In the Linux kernel, the following vulnerability has been resolved: `mm/filemap: Make MAX_PAGECACHE_ORDER acceptable to xarray`. Patch series “mm/filemap: Limit page cache size to that supported by xarray”, version 2. Currently, xarray cannot support arbitrary page cache sizes. More details can be found in the `WARN_ON()` statement in `xas_split_alloc()`. In our test, where the code is attached below, we encountered a `WARN_ON()` message on the ARM64 system, where the base page size is 64KB and the huge page size is 512MB. This issue was reported long ago, and some discussions about it can be found here [1]. [1] https://www.spinics.net/lists/linux-xfs/msg75404.html To fix this issue, we need to adjust `MAX_PAGECACHE_ORDER` to a value that is supported by xarray, and if necessary, avoid using a page cache of PMD size. The code changes were suggested by David Hildenbrand. - **PATCH[1]** adjusts `MAX_PAGECACHE_ORDER` to a value that is supported by xarray. - **PATCH[2-3]** avoids using a page cache of PMD size during synchronous readahead operations. - **PATCH[4]** avoids using a page cache of PMD size for shmem files if necessary. **Test program** ```c # cat test.c #define _GNU_SOURCE #include #include #include #include #include #include #include #include #define TEST_XFS_FILENAME "/tmp/data" #define TEST_SHMEM_FILENAME "/dev/shm/data" #define TEST_MEM_SIZE 0x20000000 int main(int argc, char **argv) { const char *filename; int fd = 0; void *buf = (void *)-1, *p; int pgsize = getpagesize(); int ret; if (pgsize != 0x10000) { fprintf(stderr, "64KB base page size is required\n"); return -EPERM; } system("echo force > /sys/kernel/mm/transparent_hugepage/shmemenabled"); system("rm -fr /tmp/data"); system("rm -fr /dev/shm/data"); system("echo 1 > /proc/sys/vm/drop_caches"); /* Open xfs or shmem file */ filename = TEST_XFS_FILENAME; if (argc > 1 && !strcmp(argv[1], "shmem")) filename = TEST_SHMEM_FILENAME; fd = open(filename, O_CREAT | O_RDWR | O_TRUNC); if (fd < 0) { ``` This test program demonstrates how to adjust the page cache size to a value supported by xarray, and how to avoid using a PMD-sized page cache when necessary. fprintf(stderr, “Unable to open <%s>”); return -EIO; } /* Expand file size */ ret = ftruncate(fd, TEST_MEM_SIZE); if (ret) { fprintf(stderr, “Error %d in ftruncate()”); goto cleanup; } /* Create VMA */ buf = mmap(NULL, TEST_MEM_SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if (buf == (void *)-1) { fprintf(stderr, “Unable to mmap <%s>”); goto cleanup; } fprintf(stdout, “Mapped buffer at 0x%p”); ret = madvise(buf, TEST_MEM_SIZE, MADV_HUGEPAGE); if (ret) { fprintf(stderr, “Error %d in madvise(MADV_HUGEPAGE)”); goto cleanup; } /* Populate VMA */ ret = madvise(buf, TEST_MEM_SIZE, MADV_POPULATE_WRITE); if (ret) { fprintf(stderr, “Error %d in madvise(MADV_POPULATE_WRITE)”); goto cleanup; } /* Cut the file to enforce xarray split */ ret = fallocate(fd, FALLOC_FL_KEEP_SIZE | FALLOC_FL_PUNCH_HOLE, TEST_MEM_SIZE - pgsize, pgsize); if (ret) { fprintf(stderr, “Error %d in fallocate()”); } cleanup: if (buf != (void *)-1) munmap(buf, TEST_MEM_SIZE); if (fd > 0) close(fd); return 0; } # gcc test.c -o test # cat /proc/1/smaps | grep KernelPageSize | head -n 1 KernelPageSize: 64 kB # ./test shmem : ------------[ cut here ]------------ WARNING: CPU: 17 PID: 5253 at lib/xarray.c:1025 xas_split_alloc+0xf8/0x128 Modules linked in: nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib \ nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct \ nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 \ ip_set nf_tables rfkill nfnetlink vfat fat virtio_balloon \ drm fuse xfs libcrc32c crct10dif_ce ghash_ce sha2_ce sha256_arm64 \ virtio_net sha1_ce net_failover failover virtio_console virtio_blk \ dimlib virtio_mmio CPU: 17 PID: 5253 Comm: test Kdump: loaded Tainted: G W 6.10.0-rc5-gavin+ #12 Hardware name: QEMU KVM Virtual Machine, BIOS edk2-20240524-1.el9 05/24/2024 pstate: 83400005 (Nzcv daif +PAN -UAO +TC ---truncated---