Re: [PATCH v4 13/13] mm/gup: Handle hugetlb in the generic follow_page_mask code

Ryan Roberts
2024-04-02T14:48:36Z

Hi Peter,

On 27/03/2024 15:23, peterx@redhat.com wrote:
> From: Peter Xu <peterx@redhat.com>
>
> Now follow_page() is ready to handle hugetlb pages in whatever form, and
> over all architectures. Switch to the generic code path.
>
> Time to retire hugetlb_follow_page_mask(), following the previous
> retirement of follow_hugetlb_page() in 4849807114b8.
>
> There may be a slight difference of how the loops run when processing slow
> GUP over a large hugetlb range on cont_pte/cont_pmd supported archs: each
> loop of __get_user_pages() will resolve one pgtable entry with the patch
> applied, rather than relying on the size of hugetlb hstate, the latter may
> cover multiple entries in one loop.
>
> A quick performance test on an aarch64 VM on M1 chip shows 15% degrade over
> a tight loop of slow gup after the path switched. That shouldn't be a
> problem because slow-gup should not be a hot path for GUP in general: when
> page is commonly present, fast-gup will already succeed, while when the
> page is indeed missing and require a follow up page fault, the slow gup
> degrade will probably buried in the fault paths anyway. It also explains
> why slow gup for THP used to be very slow before 57edfcfd3419 ("mm/gup:
> accelerate thp gup even for "pages != NULL"") lands, the latter not part of
> a performance analysis but a side benefit. If the performance will be a
> concern, we can consider handle CONT_PTE in follow_page().
>
> Before that is justified to be necessary, keep everything clean and simple.
>
> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>

Afraid I'm seeing an oops when running gup_longterm test on arm64 with current mm-unstable. Git bisect blames this patch. The oops reproduces for me every time on 2 different machines:

[ 9.340416] kernel BUG at mm/gup.c:778!
[ 9.340746] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP
[ 9.341199] Modules linked in:
[ 9.341481] CPU: 1 PID: 1159 Comm: gup_longterm Not tainted 6.9.0-rc2-00210-g910ff1a347e4 #11
[ 9.342232] Hardware name: linux,dummy-virt (DT)
[ 9.342647] pstate: 60400005 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 9.343195] pc : follow_page_mask+0x4d4/0x880
[ 9.343580] lr : follow_page_mask+0x4d4/0x880
[ 9.344018] sp : ffff8000898b3aa0
[ 9.344345] x29: ffff8000898b3aa0 x28: fffffdffc53973e8 x27: 00003c0005d08000
[ 9.345028] x26: ffff00014e5cfd08 x25: ffffd3513a40c000 x24: fffffdffc5d08000
[ 9.345682] x23: ffffc1ffc0000000 x22: 0000000000080101 x21: ffff8000898b3ba8
[ 9.346337] x20: 0000fffff4200000 x19: ffff00014e52d508 x18: 0000000000000010
[ 9.347005] x17: 5f656e6f7a5f7369 x16: 2120262620296567 x15: 6170286461654865
[ 9.347713] x14: 6761502128454741 x13: 2929656761702865 x12: 6761705f65636976
[ 9.348371] x11: 65645f656e6f7a5f x10: ffffd3513b31d6e0 x9 : ffffd3513852f090
[ 9.349062] x8 : 00000000ffffefff x7 : ffffd3513b31d6e0 x6 : 0000000000000000
[ 9.349753] x5 : ffff00017ff98cc8 x4 : 0000000000000fff x3 : 0000000000000000
[ 9.350397] x2 : 0000000000000000 x1 : ffff000190e8b480 x0 : 0000000000000052
[ 9.351097] Call trace:
[ 9.351312] follow_page_mask+0x4d4/0x880
[ 9.351700] __get_user_pages+0xf4/0x3e8
[ 9.352089] __gup_longterm_locked+0x204/0xa70
[ 9.352516] pin_user_pages+0x88/0xc0
[ 9.352873] gup_test_ioctl+0x860/0xc40
[ 9.353249] __arm64_sys_ioctl+0xb0/0x100
[ 9.353648] invoke_syscall+0x50/0x128
[ 9.354022] el0_svc_common.constprop.0+0x48/0xf8
[ 9.354488] do_el0_svc+0x28/0x40
[ 9.354822] el0_svc+0x34/0xe0
[ 9.355128] el0t_64_sync_handler+0x13c/0x158
[ 9.355489] el0t_64_sync+0x190/0x198
[ 9.355793] Code: aa1803e0 d000d8e1 91220021 97fff560 (d4210000)
[ 9.356280] ---[ end trace 0000000000000000 ]---
[ 9.356651] note: gup_longterm[1159] exited with irqs disabled
[ 9.357141] note: gup_longterm[1159] exited with preempt_count 2
[ 9.358033] ------------[ cut here ]------------
[ 9.358800] WARNING: CPU: 1 PID: 0 at kernel/context_tracking.c:128 ct_kernel_exit.constprop.0+0x108/0x120
[ 9.360157] Modules linked in:
[ 9.360541] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G D 6.9.0-rc2-00210-g910ff1a347e4 #11
[ 9.361626] Hardware name: linux,dummy-virt (DT)
[ 9.362087] pstate: 204003c5 (nzCv DAIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 9.362758] pc : ct_kernel_exit.constprop.0+0x108/0x120
[ 9.363306] lr : ct_idle_enter+0x10/0x20
[ 9.363845] sp : ffff8000801abdc0
[ 9.364222] x29: ffff8000801abdc0 x28: 0000000000000000 x27: 0000000000000000
[ 9.364961] x26: 0000000000000000 x25: ffff00014149d780 x24: 0000000000000000
[ 9.365557] x23: 0000000000000000 x22: ffffd3513b299d48 x21: ffffd3513a785730
[ 9.366239] x20: ffffd3513b299c28 x19: ffff00017ffa7da0 x18: 0000fffff5ffffff
[ 9.366869] x17: 0000000000000000 x16: 1fffe0002a21a8c1 x15: 0000000000000000
[ 9.367524] x14: 0000000000000000 x13: 0000000000000000 x12: 0000000000000002
[ 9.368207] x11: 0000000000000001 x10: 0000000000000ad0 x9 : ffffd35138589230
[ 9.369123] x8 : ffff00014149e2b0 x7 : 0000000000000000 x6 : 000000000f8c0fb2
[ 9.370403] x5 : 4000000000000002 x4 : ffff2cb045825000 x3 : ffff8000801abdc0
[ 9.371170] x2 : ffffd3513a782da0 x1 : 4000000000000000 x0 : ffffd3513a782da0
[ 9.372279] Call trace:
[ 9.372519] ct_kernel_exit.constprop.0+0x108/0x120
[ 9.373216] ct_idle_enter+0x10/0x20
[ 9.373562] default_idle_call+0x3c/0x160
[ 9.374055] do_idle+0x21c/0x280
[ 9.374394] cpu_startup_entry+0x3c/0x50
[ 9.374797] secondary_start_kernel+0x140/0x168
[ 9.375220] __secondary_switched+0xb8/0xc0
[ 9.375875] ---[ end trace 0000000000000000 ]---

The oops trigger is at mm/gup.c:778:
VM_BUG_ON_PAGE(!PageHead(page) && !is_zone_device_page(page), page);

This is the output of gup_longterm (last output is just before oops):

# [INFO] detected hugetlb page size: 2048 KiB
# [INFO] detected hugetlb page size: 32768 KiB
# [INFO] detected hugetlb page size: 64 KiB
# [INFO] detected hugetlb page size: 1048576 KiB
TAP version 13
1..70
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd
ok 1 Should have worked
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with tmpfile
ok 2 Should have failed
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with local tmpfile
ok 3 Should have failed
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (2048 kB)
ok 4 Should have worked
# [RUN] R/W longterm GUP pin in MAP_SHARED file mapping ... with memfd hugetlb (32768 kB)

So 2M passed ok, and its failing for 32M, which is cont-pmd. I'm guessing you're trying to iterate 2M into a cont-pmd folio and ending up with an unexpected tail page?

I'm running with defconfig plus these:

./scripts/config --enable CONFIG_SQUASHFS_LZ4
./scripts/config --enable CONFIG_SQUASHFS_LZO
./scripts/config --enable CONFIG_SQUASHFS_XZ
./scripts/config --enable CONFIG_SQUASHFS_ZSTD
./scripts/config --enable CONFIG_XFS_FS
./scripts/config --enable CONFIG_FTRACE
./scripts/config --enable CONFIG_FUNCTION_TRACER
./scripts/config --enable CONFIG_KPROBES
./scripts/config --enable CONFIG_HIST_TRIGGERS
./scripts/config --enable CONFIG_FTRACE_SYSCALLS
./scripts/config --enable CONFIG_DEBUG_VM
./scripts/config --enable CONFIG_DEBUG_VM_MAPLE_TREE
./scripts/config --enable CONFIG_DEBUG_VM_RB
./scripts/config --enable CONFIG_DEBUG_VM_PGFLAGS
./scripts/config --enable CONFIG_DEBUG_VM_PGTABLE
./scripts/config --enable CONFIG_PAGE_TABLE_CHECK
./scripts/config --enable CONFIG_USERFAULTFD
./scripts/config --enable CONFIG_TEST_VMALLOC
./scripts/config --enable CONFIG_GUP_TEST

Thanks,
Ryan

> ---
> include/linux/hugetlb.h | 7 ----
> mm/gup.c | 15 +++------
> mm/hugetlb.c | 71 -----------------------------------------
> 3 files changed, 5 insertions(+), 88 deletions(-)
>
> diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
> index 294c78b3549f..a546140f89cd 100644
> --- a/include/linux/hugetlb.h
> +++ b/include/linux/hugetlb.h
> @@ -328,13 +328,6 @@ static inline void hugetlb_zap_end(
> {
> }
>
> -static inline struct page *hugetlb_follow_page_mask(
> - struct vm_area_struct *vma, unsigned long address, unsigned int flags,
> - unsigned int *page_mask)
> -{
> - BUILD_BUG(); /* should never be compiled in if !CONFIG_HUGETLB_PAGE*/
> -}
> -
> static inline int copy_hugetlb_page_range(struct mm_struct *dst,
> struct mm_struct *src,
> struct vm_area_struct *dst_vma,
> diff --git a/mm/gup.c b/mm/gup.c
> index a02463c9420e..c803d0b0f358 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1135,18 +1135,11 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
> {
> pgd_t *pgd;
> struct mm_struct *mm = vma->vm_mm;
> + struct page *page;
>
> - ctx->page_mask = 0;
> -
> - /*
> - * Call hugetlb_follow_page_mask for hugetlb vmas as it will use
> - * special hugetlb page table walking code. This eliminates the
> - * need to check for hugetlb entries in the general walking code.
> - */
> - if (is_vm_hugetlb_page(vma))
> - return hugetlb_follow_page_mask(vma, address, flags,
> - &ctx->page_mask);
> + vma_pgtable_walk_begin(vma);
>
> + ctx->page_mask = 0;
> pgd = pgd_offset(mm, address);
>
> if (unlikely(is_hugepd(__hugepd(pgd_val(*pgd)))))
> @@ -1157,6 +1150,8 @@ static struct page *follow_page_mask(struct vm_area_struct *vma,
> else
> page = follow_p4d_mask(vma, address, pgd, flags, ctx);
>
> + vma_pgtable_walk_end(vma);
> +
> return page;
> }
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 65b9c9a48fd2..cc79891a3597 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -6870,77 +6870,6 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
> }
> #endif /* CONFIG_USERFAULTFD */
>
> -struct page *hugetlb_follow_page_mask(struct vm_area_struct *vma,
> - unsigned long address, unsigned int flags,
> - unsigned int *page_mask)
> -{
> - struct hstate *h = hstate_vma(vma);
> - struct mm_struct *mm = vma->vm_mm;
> - unsigned long haddr = address & huge_page_mask(h);
> - struct page *page = NULL;
> - spinlock_t *ptl;
> - pte_t *pte, entry;
> - int ret;
> -
> - hugetlb_vma_lock_read(vma);
> - pte = hugetlb_walk(vma, haddr, huge_page_size(h));
> - if (!pte)
> - goto out_unlock;
> -
> - ptl = huge_pte_lock(h, mm, pte);
> - entry = huge_ptep_get(pte);
> - if (pte_present(entry)) {
> - page = pte_page(entry);
> -
> - if (!huge_pte_write(entry)) {
> - if (flags & FOLL_WRITE) {
> - page = NULL;
> - goto out;
> - }
> -
> - if (gup_must_unshare(vma, flags, page)) {
> - /* Tell the caller to do unsharing */
> - page = ERR_PTR(-EMLINK);
> - goto out;
> - }
> - }
> -
> - page = nth_page(page, ((address & ~huge_page_mask(h)) >> PAGE_SHIFT));
> -
> - /*
> - * Note that page may be a sub-page, and with vmemmap
> - * optimizations the page struct may be read only.
> - * try_grab_page() will increase the ref count on the
> - * head page, so this will be OK.
> - *
> - * try_grab_page() should always be able to get the page here,
> - * because we hold the ptl lock and have verified pte_present().
> - */
> - ret = try_grab_page(page, flags);
> -
> - if (WARN_ON_ONCE(ret)) {
> - page = ERR_PTR(ret);
> - goto out;
> - }
> -
> - *page_mask = (1U << huge_page_order(h)) - 1;
> - }
> -out:
> - spin_unlock(ptl);
> -out_unlock:
> - hugetlb_vma_unlock_read(vma);
> -
> - /*
> - * Fixup retval for dump requests: if pagecache doesn't exist,
> - * don't try to allocate a new page but just skip it.
> - */
> - if (!page && (flags & FOLL_DUMP) &&
> - !hugetlbfs_pagecache_present(h, vma, address))
> - page = ERR_PTR(-EFAULT);
> -
> - return page;
> -}
> -
> long hugetlb_change_protection(struct vm_area_struct *vma,
> unsigned long address, unsigned long end,
> pgprot_t newprot, unsigned long cp_flags)