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Message-ID: <22b6346d-b8a0-4d89-b886-1ac2c9ce6ba4@lucifer.local>
Date: Wed, 21 Aug 2024 12:02:06 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
linux-kernel@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
Matthew Wilcox <willy@...radead.org>, Vlastimil Babka <vbabka@...e.cz>,
sidhartha.kumar@...cle.com, Bert Karwatzki <spasswolf@....de>,
Jiri Olsa <olsajiri@...il.com>, Kees Cook <kees@...nel.org>,
"Paul E . McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH v6 14/20] mm/mmap: Avoid zeroing vma tree in mmap_region()
On Tue, Aug 20, 2024 at 07:57:23PM GMT, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>
> Instead of zeroing the vma tree and then overwriting the area, let the
> area be overwritten and then clean up the gathered vmas using
> vms_complete_munmap_vmas().
>
> To ensure locking is downgraded correctly, the mm is set regardless of
> MAP_FIXED or not (NULL vma).
>
> If a driver is mapping over an existing vma, then clear the ptes before
> the call_mmap() invocation. This is done using the vms_clean_up_area()
> helper. If there is a close vm_ops, that must also be called to ensure
> any cleanup is done before mapping over the area. This also means that
> calling open has been added to the abort of an unmap operation, for now.
Might be worth explicitly expanding this to say that this isn't a permanent
solution because of asymmetric vm_ops->open() / close().
>
> Temporarily keep track of the number of pages that will be removed and
> reduce the charged amount.
>
> This also drops the validate_mm() call in the vma_expand() function.
> It is necessary to drop the validate as it would fail since the mm
> map_count would be incorrect during a vma expansion, prior to the
> cleanup from vms_complete_munmap_vmas().
>
> Clean up the error handing of the vms_gather_munmap_vmas() by calling
> the verification within the function.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
Broadly looks good, some nits and questions below, but generally:
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> ---
> mm/mmap.c | 62 ++++++++++++++++++++++++++-----------------------------
> mm/vma.c | 54 +++++++++++++++++++++++++++++++++++++-----------
> mm/vma.h | 22 ++++++++++++++------
> 3 files changed, 87 insertions(+), 51 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 71b2bad717b6..6550d9470d3a 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -1373,23 +1373,19 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> unsigned long merge_start = addr, merge_end = end;
> bool writable_file_mapping = false;
> pgoff_t vm_pgoff;
> - int error;
> + int error = -ENOMEM;
> VMA_ITERATOR(vmi, mm, addr);
> + unsigned long nr_pages, nr_accounted;
>
> - /* Check against address space limit. */
> - if (!may_expand_vm(mm, vm_flags, len >> PAGE_SHIFT)) {
> - unsigned long nr_pages;
> + nr_pages = count_vma_pages_range(mm, addr, end, &nr_accounted);
>
> - /*
> - * MAP_FIXED may remove pages of mappings that intersects with
> - * requested mapping. Account for the pages it would unmap.
> - */
> - nr_pages = count_vma_pages_range(mm, addr, end);
> -
> - if (!may_expand_vm(mm, vm_flags,
> - (len >> PAGE_SHIFT) - nr_pages))
> - return -ENOMEM;
> - }
> + /*
> + * Check against address space limit.
> + * MAP_FIXED may remove pages of mappings that intersects with requested
> + * mapping. Account for the pages it would unmap.
> + */
> + if (!may_expand_vm(mm, vm_flags, (len >> PAGE_SHIFT) - nr_pages))
> + return -ENOMEM;
>
> /* Find the first overlapping VMA */
> vma = vma_find(&vmi, end);
> @@ -1400,14 +1396,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> mas_init(&mas_detach, &mt_detach, /* addr = */ 0);
> /* Prepare to unmap any existing mapping in the area */
> if (vms_gather_munmap_vmas(&vms, &mas_detach))
> - goto gather_failed;
> -
> - /* Remove any existing mappings from the vma tree */
> - if (vma_iter_clear_gfp(&vmi, addr, end, GFP_KERNEL))
> - goto clear_tree_failed;
> + return -ENOMEM;
>
> - /* Unmap any existing mapping in the area */
> - vms_complete_munmap_vmas(&vms, &mas_detach);
> next = vms.next;
> prev = vms.prev;
> vma = NULL;
> @@ -1423,8 +1413,10 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> */
> if (accountable_mapping(file, vm_flags)) {
> charged = len >> PAGE_SHIFT;
> + charged -= nr_accounted;
> if (security_vm_enough_memory_mm(mm, charged))
> - return -ENOMEM;
> + goto abort_munmap;
> + vms.nr_accounted = 0;
> vm_flags |= VM_ACCOUNT;
> }
>
> @@ -1473,10 +1465,8 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> * not unmapped, but the maps are removed from the list.
> */
> vma = vm_area_alloc(mm);
> - if (!vma) {
> - error = -ENOMEM;
> + if (!vma)
> goto unacct_error;
> - }
>
> vma_iter_config(&vmi, addr, end);
> vma_set_range(vma, addr, end, pgoff);
> @@ -1485,6 +1475,11 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>
> if (file) {
> vma->vm_file = get_file(file);
> + /*
> + * call_mmap() may map PTE, so ensure there are no existing PTEs
> + * call the vm_ops close function if one exists.
Super-nit, but maybe add an 'and' here.
> + */
> + vms_clean_up_area(&vms, &mas_detach, true);
I hate that we have to do this. These kind of hooks are the devil's works...
> error = call_mmap(file, vma);
> if (error)
> goto unmap_and_free_vma;
> @@ -1575,6 +1570,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> expanded:
> perf_event_mmap(vma);
>
> + /* Unmap any existing mapping in the area */
> + vms_complete_munmap_vmas(&vms, &mas_detach);
> +
> vm_stat_account(mm, vm_flags, len >> PAGE_SHIFT);
> if (vm_flags & VM_LOCKED) {
> if ((vm_flags & VM_SPECIAL) || vma_is_dax(vma) ||
> @@ -1603,7 +1601,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> return addr;
>
> close_and_free_vma:
> - if (file && vma->vm_ops && vma->vm_ops->close)
> + if (file && !vms.closed_vm_ops && vma->vm_ops && vma->vm_ops->close)
> vma->vm_ops->close(vma);
>
> if (file || vma->vm_file) {
> @@ -1622,14 +1620,12 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
> unacct_error:
> if (charged)
> vm_unacct_memory(charged);
> - validate_mm(mm);
> - return error;
>
> -clear_tree_failed:
> - abort_munmap_vmas(&mas_detach);
> -gather_failed:
> +abort_munmap:
> + if (vms.nr_pages)
> + abort_munmap_vmas(&mas_detach, vms.closed_vm_ops);
> validate_mm(mm);
> - return -ENOMEM;
> + return error;
> }
>
> static int __vm_munmap(unsigned long start, size_t len, bool unlock)
> @@ -1959,7 +1955,7 @@ void exit_mmap(struct mm_struct *mm)
> do {
> if (vma->vm_flags & VM_ACCOUNT)
> nr_accounted += vma_pages(vma);
> - remove_vma(vma, true);
> + remove_vma(vma, /* unreachable = */ true, /* closed = */ false);
> count++;
> cond_resched();
> vma = vma_next(&vmi);
> diff --git a/mm/vma.c b/mm/vma.c
> index 7104c2c080bb..5b33f7460ab7 100644
> --- a/mm/vma.c
> +++ b/mm/vma.c
> @@ -136,10 +136,10 @@ can_vma_merge_after(struct vm_area_struct *vma, unsigned long vm_flags,
> /*
> * Close a vm structure and free it.
> */
> -void remove_vma(struct vm_area_struct *vma, bool unreachable)
> +void remove_vma(struct vm_area_struct *vma, bool unreachable, bool closed)
> {
> might_sleep();
> - if (vma->vm_ops && vma->vm_ops->close)
> + if (!closed && vma->vm_ops && vma->vm_ops->close)
> vma->vm_ops->close(vma);
> if (vma->vm_file)
> fput(vma->vm_file);
> @@ -521,7 +521,6 @@ int vma_expand(struct vma_iterator *vmi, struct vm_area_struct *vma,
> vma_iter_store(vmi, vma);
>
> vma_complete(&vp, vmi, vma->vm_mm);
> - validate_mm(vma->vm_mm);
> return 0;
>
> nomem:
> @@ -645,11 +644,14 @@ void vma_complete(struct vma_prepare *vp,
> uprobe_mmap(vp->insert);
> }
>
> -static void vms_complete_pte_clear(struct vma_munmap_struct *vms,
> - struct ma_state *mas_detach, bool mm_wr_locked)
> +static inline void vms_clear_ptes(struct vma_munmap_struct *vms,
> + struct ma_state *mas_detach, bool mm_wr_locked)
> {
> struct mmu_gather tlb;
>
> + if (!vms->clear_ptes) /* Nothing to do */
> + return;
> +
> /*
> * We can free page tables without write-locking mmap_lock because VMAs
> * were isolated before we downgraded mmap_lock.
> @@ -658,11 +660,31 @@ static void vms_complete_pte_clear(struct vma_munmap_struct *vms,
> lru_add_drain();
> tlb_gather_mmu(&tlb, vms->mm);
> update_hiwater_rss(vms->mm);
> - unmap_vmas(&tlb, mas_detach, vms->vma, vms->start, vms->end, vms->vma_count, mm_wr_locked);
> + unmap_vmas(&tlb, mas_detach, vms->vma, vms->start, vms->end,
> + vms->vma_count, mm_wr_locked);
> +
> mas_set(mas_detach, 1);
> /* start and end may be different if there is no prev or next vma. */
> - free_pgtables(&tlb, mas_detach, vms->vma, vms->unmap_start, vms->unmap_end, mm_wr_locked);
> + free_pgtables(&tlb, mas_detach, vms->vma, vms->unmap_start,
> + vms->unmap_end, mm_wr_locked);
> tlb_finish_mmu(&tlb);
> + vms->clear_ptes = false;
> +}
> +
> +void vms_clean_up_area(struct vma_munmap_struct *vms,
> + struct ma_state *mas_detach, bool mm_wr_locked)
The only invocation of this function has mm_wr_locked set, is this
parameter necessary?
> +{
> + struct vm_area_struct *vma;
> +
> + if (!vms->nr_pages)
> + return;
> +
> + vms_clear_ptes(vms, mas_detach, mm_wr_locked);
> + mas_set(mas_detach, 0);
> + mas_for_each(mas_detach, vma, ULONG_MAX)
> + if (vma->vm_ops && vma->vm_ops->close)
> + vma->vm_ops->close(vma);
> + vms->closed_vm_ops = true;
> }
>
> /*
> @@ -686,7 +708,10 @@ void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
> if (vms->unlock)
> mmap_write_downgrade(mm);
>
> - vms_complete_pte_clear(vms, mas_detach, !vms->unlock);
> + if (!vms->nr_pages)
> + return;
> +
> + vms_clear_ptes(vms, mas_detach, !vms->unlock);
> /* Update high watermark before we lower total_vm */
> update_hiwater_vm(mm);
> /* Stat accounting */
> @@ -697,7 +722,7 @@ void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
> /* Remove and clean up vmas */
> mas_set(mas_detach, 0);
> mas_for_each(mas_detach, vma, ULONG_MAX)
> - remove_vma(vma, false);
> + remove_vma(vma, /* = */ false, vms->closed_vm_ops);
>
> vm_unacct_memory(vms->nr_accounted);
> validate_mm(mm);
> @@ -849,13 +874,14 @@ int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> while (vma_iter_addr(vms->vmi) > vms->start)
> vma_iter_prev_range(vms->vmi);
>
> + vms->clear_ptes = true;
> return 0;
>
> userfaultfd_error:
> munmap_gather_failed:
> end_split_failed:
> modify_vma_failed:
> - abort_munmap_vmas(mas_detach);
> + abort_munmap_vmas(mas_detach, /* closed = */ false);
> start_split_failed:
> map_count_exceeded:
> return error;
> @@ -900,7 +926,7 @@ int do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
> return 0;
>
> clear_tree_failed:
> - abort_munmap_vmas(&mas_detach);
> + abort_munmap_vmas(&mas_detach, /* closed = */ false);
> gather_failed:
> validate_mm(mm);
> return error;
> @@ -1618,17 +1644,21 @@ bool vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot)
> }
>
> unsigned long count_vma_pages_range(struct mm_struct *mm,
> - unsigned long addr, unsigned long end)
> + unsigned long addr, unsigned long end,
> + unsigned long *nr_accounted)
> {
> VMA_ITERATOR(vmi, mm, addr);
> struct vm_area_struct *vma;
> unsigned long nr_pages = 0;
>
> + *nr_accounted = 0;
> for_each_vma_range(vmi, vma, end) {
> unsigned long vm_start = max(addr, vma->vm_start);
> unsigned long vm_end = min(end, vma->vm_end);
>
> nr_pages += PHYS_PFN(vm_end - vm_start);
> + if (vma->vm_flags & VM_ACCOUNT)
> + *nr_accounted += PHYS_PFN(vm_end - vm_start);
Nitty, but maybe:
...
unsigned long pages = PHYS_PFN(vm_end - vm_start);
nr_pages += pages;
if (vma->vm_flags & VM_ACCOUNT)
*nr_accounted += pages;
> }
>
> return nr_pages;
> diff --git a/mm/vma.h b/mm/vma.h
> index 6028fdf79257..756dd42a6ec4 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -48,6 +48,8 @@ struct vma_munmap_struct {
> unsigned long stack_vm;
> unsigned long data_vm;
> bool unlock; /* Unlock after the munmap */
> + bool clear_ptes; /* If there are outstanding PTE to be cleared */
> + bool closed_vm_ops; /* call_mmap() was encountered, so vmas may be closed */
> };
>
> #ifdef CONFIG_DEBUG_VM_MAPLE_TREE
> @@ -95,14 +97,13 @@ static inline void init_vma_munmap(struct vma_munmap_struct *vms,
> unsigned long start, unsigned long end, struct list_head *uf,
> bool unlock)
> {
> + vms->mm = current->mm;
I'm guessing there's no circumstances under which we'd be looking at a
remote mm_struct?
This does sort of beg the question as to why we're bothering to store the
field if we can't just grab it from current->mm? Perhaps because the cache
line for the start of vms will be populated and current's containing ->mm
may not?
> vms->vmi = vmi;
> vms->vma = vma;
> if (vma) {
> - vms->mm = vma->vm_mm;
> vms->start = start;
> vms->end = end;
> } else {
> - vms->mm = NULL;
I guess as well there's no drawback to having an otherwise empty vms have a
populated mm?
> vms->start = vms->end = 0;
> }
> vms->unlock = unlock;
> @@ -112,6 +113,8 @@ static inline void init_vma_munmap(struct vma_munmap_struct *vms,
> vms->exec_vm = vms->stack_vm = vms->data_vm = 0;
> vms->unmap_start = FIRST_USER_ADDRESS;
> vms->unmap_end = USER_PGTABLES_CEILING;
> + vms->clear_ptes = false;
> + vms->closed_vm_ops = false;
> }
>
> int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> @@ -120,18 +123,24 @@ int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
> struct ma_state *mas_detach);
>
> +void vms_clean_up_area(struct vma_munmap_struct *vms,
> + struct ma_state *mas_detach, bool mm_wr_locked);
> +
> /*
> * abort_munmap_vmas - Undo any munmap work and free resources
> *
> * Reattach any detached vmas and free up the maple tree used to track the vmas.
> */
> -static inline void abort_munmap_vmas(struct ma_state *mas_detach)
> +static inline void abort_munmap_vmas(struct ma_state *mas_detach, bool closed)
> {
> struct vm_area_struct *vma;
>
> mas_set(mas_detach, 0);
> - mas_for_each(mas_detach, vma, ULONG_MAX)
> + mas_for_each(mas_detach, vma, ULONG_MAX) {
> vma_mark_detached(vma, false);
> + if (closed && vma->vm_ops && vma->vm_ops->open)
> + vma->vm_ops->open(vma);
> + }
Hang on, I thought we eliminated this approach? OK I see you change this in
the next commmit.
Not necessarily a huge fan of having a commit in the tree that's broken for
(hideous, asymmetric) drivers + such but I guess it's okay given we address
it immediately and it helps document the thinking process + split up the
code.
>
> __mt_destroy(mas_detach->tree);
> }
> @@ -145,7 +154,7 @@ int do_vmi_munmap(struct vma_iterator *vmi, struct mm_struct *mm,
> unsigned long start, size_t len, struct list_head *uf,
> bool unlock);
>
> -void remove_vma(struct vm_area_struct *vma, bool unreachable);
> +void remove_vma(struct vm_area_struct *vma, bool unreachable, bool closed);
>
> void unmap_region(struct ma_state *mas, struct vm_area_struct *vma,
> struct vm_area_struct *prev, struct vm_area_struct *next);
> @@ -259,7 +268,8 @@ bool vma_wants_writenotify(struct vm_area_struct *vma, pgprot_t vm_page_prot);
> int mm_take_all_locks(struct mm_struct *mm);
> void mm_drop_all_locks(struct mm_struct *mm);
> unsigned long count_vma_pages_range(struct mm_struct *mm,
> - unsigned long addr, unsigned long end);
> + unsigned long addr, unsigned long end,
> + unsigned long *nr_accounted);
>
> static inline bool vma_wants_manual_pte_write_upgrade(struct vm_area_struct *vma)
> {
> --
> 2.43.0
>
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