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

[kernel test robot <lkp@intel.com>]

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]
     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	

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