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Date: Wed, 5 Jun 2024 10:57:13 +0800
From: kernel test robot <lkp@...el.com>
To: Yang Shi <yang@...amperecomputing.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, yang@...amperecomputing.com,
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
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]
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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