Re: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI

From: kernel test robot
Date: Tue Oct 17 2023 - 15:40:32 EST

Hi Suren,

kernel test robot noticed the following build warnings:

[auto build test WARNING on akpm-mm/mm-everything]
[also build test WARNING on next-20231017]
[cannot apply to linus/master v6.6-rc6]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url: https://github.com/intel-lab-lkp/linux/commits/Suren-Baghdasaryan/mm-rmap-support-move-to-different-root-anon_vma-in-folio_move_anon_rmap/20231009-144552
base: https://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm.git mm-everything
patch link: https://lore.kernel.org/r/20231009064230.2952396-3-surenb%40google.com
patch subject: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI
config: i386-randconfig-141-20231017 (https://download.01.org/0day-ci/archive/20231018/202310180338.zTpcYECK-lkp@xxxxxxxxx/config)
compiler: gcc-12 (Debian 12.2.0-14) 12.2.0
reproduce: (https://download.01.org/0day-ci/archive/20231018/202310180338.zTpcYECK-lkp@xxxxxxxxx/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@xxxxxxxxx>
| Closes: https://lore.kernel.org/oe-kbuild-all/202310180338.zTpcYECK-lkp@xxxxxxxxx/

smatch warnings:
mm/userfaultfd.c:1380 remap_pages() warn: unsigned 'src_start + len - src_addr' is never less than zero.

vim +1380 mm/userfaultfd.c

1195
1196 /**
1197 * remap_pages - remap arbitrary anonymous pages of an existing vma
1198 * @dst_start: start of the destination virtual memory range
1199 * @src_start: start of the source virtual memory range
1200 * @len: length of the virtual memory range
1201 *
1202 * remap_pages() remaps arbitrary anonymous pages atomically in zero
1203 * copy. It only works on non shared anonymous pages because those can
1204 * be relocated without generating non linear anon_vmas in the rmap
1205 * code.
1206 *
1207 * It provides a zero copy mechanism to handle userspace page faults.
1208 * The source vma pages should have mapcount == 1, which can be
1209 * enforced by using madvise(MADV_DONTFORK) on src vma.
1210 *
1211 * The thread receiving the page during the userland page fault
1212 * will receive the faulting page in the source vma through the network,
1213 * storage or any other I/O device (MADV_DONTFORK in the source vma
1214 * avoids remap_pages() to fail with -EBUSY if the process forks before
1215 * remap_pages() is called), then it will call remap_pages() to map the
1216 * page in the faulting address in the destination vma.
1217 *
1218 * This userfaultfd command works purely via pagetables, so it's the
1219 * most efficient way to move physical non shared anonymous pages
1220 * across different virtual addresses. Unlike mremap()/mmap()/munmap()
1221 * it does not create any new vmas. The mapping in the destination
1222 * address is atomic.
1223 *
1224 * It only works if the vma protection bits are identical from the
1225 * source and destination vma.
1226 *
1227 * It can remap non shared anonymous pages within the same vma too.
1228 *
1229 * If the source virtual memory range has any unmapped holes, or if
1230 * the destination virtual memory range is not a whole unmapped hole,
1231 * remap_pages() will fail respectively with -ENOENT or -EEXIST. This
1232 * provides a very strict behavior to avoid any chance of memory
1233 * corruption going unnoticed if there are userland race conditions.
1234 * Only one thread should resolve the userland page fault at any given
1235 * time for any given faulting address. This means that if two threads
1236 * try to both call remap_pages() on the same destination address at the
1237 * same time, the second thread will get an explicit error from this
1238 * command.
1239 *
1240 * The command retval will return "len" is successful. The command
1241 * however can be interrupted by fatal signals or errors. If
1242 * interrupted it will return the number of bytes successfully
1243 * remapped before the interruption if any, or the negative error if
1244 * none. It will never return zero. Either it will return an error or
1245 * an amount of bytes successfully moved. If the retval reports a
1246 * "short" remap, the remap_pages() command should be repeated by
1247 * userland with src+retval, dst+reval, len-retval if it wants to know
1248 * about the error that interrupted it.
1249 *
1250 * The UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES flag can be specified to
1251 * prevent -ENOENT errors to materialize if there are holes in the
1252 * source virtual range that is being remapped. The holes will be
1253 * accounted as successfully remapped in the retval of the
1254 * command. This is mostly useful to remap hugepage naturally aligned
1255 * virtual regions without knowing if there are transparent hugepage
1256 * in the regions or not, but preventing the risk of having to split
1257 * the hugepmd during the remap.
1258 *
1259 * If there's any rmap walk that is taking the anon_vma locks without
1260 * first obtaining the folio lock (the only current instance is
1261 * folio_referenced), they will have to verify if the folio->mapping
1262 * has changed after taking the anon_vma lock. If it changed they
1263 * should release the lock and retry obtaining a new anon_vma, because
1264 * it means the anon_vma was changed by remap_pages() before the lock
1265 * could be obtained. This is the only additional complexity added to
1266 * the rmap code to provide this anonymous page remapping functionality.
1267 */
1268 ssize_t remap_pages(struct mm_struct *dst_mm, struct mm_struct *src_mm,
1269 unsigned long dst_start, unsigned long src_start,
1270 unsigned long len, __u64 mode)
1271 {
1272 struct vm_area_struct *src_vma, *dst_vma;
1273 unsigned long src_addr, dst_addr;
1274 pmd_t *src_pmd, *dst_pmd;
1275 long err = -EINVAL;
1276 ssize_t moved = 0;
1277
1278 /*
1279 * Sanitize the command parameters:
1280 */
1281 BUG_ON(src_start & ~PAGE_MASK);
1282 BUG_ON(dst_start & ~PAGE_MASK);
1283 BUG_ON(len & ~PAGE_MASK);
1284
1285 /* Does the address range wrap, or is the span zero-sized? */
1286 BUG_ON(src_start + len <= src_start);
1287 BUG_ON(dst_start + len <= dst_start);
1288
1289 /*
1290 * Because these are read sempahores there's no risk of lock
1291 * inversion.
1292 */
1293 mmap_read_lock(dst_mm);
1294 if (dst_mm != src_mm)
1295 mmap_read_lock(src_mm);
1296
1297 /*
1298 * Make sure the vma is not shared, that the src and dst remap
1299 * ranges are both valid and fully within a single existing
1300 * vma.
1301 */
1302 src_vma = find_vma(src_mm, src_start);
1303 if (!src_vma || (src_vma->vm_flags & VM_SHARED))
1304 goto out;
1305 if (src_start < src_vma->vm_start ||
1306 src_start + len > src_vma->vm_end)
1307 goto out;
1308
1309 dst_vma = find_vma(dst_mm, dst_start);
1310 if (!dst_vma || (dst_vma->vm_flags & VM_SHARED))
1311 goto out;
1312 if (dst_start < dst_vma->vm_start ||
1313 dst_start + len > dst_vma->vm_end)
1314 goto out;
1315
1316 err = validate_remap_areas(src_vma, dst_vma);
1317 if (err)
1318 goto out;
1319
1320 for (src_addr = src_start, dst_addr = dst_start;
1321 src_addr < src_start + len;) {
1322 spinlock_t *ptl;
1323 pmd_t dst_pmdval;
1324 unsigned long step_size;
1325
1326 BUG_ON(dst_addr >= dst_start + len);
1327 /*
1328 * Below works because anonymous area would not have a
1329 * transparent huge PUD. If file-backed support is added,
1330 * that case would need to be handled here.
1331 */
1332 src_pmd = mm_find_pmd(src_mm, src_addr);
1333 if (unlikely(!src_pmd)) {
1334 if (!(mode & UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES)) {
1335 err = -ENOENT;
1336 break;
1337 }
1338 src_pmd = mm_alloc_pmd(src_mm, src_addr);
1339 if (unlikely(!src_pmd)) {
1340 err = -ENOMEM;
1341 break;
1342 }
1343 }
1344 dst_pmd = mm_alloc_pmd(dst_mm, dst_addr);
1345 if (unlikely(!dst_pmd)) {
1346 err = -ENOMEM;
1347 break;
1348 }
1349
1350 dst_pmdval = pmdp_get_lockless(dst_pmd);
1351 /*
1352 * If the dst_pmd is mapped as THP don't override it and just
1353 * be strict. If dst_pmd changes into TPH after this check, the
1354 * remap_pages_huge_pmd() will detect the change and retry
1355 * while remap_pages_pte() will detect the change and fail.
1356 */
1357 if (unlikely(pmd_trans_huge(dst_pmdval))) {
1358 err = -EEXIST;
1359 break;
1360 }
1361
1362 ptl = pmd_trans_huge_lock(src_pmd, src_vma);
1363 if (ptl) {
1364 if (pmd_devmap(*src_pmd)) {
1365 spin_unlock(ptl);
1366 err = -ENOENT;
1367 break;
1368 }
1369
1370 /*
1371 * Check if we can move the pmd without
1372 * splitting it. First check the address
1373 * alignment to be the same in src/dst. These
1374 * checks don't actually need the PT lock but
1375 * it's good to do it here to optimize this
1376 * block away at build time if
1377 * CONFIG_TRANSPARENT_HUGEPAGE is not set.
1378 */
1379 if ((src_addr & ~HPAGE_PMD_MASK) || (dst_addr & ~HPAGE_PMD_MASK) ||
> 1380 src_start + len - src_addr < HPAGE_PMD_SIZE || !pmd_none(dst_pmdval)) {

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