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Message-ID: <f12e65e3-adfb-4140-9aaa-e7b11cab972d@lucifer.local>
Date: Fri, 5 Jul 2024 20:27:17 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
Suren Baghdasaryan <surenb@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>, Lorenzo Stoakes <lstoakes@...il.com>,
Matthew Wilcox <willy@...radead.org>, sidhartha.kumar@...cle.com,
"Paul E . McKenney" <paulmck@...nel.org>,
Bert Karwatzki <spasswolf@....de>, Jiri Olsa <olsajiri@...il.com>,
linux-kernel@...r.kernel.org, Kees Cook <kees@...nel.org>
Subject: Re: [PATCH v3 06/16] mm/mmap: Change munmap to use
vma_munmap_struct() for accounting and surrounding vmas
On Thu, Jul 04, 2024 at 02:27:08PM GMT, Liam R. Howlett wrote:
> Clean up the code by changing the munmap operation to use a structure
> for the accounting and munmap variables.
>
> Since remove_mt() is only called in one location and the contents will
> be reduce to almost nothing. The remains of the function can be added
> to vms_complete_munmap_vmas().
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
> mm/internal.h | 6 ++++
> mm/mmap.c | 81 ++++++++++++++++++++++++++-------------------------
> 2 files changed, 47 insertions(+), 40 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index f1e6dea2efcf..8cbbbe7d40f3 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1488,12 +1488,18 @@ struct vma_munmap_struct {
> struct vma_iterator *vmi;
> struct mm_struct *mm;
> struct vm_area_struct *vma; /* The first vma to munmap */
> + struct vm_area_struct *next; /* vma after the munmap area */
> + struct vm_area_struct *prev; /* vma before the munmap area */
I mean this is about as pedantic as it gets, and, admittedly an annoying
comment to make, but the ordering... can't we at least put prev before
next? ;)
This is actually a comment you can fully disregard, by the way!
> struct list_head *uf; /* Userfaultfd list_head */
> unsigned long start; /* Aligned start addr */
> unsigned long end; /* Aligned end addr */
> int vma_count; /* Number of vmas that will be removed */
> unsigned long nr_pages; /* Number of pages being removed */
> unsigned long locked_vm; /* Number of locked pages */
> + unsigned long nr_accounted; /* Number of VM_ACCOUNT pages */
> + unsigned long exec_vm;
> + unsigned long stack_vm;
> + unsigned long data_vm;
> bool unlock; /* Unlock after the munmap */
> };
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 76e93146ee9d..2a1a49f98fa3 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -523,7 +523,8 @@ static inline void init_vma_munmap(struct vma_munmap_struct *vms,
> vms->unlock = unlock;
> vms->uf = uf;
> vms->vma_count = 0;
> - vms->nr_pages = vms->locked_vm = 0;
> + vms->nr_pages = vms->locked_vm = vms->nr_accounted = 0;
> + vms->exec_vm = vms->stack_vm = vms->data_vm = 0;
> }
>
> /*
> @@ -2388,30 +2389,6 @@ struct vm_area_struct *expand_stack(struct mm_struct *mm, unsigned long addr)
> return vma;
> }
>
> -/*
> - * Ok - we have the memory areas we should free on a maple tree so release them,
> - * and do the vma updates.
> - *
> - * Called with the mm semaphore held.
> - */
> -static inline void remove_mt(struct mm_struct *mm, struct ma_state *mas)
> -{
> - unsigned long nr_accounted = 0;
> - struct vm_area_struct *vma;
> -
> - /* Update high watermark before we lower total_vm */
> - update_hiwater_vm(mm);
> - mas_for_each(mas, vma, ULONG_MAX) {
> - long nrpages = vma_pages(vma);
> -
> - if (vma->vm_flags & VM_ACCOUNT)
> - nr_accounted += nrpages;
> - vm_stat_account(mm, vma->vm_flags, -nrpages);
> - remove_vma(vma, false);
> - }
> - vm_unacct_memory(nr_accounted);
> -}
> -
> /*
> * Get rid of page table information in the indicated region.
> *
> @@ -2634,12 +2611,15 @@ static inline void abort_munmap_vmas(struct ma_state *mas_detach)
> * vms_complete_munmap_vmas() - Finish the munmap() operation
> * @vms: The vma munmap struct
> * @mas_detach: The maple state of the detached vmas
> + *
> + * This function updates the mm_struct, unmaps the region, frees the resources
> + * used for the munmap() and may downgrade the lock - if requested. Everything
> + * needed to be done once the vma maple tree is updated.
> */
> -
> static void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
> struct ma_state *mas_detach)
> {
> - struct vm_area_struct *prev, *next;
> + struct vm_area_struct *vma;
> struct mm_struct *mm;
>
> mm = vms->mm;
> @@ -2648,21 +2628,26 @@ static void vms_complete_munmap_vmas(struct vma_munmap_struct *vms,
> if (vms->unlock)
> mmap_write_downgrade(mm);
>
> - prev = vma_iter_prev_range(vms->vmi);
> - next = vma_next(vms->vmi);
> - if (next)
> - vma_iter_prev_range(vms->vmi);
> -
> /*
> * We can free page tables without write-locking mmap_lock because VMAs
> * were isolated before we downgraded mmap_lock.
> */
> mas_set(mas_detach, 1);
> - unmap_region(mm, mas_detach, vms->vma, prev, next, vms->start, vms->end,
> - vms->vma_count, !vms->unlock);
> - /* Statistics and freeing VMAs */
> + unmap_region(mm, mas_detach, vms->vma, vms->prev, vms->next,
> + vms->start, vms->end, vms->vma_count, !vms->unlock);
> + /* Update high watermark before we lower total_vm */
> + update_hiwater_vm(mm);
> + /* Stat accounting */
> + WRITE_ONCE(mm->total_vm, READ_ONCE(mm->total_vm) - vms->nr_pages);
> + mm->exec_vm -= vms->exec_vm;
> + mm->stack_vm -= vms->stack_vm;
> + mm->data_vm -= vms->data_vm;
OK I was going to ask you're READ_ONCE()'ing and WRITE_ONCE()'ing
mm->total_vm but not the other statistics, but the original
vm_stat_account() does this so. Yeah :)
> + /* Remove and clean up vmas */
> mas_set(mas_detach, 0);
> - remove_mt(mm, mas_detach);
> + mas_for_each(mas_detach, vma, ULONG_MAX)
> + remove_vma(vma, false);
> +
> + vm_unacct_memory(vms->nr_accounted);
> validate_mm(mm);
> if (vms->unlock)
> mmap_read_unlock(mm);
> @@ -2710,13 +2695,14 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> if (error)
> goto start_split_failed;
> }
> + vms->prev = vma_prev(vms->vmi);
>
> /*
> * Detach a range of VMAs from the mm. Using next as a temp variable as
> * it is always overwritten.
> */
> - next = vms->vma;
> - do {
> + for_each_vma_range(*(vms->vmi), next, vms->end) {
> + long nrpages;
> /* Does it split the end? */
> if (next->vm_end > vms->end) {
> error = __split_vma(vms->vmi, next, vms->end, 0);
> @@ -2725,8 +2711,21 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> }
> vma_start_write(next);
> mas_set(mas_detach, vms->vma_count++);
> + nrpages = vma_pages(next);
> +
> + vms->nr_pages += nrpages;
> if (next->vm_flags & VM_LOCKED)
> - vms->locked_vm += vma_pages(next);
> + vms->locked_vm += nrpages;
> +
> + if (next->vm_flags & VM_ACCOUNT)
> + vms->nr_accounted += nrpages;
> +
> + if (is_exec_mapping(next->vm_flags))
> + vms->exec_vm += nrpages;
> + else if (is_stack_mapping(next->vm_flags))
> + vms->stack_vm += nrpages;
> + else if (is_data_mapping(next->vm_flags))
> + vms->data_vm += nrpages;
>
> error = mas_store_gfp(mas_detach, next, GFP_KERNEL);
> if (error)
> @@ -2752,7 +2751,9 @@ static int vms_gather_munmap_vmas(struct vma_munmap_struct *vms,
> BUG_ON(next->vm_start < vms->start);
> BUG_ON(next->vm_start > vms->end);
> #endif
> - } for_each_vma_range(*(vms->vmi), next, vms->end);
> + }
> +
> + vms->next = vma_next(vms->vmi);
>
> #if defined(CONFIG_DEBUG_VM_MAPLE_TREE)
> /* Make sure no VMAs are about to be lost. */
> --
> 2.43.0
>
This is a big improvement overall, very fiddly code.
Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
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