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Message-ID: <crakkuynbh373evxs5tbqp2jeq4h3rmvuprjq73sc3gjex6w5q@tnzempendkz7>
Date: Mon, 7 Oct 2024 11:41:30 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: Anthony Yznaga <anthony.yznaga@...cle.com>
Cc: akpm@...ux-foundation.org, willy@...radead.org,
markhemm@...glemail.com, viro@...iv.linux.org.uk, david@...hat.com, khalid@...nel.org,
andreyknvl@...il.com, dave.hansen@...el.com, luto@...nel.org, brauner@...nel.org,
arnd@...db.de, ebiederm@...ssion.com, catalin.marinas@....com,
linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
mhiramat@...nel.org, rostedt@...dmis.org, vasily.averin@...ux.dev,
xhao@...ux.alibaba.com, pcc@...gle.com, neilb@...e.de, maz@...nel.org
Subject: Re: [RFC PATCH v3 08/10] mm/mshare: Add basic page table sharing
support
On Tue, Sep 03, 2024 at 04:22:39PM -0700, Anthony Yznaga wrote:
> From: Khalid Aziz <khalid@...nel.org>
>
> Add support for handling page faults in an mshare region by
> redirecting the faults to operate on the mshare mm_struct and
> vmas contained in it and to link the page tables of the faulting
> process with the shared page tables in the mshare mm.
> Modify the unmap path to ensure that page tables in mshare regions
> are kept intact when a process exits. Note that they are also
> kept intact and not unlinked from a process when an mshare region
> is explicitly unmapped which is bug to be addressed.
>
> Signed-off-by: Khalid Aziz <khalid@...nel.org>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> Signed-off-by: Anthony Yznaga <anthony.yznaga@...cle.com>
> ---
> mm/internal.h | 1 +
> mm/memory.c | 62 ++++++++++++++++++++++++++++++++++++++++++++++++---
> mm/mshare.c | 38 +++++++++++++++++++++++++++++++
> 3 files changed, 98 insertions(+), 3 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 8005d5956b6e..8ac224d96806 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1578,6 +1578,7 @@ void unlink_file_vma_batch_init(struct unlink_vma_file_batch *);
> void unlink_file_vma_batch_add(struct unlink_vma_file_batch *, struct vm_area_struct *);
> void unlink_file_vma_batch_final(struct unlink_vma_file_batch *);
>
> +extern vm_fault_t find_shared_vma(struct vm_area_struct **vma, unsigned long *addrp);
> static inline bool vma_is_shared(const struct vm_area_struct *vma)
> {
> return VM_SHARED_PT && (vma->vm_flags & VM_SHARED_PT);
> diff --git a/mm/memory.c b/mm/memory.c
> index 3c01d68065be..f526aef71a61 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -387,11 +387,15 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
> vma_start_write(vma);
> unlink_anon_vmas(vma);
>
> + /*
> + * There is no page table to be freed for vmas that
> + * are mapped in mshare regions
> + */
> if (is_vm_hugetlb_page(vma)) {
> unlink_file_vma(vma);
> hugetlb_free_pgd_range(tlb, addr, vma->vm_end,
> floor, next ? next->vm_start : ceiling);
> - } else {
> + } else if (!vma_is_shared(vma)) {
> unlink_file_vma_batch_init(&vb);
> unlink_file_vma_batch_add(&vb, vma);
>
> @@ -399,7 +403,8 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
> * Optimization: gather nearby vmas into one call down
> */
> while (next && next->vm_start <= vma->vm_end + PMD_SIZE
> - && !is_vm_hugetlb_page(next)) {
> + && !is_vm_hugetlb_page(next)
> + && !vma_is_shared(next)) {
> vma = next;
> next = mas_find(mas, ceiling - 1);
> if (unlikely(xa_is_zero(next)))
> @@ -412,7 +417,9 @@ void free_pgtables(struct mmu_gather *tlb, struct ma_state *mas,
> unlink_file_vma_batch_final(&vb);
> free_pgd_range(tlb, addr, vma->vm_end,
> floor, next ? next->vm_start : ceiling);
> - }
> + } else
> + unlink_file_vma(vma);
> +
> vma = next;
I would rather have vma->vm_ops->free_pgtables() hook that would be defined
to non-NULL for mshared and hugetlb VMAs
> } while (vma);
> }
> @@ -1797,6 +1804,13 @@ void unmap_page_range(struct mmu_gather *tlb,
> pgd_t *pgd;
> unsigned long next;
>
> + /*
> + * No need to unmap vmas that share page table through
> + * mshare region
> + */
> + if (vma_is_shared(vma))
> + return;
> +
Ditto. vma->vm_ops->unmap_page_range().
> BUG_ON(addr >= end);
> tlb_start_vma(tlb, vma);
> pgd = pgd_offset(vma->vm_mm, addr);
> @@ -5801,6 +5815,7 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> struct mm_struct *mm = vma->vm_mm;
> vm_fault_t ret;
> bool is_droppable;
> + bool shared = false;
>
> __set_current_state(TASK_RUNNING);
>
> @@ -5808,6 +5823,21 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> if (ret)
> goto out;
>
> + if (unlikely(vma_is_shared(vma))) {
> + /* XXX mshare does not support per-VMA locks yet so fallback to mm lock */
> + if (flags & FAULT_FLAG_VMA_LOCK) {
> + vma_end_read(vma);
> + return VM_FAULT_RETRY;
> + }
> +
> + ret = find_shared_vma(&vma, &address);
> + if (ret)
> + return ret;
> + if (!vma)
> + return VM_FAULT_SIGSEGV;
> + shared = true;
Do we need to update 'mm' variable here?
It is going to be used to account the fault below. Not sure which mm has
to account such faults.
> + }
> +
> if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
> flags & FAULT_FLAG_INSTRUCTION,
> flags & FAULT_FLAG_REMOTE)) {
> @@ -5843,6 +5873,32 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> if (is_droppable)
> ret &= ~VM_FAULT_OOM;
>
> + /*
> + * Release the read lock on the shared mm of a shared VMA unless
> + * unless the lock has already been released.
> + * The mmap lock will already have been released if __handle_mm_fault
> + * returns VM_FAULT_COMPLETED or if it returns VM_FAULT_RETRY and
> + * the flags FAULT_FLAG_ALLOW_RETRY and FAULT_FLAG_RETRY_NOWAIT are
> + * _not_ both set.
> + * If the lock was released earlier, release the lock on the task's
> + * mm now to keep lock state consistent.
> + */
> + if (shared) {
> + int release_mmlock = 1;
> +
> + if ((ret & (VM_FAULT_RETRY | VM_FAULT_COMPLETED)) == 0) {
> + mmap_read_unlock(vma->vm_mm);
> + release_mmlock = 0;
> + } else if ((flags & FAULT_FLAG_ALLOW_RETRY) &&
> + (flags & FAULT_FLAG_RETRY_NOWAIT)) {
> + mmap_read_unlock(vma->vm_mm);
> + release_mmlock = 0;
> + }
> +
> + if (release_mmlock)
> + mmap_read_unlock(mm);
> + }
> +
> if (flags & FAULT_FLAG_USER) {
> mem_cgroup_exit_user_fault();
> /*
> diff --git a/mm/mshare.c b/mm/mshare.c
> index f3f6ed9c3761..8f47c8d6e6a4 100644
> --- a/mm/mshare.c
> +++ b/mm/mshare.c
> @@ -19,6 +19,7 @@
> #include <linux/spinlock_types.h>
> #include <uapi/linux/magic.h>
> #include <uapi/linux/msharefs.h>
> +#include "internal.h"
>
> struct mshare_data {
> struct mm_struct *mm;
> @@ -33,6 +34,43 @@ struct msharefs_info {
> static const struct inode_operations msharefs_dir_inode_ops;
> static const struct inode_operations msharefs_file_inode_ops;
>
> +/* Returns holding the host mm's lock for read. Caller must release. */
> +vm_fault_t
> +find_shared_vma(struct vm_area_struct **vmap, unsigned long *addrp)
> +{
> + struct vm_area_struct *vma, *guest = *vmap;
> + struct mshare_data *m_data = guest->vm_private_data;
> + struct mm_struct *host_mm = m_data->mm;
> + unsigned long host_addr;
> + pgd_t *pgd, *guest_pgd;
> +
> + mmap_read_lock(host_mm);
Hm. So we have current->mm locked here, right? So this is nested mmap
lock. Have you tested it under lockdep? I expected it to complain.
> + host_addr = *addrp - guest->vm_start + host_mm->mmap_base;
> + pgd = pgd_offset(host_mm, host_addr);
> + guest_pgd = pgd_offset(guest->vm_mm, *addrp);
> + if (!pgd_same(*guest_pgd, *pgd)) {
> + set_pgd(guest_pgd, *pgd);
> + mmap_read_unlock(host_mm);
> + return VM_FAULT_NOPAGE;
> + }
> +
> + *addrp = host_addr;
> + vma = find_vma(host_mm, host_addr);
> +
> + /* XXX: expand stack? */
> + if (vma && vma->vm_start > host_addr)
> + vma = NULL;
> +
> + *vmap = vma;
> +
> + /*
> + * release host mm lock unless a matching vma is found
> + */
> + if (!vma)
> + mmap_read_unlock(host_mm);
> + return 0;
> +}
> +
> /*
> * Disallow partial unmaps of an mshare region for now. Unmapping at
> * boundaries aligned to the level page tables are shared at could
> --
> 2.43.5
>
--
Kiryl Shutsemau / Kirill A. Shutemov
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