linux-kernel - Re: [PATCH v3 09/16] mm/mmap: Expand mmap

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Message-ID: <b283fae7-fe72-4bdf-a7d9-7508c3f93293@lucifer.local>
Date: Fri, 5 Jul 2024 21:06:49 +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>, 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 09/16] mm/mmap: Expand mmap_region() munmap call

On Thu, Jul 04, 2024 at 02:27:11PM GMT, Liam R. Howlett wrote:
> From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
>
> Open code the do_vmi_align_munmap() call so that it can be broken up
> later in the series.
>
> Signed-off-by: Liam R. Howlett <Liam.Howlett@...cle.com>
> ---
>  mm/mmap.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/mm/mmap.c b/mm/mmap.c
> index e9858ca8bbd4..f5b33de4e717 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2915,6 +2915,9 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	struct vm_area_struct *next, *prev, *merge;
>  	pgoff_t pglen = len >> PAGE_SHIFT;
>  	unsigned long charged = 0;
> +	struct vma_munmap_struct vms;
> +	struct ma_state mas_detach;
> +	struct maple_tree mt_detach;
>  	unsigned long end = addr + len;
>  	unsigned long merge_start = addr, merge_end = end;
>  	bool writable_file_mapping = false;
> @@ -2947,9 +2950,24 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  	/* Find the first overlapping VMA */
>  	vma = vma_find(&vmi, end);
>  	if (vma) {
> -		if (do_vmi_align_munmap(&vmi, vma, mm, addr, end, uf, false))
> +		mt_init_flags(&mt_detach, vmi.mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> +		mt_on_stack(mt_detach);
> +		mas_init(&mas_detach, &mt_detach, /* addr = */ 0);

I'm guessing this is exactly equivalent (or equivalent enough for our purposes
here) to the MA_STATE() call in do_vmi_align_munmap()?

Checking the two against each other it seems that it is indeed.

> +		init_vma_munmap(&vms, &vmi, vma, addr, end, uf, /* unlock = */ false);
> +		if (vms_gather_munmap_vmas(&vms, &mas_detach))
> +			return -ENOMEM;

In do_vmi_align_munmap() we also invoke vmalidate_mm(), why aren't we doing that
here?

> +
> +		if (vma_iter_clear_gfp(&vmi, addr, end, GFP_KERNEL))
>  			return -ENOMEM;

Same here.

> +
> +		vms_complete_munmap_vmas(&vms, &mas_detach);
> +		next = vms.next;
> +		prev = vms.prev;
> +		vma_prev(&vmi);

I'm sure this is correct, but just to double-check - we want to set the VMI to
prev here right?

It might be worth adding a small cmoment saying '/* vmi now points at prev */'
or similar, I've found it can get quite hard to follow where the iterator is at
sometimes.

>  		vma = NULL;
> +	} else {
> +		next = vma_next(&vmi);
> +		prev = vma_prev(&vmi);

But here we move forward to the next VMA and set this to next, then go back to
the _original_ one and this is prev?

Actually I guess if vma == NULL, next gets you to the next, and prev jumps back
to prev, with nothing between, and so that makes sense.


>  	}
>
>  	/*
> @@ -2962,8 +2980,6 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
>  		vm_flags |= VM_ACCOUNT;
>  	}
>
> -	next = vma_next(&vmi);
> -	prev = vma_prev(&vmi);
>  	if (vm_flags & VM_SPECIAL) {
>  		if (prev)
>  			vma_iter_next_range(&vmi);
> --
> 2.43.0
>
>