Re: [PATCH 2/2] mm: gup: do not call try_grab_folio() in slow path

[Yang Shi <yang@os.amperecomputing.com>]

On 6/4/24 7:57 PM, kernel test robot wrote:
> Hi Yang,
>
> kernel test robot noticed the following build warnings:
>
> [auto build test WARNING on akpm-mm/mm-everything]
>
> url:    https://github.com/intel-lab-lkp/linux/commits/Yang-Shi/mm-gup-do-not-call-try_grab_folio-in-slow-path/20240605-075027
> base:   https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
> patch link:    https://lore.kernel.org/r/20240604234858.948986-2-yang%40os.amperecomputing.com
> patch subject: [PATCH 2/2] mm: gup: do not call try_grab_folio() in slow path
> config: openrisc-allnoconfig (https://download.01.org/0day-ci/archive/20240605/202406051039.9m00gwIx-lkp@intel.com/config)
> compiler: or1k-linux-gcc (GCC) 13.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240605/202406051039.9m00gwIx-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202406051039.9m00gwIx-lkp@intel.com/
>
> All warnings (new ones prefixed by >>):
>
>>> mm/gup.c:131:22: warning: 'try_grab_folio_fast' defined but not used [-Wunused-function]

Thanks for reporting the problem. It seems try_grab_folio_fast() 
definition should be protected by CONFIG_HAVE_FAST_GUP, will fix it in v2.

>       131 | static struct folio *try_grab_folio_fast(struct page *page, int refs,
>           |                      ^~~~~~~~~~~~~~~~~~~
>
>
> vim +/try_grab_folio_fast +131 mm/gup.c
>
>     101	
>     102	/**
>     103	 * try_grab_folio_fast() - Attempt to get or pin a folio in fast path.
>     104	 * @page:  pointer to page to be grabbed
>     105	 * @refs:  the value to (effectively) add to the folio's refcount
>     106	 * @flags: gup flags: these are the FOLL_* flag values.
>     107	 *
>     108	 * "grab" names in this file mean, "look at flags to decide whether to use
>     109	 * FOLL_PIN or FOLL_GET behavior, when incrementing the folio's refcount.
>     110	 *
>     111	 * Either FOLL_PIN or FOLL_GET (or neither) must be set, but not both at the
>     112	 * same time. (That's true throughout the get_user_pages*() and
>     113	 * pin_user_pages*() APIs.) Cases:
>     114	 *
>     115	 *    FOLL_GET: folio's refcount will be incremented by @refs.
>     116	 *
>     117	 *    FOLL_PIN on large folios: folio's refcount will be incremented by
>     118	 *    @refs, and its pincount will be incremented by @refs.
>     119	 *
>     120	 *    FOLL_PIN on single-page folios: folio's refcount will be incremented by
>     121	 *    @refs * GUP_PIN_COUNTING_BIAS.
>     122	 *
>     123	 * Return: The folio containing @page (with refcount appropriately
>     124	 * incremented) for success, or NULL upon failure. If neither FOLL_GET
>     125	 * nor FOLL_PIN was set, that's considered failure, and furthermore,
>     126	 * a likely bug in the caller, so a warning is also emitted.
>     127	 *
>     128	 * It uses add ref unless zero to elevate the folio refcount and must be called
>     129	 * in fast path only.
>     130	 */
>   > 131	static struct folio *try_grab_folio_fast(struct page *page, int refs,
>     132						 unsigned int flags)
>     133	{
>     134		struct folio *folio;
>     135	
>     136		/* Raise warn if it is not called in fast GUP */
>     137		VM_WARN_ON_ONCE(!irqs_disabled());
>     138	
>     139		if (WARN_ON_ONCE((flags & (FOLL_GET | FOLL_PIN)) == 0))
>     140			return NULL;
>     141	
>     142		if (unlikely(!(flags & FOLL_PCI_P2PDMA) && is_pci_p2pdma_page(page)))
>     143			return NULL;
>     144	
>     145		if (flags & FOLL_GET)
>     146			return try_get_folio(page, refs);
>     147	
>     148		/* FOLL_PIN is set */
>     149	
>     150		/*
>     151		 * Don't take a pin on the zero page - it's not going anywhere
>     152		 * and it is used in a *lot* of places.
>     153		 */
>     154		if (is_zero_page(page))
>     155			return page_folio(page);
>     156	
>     157		folio = try_get_folio(page, refs);
>     158		if (!folio)
>     159			return NULL;
>     160	
>     161		/*
>     162		 * Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
>     163		 * right zone, so fail and let the caller fall back to the slow
>     164		 * path.
>     165		 */
>     166		if (unlikely((flags & FOLL_LONGTERM) &&
>     167			     !folio_is_longterm_pinnable(folio))) {
>     168			if (!put_devmap_managed_folio_refs(folio, refs))
>     169				folio_put_refs(folio, refs);
>     170			return NULL;
>     171		}
>     172	
>     173		/*
>     174		 * When pinning a large folio, use an exact count to track it.
>     175		 *
>     176		 * However, be sure to *also* increment the normal folio
>     177		 * refcount field at least once, so that the folio really
>     178		 * is pinned.  That's why the refcount from the earlier
>     179		 * try_get_folio() is left intact.
>     180		 */
>     181		if (folio_test_large(folio))
>     182			atomic_add(refs, &folio->_pincount);
>     183		else
>     184			folio_ref_add(folio,
>     185					refs * (GUP_PIN_COUNTING_BIAS - 1));
>     186		/*
>     187		 * Adjust the pincount before re-checking the PTE for changes.
>     188		 * This is essentially a smp_mb() and is paired with a memory
>     189		 * barrier in folio_try_share_anon_rmap_*().
>     190		 */
>     191		smp_mb__after_atomic();
>     192	
>     193		node_stat_mod_folio(folio, NR_FOLL_PIN_ACQUIRED, refs);
>     194	
>     195		return folio;
>     196	}
>     197	
>