The patch titled Subject: mm/gup: reject FOLL_SPLIT_PMD with hugetlb VMAs has been added to the -mm mm-unstable branch. Its filename is mm-gup-reject-foll_split_pmd-with-hugetlb-vmas.patch

This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...

This patch will later appear in the mm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------ From: David Hildenbrand david@redhat.com Subject: mm/gup: reject FOLL_SPLIT_PMD with hugetlb VMAs Date: Mon, 10 Feb 2025 20:37:43 +0100

Patch series "mm: fixes for device-exclusive entries (hmm)", v2.

Discussing the PageTail() call in make_device_exclusive_range() with Willy, I recently discovered [1] that device-exclusive handling does not properly work with THP, making the hmm-tests selftests fail if THPs are enabled on the system.

Looking into more details, I found that hugetlb is not properly fenced, and I realized that something that was bugging me for longer -- how device-exclusive entries interact with mapcounts -- completely breaks migration/swapout/split/hwpoison handling of these folios while they have device-exclusive PTEs.

The program below can be used to allocate 1 GiB worth of pages and making them device-exclusive on a kernel with CONFIG_TEST_HMM.

Once they are device-exclusive, these folios cannot get swapped out (proc$pid/smaps_rollup will always indicate 1 GiB RSS no matter how much one forces memory reclaim), and when having a memory block onlined to ZONE_MOVABLE, trying to offline it will loop forever and complain about failed migration of a page that should be movable.

# echo offline > /sys/devices/system/memory/memory136/state # echo online_movable > /sys/devices/system/memory/memory136/state # ./hmm-swap & ... wait until everything is device-exclusive # echo offline > /sys/devices/system/memory/memory136/state [ 285.193431][T14882] page: refcount:2 mapcount:0 mapping:0000000000000000 index:0x7f20671f7 pfn:0x442b6a [ 285.196618][T14882] memcg:ffff888179298000 [ 285.198085][T14882] anon flags: 0x5fff0000002091c(referenced|uptodate| dirty|active|owner_2|swapbacked|node=1|zone=3|lastcpupid=0x7ff) [ 285.201734][T14882] raw: ... [ 285.204464][T14882] raw: ... [ 285.207196][T14882] page dumped because: migration failure [ 285.209072][T14882] page_owner tracks the page as allocated [ 285.210915][T14882] page last allocated via order 0, migratetype Movable, gfp_mask 0x140dca(GFP_HIGHUSER_MOVABLE|__GFP_COMP|__GFP_ZERO), id 14926, tgid 14926 (hmm-swap), ts 254506295376, free_ts 227402023774 [ 285.216765][T14882] post_alloc_hook+0x197/0x1b0 [ 285.218874][T14882] get_page_from_freelist+0x76e/0x3280 [ 285.220864][T14882] __alloc_frozen_pages_noprof+0x38e/0x2740 [ 285.223302][T14882] alloc_pages_mpol+0x1fc/0x540 [ 285.225130][T14882] folio_alloc_mpol_noprof+0x36/0x340 [ 285.227222][T14882] vma_alloc_folio_noprof+0xee/0x1a0 [ 285.229074][T14882] __handle_mm_fault+0x2b38/0x56a0 [ 285.230822][T14882] handle_mm_fault+0x368/0x9f0 ...

This series fixes all issues I found so far. There is no easy way to fix without a bigger rework/cleanup. I have a bunch of cleanups on top (some previous sent, some the result of the discussion in v1) that I will send out separately once this landed and I get to it.

I wish we could just use some special present PROT_NONE PTEs instead of these (non-present, non-none) fake-swap entries; but that just results in the same problem we keep having (lack of spare PTE bits), and staring at other similar fake-swap entries, that ship has sailed.

With this series, make_device_exclusive() doesn't actually belong into mm/rmap.c anymore, but I'll leave moving that for another day.

I only tested this series with the hmm-tests selftests due to lack of HW, so I'd appreciate some testing, especially if the interaction between two GPUs wanting a device-exclusive entry works as expected.

<program> #include <stdio.h> #include <fcntl.h> #include <stdint.h> #include <unistd.h> #include <stdlib.h> #include <string.h> #include <sys/mman.h> #include <sys/ioctl.h> #include <linux/types.h> #include <linux/ioctl.h>

#define HMM_DMIRROR_EXCLUSIVE _IOWR('H', 0x05, struct hmm_dmirror_cmd)

struct hmm_dmirror_cmd { __u64 addr; __u64 ptr; __u64 npages; __u64 cpages; __u64 faults; };

const size_t size = 1 * 1024 * 1024 * 1024ul; const size_t chunk_size = 2 * 1024 * 1024ul;

int main(void) { struct hmm_dmirror_cmd cmd; size_t cur_size; int fd, ret; char *addr, *mirror;

fd = open("/dev/hmm_dmirror1", O_RDWR, 0); if (fd < 0) { perror("open failed\n"); exit(1); }

addr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (addr == MAP_FAILED) { perror("mmap failed\n"); exit(1); } madvise(addr, size, MADV_NOHUGEPAGE); memset(addr, 1, size);

mirror = malloc(chunk_size);

for (cur_size = 0; cur_size < size; cur_size += chunk_size) { cmd.addr = (uintptr_t)addr + cur_size; cmd.ptr = (uintptr_t)mirror; cmd.npages = chunk_size / getpagesize(); ret = ioctl(fd, HMM_DMIRROR_EXCLUSIVE, &cmd); if (ret) { perror("ioctl failed\n"); exit(1); } } pause(); return 0; } </program>

[1] https://lkml.kernel.org/r/25e02685-4f1d-47fa-be5b-01ff85bb0ce2@redhat.com

This patch (of 17):

We only have two FOLL_SPLIT_PMD users. While uprobe refuses hugetlb early, make_device_exclusive_range() can end up getting called on hugetlb VMAs.

Right now, this means that with a PMD-sized hugetlb page, we can end up calling split_huge_pmd(), because pmd_trans_huge() also succeeds with hugetlb PMDs.

For example, using a modified hmm-test selftest one can trigger:

[ 207.017134][T14945] ------------[ cut here ]------------ [ 207.018614][T14945] kernel BUG at mm/page_table_check.c:87! [ 207.019716][T14945] Oops: invalid opcode: 0000 [#1] PREEMPT SMP KASAN NOPTI [ 207.021072][T14945] CPU: 3 UID: 0 PID: ... [ 207.023036][T14945] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-2.fc40 04/01/2014 [ 207.024834][T14945] RIP: 0010:page_table_check_clear.part.0+0x488/0x510 [ 207.026128][T14945] Code: ... [ 207.029965][T14945] RSP: 0018:ffffc9000cb8f348 EFLAGS: 00010293 [ 207.031139][T14945] RAX: 0000000000000000 RBX: 00000000ffffffff RCX: ffffffff8249a0cd [ 207.032649][T14945] RDX: ffff88811e883c80 RSI: ffffffff8249a357 RDI: ffff88811e883c80 [ 207.034183][T14945] RBP: ffff888105c0a050 R08: 0000000000000005 R09: 0000000000000000 [ 207.035688][T14945] R10: 00000000ffffffff R11: 0000000000000003 R12: 0000000000000001 [ 207.037203][T14945] R13: 0000000000000200 R14: 0000000000000001 R15: dffffc0000000000 [ 207.038711][T14945] FS: 00007f2783275740(0000) GS:ffff8881f4980000(0000) knlGS:0000000000000000 [ 207.040407][T14945] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 207.041660][T14945] CR2: 00007f2782c00000 CR3: 0000000132356000 CR4: 0000000000750ef0 [ 207.043196][T14945] PKRU: 55555554 [ 207.043880][T14945] Call Trace: [ 207.044506][T14945] <TASK> [ 207.045086][T14945] ? __die+0x51/0x92 [ 207.045864][T14945] ? die+0x29/0x50 [ 207.046596][T14945] ? do_trap+0x250/0x320 [ 207.047430][T14945] ? do_error_trap+0xe7/0x220 [ 207.048346][T14945] ? page_table_check_clear.part.0+0x488/0x510 [ 207.049535][T14945] ? handle_invalid_op+0x34/0x40 [ 207.050494][T14945] ? page_table_check_clear.part.0+0x488/0x510 [ 207.051681][T14945] ? exc_invalid_op+0x2e/0x50 [ 207.052589][T14945] ? asm_exc_invalid_op+0x1a/0x20 [ 207.053596][T14945] ? page_table_check_clear.part.0+0x1fd/0x510 [ 207.054790][T14945] ? page_table_check_clear.part.0+0x487/0x510 [ 207.055993][T14945] ? page_table_check_clear.part.0+0x488/0x510 [ 207.057195][T14945] ? page_table_check_clear.part.0+0x487/0x510 [ 207.058384][T14945] __page_table_check_pmd_clear+0x34b/0x5a0 [ 207.059524][T14945] ? __pfx___page_table_check_pmd_clear+0x10/0x10 [ 207.060775][T14945] ? __pfx___mutex_unlock_slowpath+0x10/0x10 [ 207.061940][T14945] ? __pfx___lock_acquire+0x10/0x10 [ 207.062967][T14945] pmdp_huge_clear_flush+0x279/0x360 [ 207.064024][T14945] split_huge_pmd_locked+0x82b/0x3750 ...

Before commit 9cb28da54643 ("mm/gup: handle hugetlb in the generic follow_page_mask code"), we would have ignored the flag; instead, let's simply refuse the combination completely in check_vma_flags(): the caller is likely not prepared to handle any hugetlb folios.

We'll teach make_device_exclusive_range() separately to ignore any hugetlb folios as a future-proof safety net.

Link: https://lkml.kernel.org/r/20250210193801.781278-1-david@redhat.com Link: https://lkml.kernel.org/r/20250210193801.781278-2-david@redhat.com Fixes: 9cb28da54643 ("mm/gup: handle hugetlb in the generic follow_page_mask code") Signed-off-by: David Hildenbrand david@redhat.com Reviewed-by: John Hubbard jhubbard@nvidia.com Reviewed-by: Alistair Popple apopple@nvidia.com Cc: Alex Shi alexs@kernel.org Cc: Danilo Krummrich dakr@kernel.org Cc: Dave Airlie airlied@gmail.com Cc: Jann Horn jannh@google.com Cc: Jason Gunthorpe jgg@nvidia.com Cc: Jerome Glisse jglisse@redhat.com Cc: Jonathan Corbet corbet@lwn.net Cc: Karol Herbst kherbst@redhat.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Lyude lyude@redhat.com Cc: "Masami Hiramatsu (Google)" mhiramat@kernel.org Cc: Oleg Nesterov oleg@redhat.com Cc: Pasha Tatashin pasha.tatashin@soleen.com Cc: Peter Xu peterx@redhat.com Cc: Peter Zijlstra (Intel) peterz@infradead.org Cc: SeongJae Park sj@kernel.org Cc: Vlastimil Babka vbabka@suse.cz Cc: Yanteng Si si.yanteng@linux.dev Cc: Simona Vetter simona.vetter@ffwll.ch Cc: Barry Song v-songbaohua@oppo.com Cc: stable@vger.kernel.org

Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/gup.c | 3 +++ 1 file changed, 3 insertions(+)

--- a/mm/gup.c~mm-gup-reject-foll_split_pmd-with-hugetlb-vmas +++ a/mm/gup.c @@ -1283,6 +1283,9 @@ static int check_vma_flags(struct vm_are if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma)) return -EOPNOTSUPP;

+ if ((gup_flags & FOLL_SPLIT_PMD) && is_vm_hugetlb_page(vma)) + return -EOPNOTSUPP; + if (vma_is_secretmem(vma)) return -EFAULT;

_

Patches currently in -mm which might be from david@redhat.com are

mm-gup-reject-foll_split_pmd-with-hugetlb-vmas.patch mm-rmap-reject-hugetlb-folios-in-folio_make_device_exclusive.patch mm-rmap-convert-make_device_exclusive_range-to-make_device_exclusive.patch mm-rmap-implement-make_device_exclusive-using-folio_walk-instead-of-rmap-walk.patch mm-memory-detect-writability-in-restore_exclusive_pte-through-can_change_pte_writable.patch mm-use-single-swp_device_exclusive-entry-type.patch mm-page_vma_mapped-device-exclusive-entries-are-not-migration-entries.patch kernel-events-uprobes-handle-device-exclusive-entries-correctly-in-__replace_page.patch mm-ksm-handle-device-exclusive-entries-correctly-in-write_protect_page.patch mm-rmap-handle-device-exclusive-entries-correctly-in-try_to_unmap_one.patch mm-rmap-handle-device-exclusive-entries-correctly-in-try_to_migrate_one.patch mm-rmap-handle-device-exclusive-entries-correctly-in-page_vma_mkclean_one.patch mm-page_idle-handle-device-exclusive-entries-correctly-in-page_idle_clear_pte_refs_one.patch mm-damon-handle-device-exclusive-entries-correctly-in-damon_folio_young_one.patch mm-damon-handle-device-exclusive-entries-correctly-in-damon_folio_mkold_one.patch mm-rmap-keep-mapcount-untouched-for-device-exclusive-entries.patch mm-rmap-avoid-ebusy-from-make_device_exclusive.patch