 1 deletion(-)
> 
> --- 5.16-rc4/include/linux/pagemap.h
> +++ linux/include/linux/pagemap.h
> @@ -285,7 +285,6 @@ static inline struct inode *folio_inode(
>  
>  static inline bool page_cache_add_speculative(struct page *page, int count)
>  {
> -	VM_BUG_ON_PAGE(PageTail(page), page);
>  	return folio_ref_try_add_rcu((struct folio *)page, count);
>  }
>