The patch titled Subject: mm/gup: handle cont-PTE hugetlb pages correctly in gup_must_unshare() via GUP-fast has been added to the -mm mm-hotfixes-unstable branch. Its filename is mm-gup-handle-cont-pte-hugetlb-pages-correctly-in-gup_must_unshare-via-gup-fast.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-hotfixes-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: handle cont-PTE hugetlb pages correctly in gup_must_unshare() via GUP-fast Date: Sat, 5 Aug 2023 12:12:56 +0200

In contrast to most other GUP code, GUP-fast common page table walking code like gup_pte_range() also handles hugetlb pages. But in contrast to other hugetlb page table walking code, it does not look at the hugetlb PTE abstraction whereby we have only a single logical hugetlb PTE per hugetlb page, even when using multiple cont-PTEs underneath -- which is for example what huge_ptep_get() abstracts.

So when we have a hugetlb page that is mapped via cont-PTEs, GUP-fast might stumble over a PTE that does not map the head page of a hugetlb page -- not the first "head" PTE of such a cont mapping.

Logically, the whole hugetlb page is mapped (entire_mapcount == 1), but we might end up calling gup_must_unshare() with a tail page of a hugetlb page.

We only maintain a single PageAnonExclusive flag per hugetlb page (as hugetlb pages cannot get partially COW-shared), stored for the head page. That flag is clear for all tail pages.

So when gup_must_unshare() ends up calling PageAnonExclusive() with a tail page of a hugetlb page:

1) With CONFIG_DEBUG_VM_PGFLAGS

Stumbles over the:

VM_BUG_ON_PGFLAGS(PageHuge(page) && !PageHead(page), page);

For example, when executing the COW selftests with 64k hugetlb pages on arm64:

[ 61.082187] page:00000000829819ff refcount:3 mapcount:1 mapping:0000000000000000 index:0x1 pfn:0x11ee11 [ 61.082842] head:0000000080f79bf7 order:4 entire_mapcount:1 nr_pages_mapped:0 pincount:2 [ 61.083384] anon flags: 0x17ffff80003000e(referenced|uptodate|dirty|head|mappedtodisk|node=0|zone=2|lastcpupid=0xfffff) [ 61.084101] page_type: 0xffffffff() [ 61.084332] raw: 017ffff800000000 fffffc00037b8401 0000000000000402 0000000200000000 [ 61.084840] raw: 0000000000000010 0000000000000000 00000000ffffffff 0000000000000000 [ 61.085359] head: 017ffff80003000e ffffd9e95b09b788 ffffd9e95b09b788 ffff0007ff63cf71 [ 61.085885] head: 0000000000000000 0000000000000002 00000003ffffffff 0000000000000000 [ 61.086415] page dumped because: VM_BUG_ON_PAGE(PageHuge(page) && !PageHead(page)) [ 61.086914] ------------[ cut here ]------------ [ 61.087220] kernel BUG at include/linux/page-flags.h:990! [ 61.087591] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP [ 61.087999] Modules linked in: ... [ 61.089404] CPU: 0 PID: 4612 Comm: cow Kdump: loaded Not tainted 6.5.0-rc4+ #3 [ 61.089917] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015 [ 61.090409] pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--) [ 61.090897] pc : gup_must_unshare.part.0+0x64/0x98 [ 61.091242] lr : gup_must_unshare.part.0+0x64/0x98 [ 61.091592] sp : ffff8000825eb940 [ 61.091826] x29: ffff8000825eb940 x28: 0000000000000000 x27: fffffc00037b8440 [ 61.092329] x26: 0400000000000001 x25: 0000000000080101 x24: 0000000000080000 [ 61.092835] x23: 0000000000080100 x22: ffff0000cffb9588 x21: ffff0000c8ec6b58 [ 61.093341] x20: 0000ffffad6b1000 x19: fffffc00037b8440 x18: ffffffffffffffff [ 61.093850] x17: 2864616548656761 x16: 5021202626202965 x15: 6761702865677548 [ 61.094358] x14: 6567615028454741 x13: 2929656761702864 x12: 6165486567615021 [ 61.094858] x11: 00000000ffff7fff x10: 00000000ffff7fff x9 : ffffd9e958b7a1c0 [ 61.095359] x8 : 00000000000bffe8 x7 : c0000000ffff7fff x6 : 00000000002bffa8 [ 61.095873] x5 : ffff0008bb19e708 x4 : 0000000000000000 x3 : 0000000000000000 [ 61.096380] x2 : 0000000000000000 x1 : ffff0000cf6636c0 x0 : 0000000000000046 [ 61.096894] Call trace: [ 61.097080] gup_must_unshare.part.0+0x64/0x98 [ 61.097392] gup_pte_range+0x3a8/0x3f0 [ 61.097662] gup_pgd_range+0x1ec/0x280 [ 61.097942] lockless_pages_from_mm+0x64/0x1a0 [ 61.098258] internal_get_user_pages_fast+0xe4/0x1d0 [ 61.098612] pin_user_pages_fast+0x58/0x78 [ 61.098917] pin_longterm_test_start+0xf4/0x2b8 [ 61.099243] gup_test_ioctl+0x170/0x3b0 [ 61.099528] __arm64_sys_ioctl+0xa8/0xf0 [ 61.099822] invoke_syscall.constprop.0+0x7c/0xd0 [ 61.100160] el0_svc_common.constprop.0+0xe8/0x100 [ 61.100500] do_el0_svc+0x38/0xa0 [ 61.100736] el0_svc+0x3c/0x198 [ 61.100971] el0t_64_sync_handler+0x134/0x150 [ 61.101280] el0t_64_sync+0x17c/0x180 [ 61.101543] Code: aa1303e0 f00074c1 912b0021 97fffeb2 (d4210000)

2) Without CONFIG_DEBUG_VM_PGFLAGS

Always detects "not exclusive" for passed tail pages and refuses to PIN the tail pages R/O, as gup_must_unshare() == true. GUP-fast will fallback to ordinary GUP. As ordinary GUP properly considers the logical hugetlb PTE abstraction in hugetlb_follow_page_mask(), pinning the page will succeed when looking at the PageAnonExclusive on the head page only.

So the only real effect of this is that with cont-PTE hugetlb pages, we'll always fallback from GUP-fast to ordinary GUP when not working on the head page, which ends up checking the head page and do the right thing.

Consequently, the cow selftests pass with cont-PTE hugetlb pages as well without CONFIG_DEBUG_VM_PGFLAGS.

Note that this only applies to anon hugetlb pages that are mapped using cont-PTEs: for example 64k hugetlb pages on a 4k arm64 kernel.

... and only when R/O-pinning (FOLL_PIN) such pages that are mapped into the page table R/O using GUP-fast.

On production kernels (and even most debug kernels, that don't set CONFIG_DEBUG_VM_PGFLAGS) this patch should theoretically not be required to be backported. But of course, it does not hurt.

Link: https://lkml.kernel.org/r/20230805101256.87306-1-david@redhat.com Fixes: a7f226604170 ("mm/gup: trigger FAULT_FLAG_UNSHARE when R/O-pinning a possibly shared anonymous page") Signed-off-by: David Hildenbrand david@redhat.com Reported-by: Ryan Roberts ryan.roberts@arm.com Cc: Vlastimil Babka vbabka@suse.cz Cc: John Hubbard jhubbard@nvidia.com Cc: Jason Gunthorpe jgg@nvidia.com Cc: Peter Xu peterx@redhat.com Cc: Mike Kravetz mike.kravetz@oracle.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/internal.h | 10 ++++++++++ 1 file changed, 10 insertions(+)

--- a/mm/internal.h~mm-gup-handle-cont-pte-hugetlb-pages-correctly-in-gup_must_unshare-via-gup-fast +++ a/mm/internal.h @@ -1005,6 +1005,16 @@ static inline bool gup_must_unshare(stru smp_rmb();

/* + * During GUP-fast we might not get called on the head page for a + * hugetlb page that is mapped using cont-PTE, because GUP-fast does + * not work with the abstracted hugetlb PTEs that always point at the + * head page. For hugetlb, PageAnonExclusive only applies on the head + * page (as it cannot be partially COW-shared), so lookup the head page. + */ + if (unlikely(!PageHead(page) && PageHuge(page))) + page = compound_head(page); + + /* * Note that PageKsm() pages cannot be exclusive, and consequently, * cannot get pinned. */ _

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

mm-gup-reintroduce-foll_numa-as-foll_honor_numa_fault.patch smaps-use-vm_normal_page_pmd-instead-of-follow_trans_huge_pmd.patch mm-gup-handle-cont-pte-hugetlb-pages-correctly-in-gup_must_unshare-via-gup-fast.patch mm-memory_hotplug-document-the-signal_pending-check-in-offline_pages.patch kvm-explicitly-set-foll_honor_numa_fault-in-hva_to_pfn_slow.patch mm-gup-dont-implicitly-set-foll_honor_numa_fault.patch pgtable-improve-pte_protnone-comment.patch selftest-mm-ksm_functional_tests-test-in-mmap_and_merge_range-if-anything-got-merged.patch selftest-mm-ksm_functional_tests-add-prot_none-test.patch