Re: [PATCHv4 15/39] thp, mm: trigger bug in replace_page_cache_page() on THP

[Dave Hansen <dave@sr71.net>]

On 05/11/2013 06:23 PM, Kirill A. Shutemov wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 
> replace_page_cache_page() is only used by FUSE. It's unlikely that we
> will support THP in FUSE page cache any soon.
> 
> Let's pospone implemetation of THP handling in replace_page_cache_page()
> until any will use it.
...
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 657ce82..3a03426 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -428,6 +428,8 @@ int replace_page_cache_page(struct page *old, struct page *new, gfp_t gfp_mask)
>  {
>  	int error;
>  
> +	VM_BUG_ON(PageTransHuge(old));
> +	VM_BUG_ON(PageTransHuge(new));
>  	VM_BUG_ON(!PageLocked(old));
>  	VM_BUG_ON(!PageLocked(new));
>  	VM_BUG_ON(new->mapping);

The code calling replace_page_cache_page() has a bunch of fallback and
error returning code.  It seems a little bit silly to bring the whole
machine down when you could just WARN_ONCE() and return an error code
like fuse already does:

>         /*
>          * This is a new and locked page, it shouldn't be mapped or
>          * have any special flags on it
>          */
>         if (WARN_ON(page_mapped(oldpage)))
>                 goto out_fallback_unlock;
>         if (WARN_ON(page_has_private(oldpage)))
>                 goto out_fallback_unlock;
>         if (WARN_ON(PageDirty(oldpage) || PageWriteback(oldpage)))
>                 goto out_fallback_unlock;
>         if (WARN_ON(PageMlocked(oldpage)))
>                 goto out_fallback_unlock;

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