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Date: Wed, 5 Jun 2024 09:19:50 -0700
From: Yang Shi <yang@...amperecomputing.com>
To: kernel test robot <lkp@...el.com>, peterx@...hat.com,
oliver.sang@...el.com, paulmck@...nel.org, david@...hat.com,
willy@...radead.org, riel@...riel.com, vivek.kasireddy@...el.com,
cl@...ux.com, akpm@...ux-foundation.org
Cc: oe-kbuild-all@...ts.linux.dev, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: gup: do not call try_grab_folio() in slow path
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@...el.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
>
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