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

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Message-ID: <pnnhq7ok3uhm7a3va2gdnpjvywziqvicxrzp64a5dhfbmydjv3@x52pq3vq7zdv>
Date: Fri, 5 Jul 2024 16:30:46 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Lorenzo Stoakes <lorenzo.stoakes@...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

* Lorenzo Stoakes <lorenzo.stoakes@...cle.com> [240705 16:06]:
> 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()?

Yes, what we are doing is inlining the function call so that it can be
split to parts.  So for reviewing, I just made it in-line.

> 
> 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?

I don't see the validate_mm() call in do_vmi_align_munmap, it is called
in the vms_complete_munmap_vmas() function  though.

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

It would fail here, our count would be wrong.

> 
> > +
> > +		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?

Yes.  In the current function, the vmi points to prev when we are done
this dance - so I put it there.
> 
> 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.

So this is about to change..

> 
> >  		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.

Yes, this is what I'm doing.

> 
> 
> >  	}
> >
> >  	/*
> > @@ -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
> >
> >