linux-kernel - Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

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Message-ID: <CAJuCfpFSJLhkv4o6ZKQKDWKsg2OKYa9kwRLERJkF=B+nzoujCQ@mail.gmail.com>
Date: Thu, 19 Dec 2024 08:14:24 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: "Liam R. Howlett" <Liam.Howlett@...cle.com>, akpm@...ux-foundation.org, willy@...radead.org, 
	lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
	hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com, 
	mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com, 
	oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org, brauner@...nel.org, 
	dhowells@...hat.com, hdanton@...a.com, hughd@...gle.com, 
	lokeshgidra@...gle.com, minchan@...gle.com, jannh@...gle.com, 
	shakeel.butt@...ux.dev, souravpanda@...gle.com, pasha.tatashin@...een.com, 
	klarasmodin@...il.com, corbet@....net, linux-doc@...r.kernel.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a
 reference count

On Thu, Dec 19, 2024 at 1:13 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Dec 18, 2024 at 01:53:17PM -0800, Suren Baghdasaryan wrote:
>
> > Ah, ok I see now. I completely misunderstood what for_each_vma_range()
> > was doing.
> >
> > Then I think vma_start_write() should remain inside
> > vms_gather_munmap_vmas() and all vmas in mas_detach should be
>
> No, it must not. You really are not modifying anything yet (except the
> split, which we've already noted mark write themselves).
>
> > write-locked, even the ones we are not modifying. Otherwise what would
> > prevent the race I mentioned before?
> >
> > __mmap_region
> >     __mmap_prepare
> >         vms_gather_munmap_vmas // adds vmas to be unmapped into mas_detach,
> >                                                       // some locked
> > by __split_vma(), some not locked
> >
> >                                      lock_vma_under_rcu()
> >                                          vma = mas_walk // finds
> > unlocked vma also in mas_detach
> >                                          vma_start_read(vma) //
> > succeeds since vma is not locked
> >                                          // vma->detached, vm_start,
> > vm_end checks pass
> >                                      // vma is successfully read-locked
> >
> >        vms_clean_up_area(mas_detach)
> >             vms_clear_ptes
> >                                      // steps on a cleared PTE
>
> So here we have the added complexity that the vma is not unhooked at
> all. Is there anything that would prevent a concurrent gup_fast() from
> doing the same -- touch a cleared PTE?
>
> AFAICT two threads, one doing overlapping mmap() and the other doing
> gup_fast() can result in exactly this scenario.
>
> If we don't care about the GUP case, when I'm thinking we should not
> care about the lockless RCU case either.
>
> >     __mmap_new_vma
> >         vma_set_range // installs new vma in the range
> >     __mmap_complete
> >         vms_complete_munmap_vmas // vmas are write-locked and detached
> > but it's too late
>
> But at this point that old vma really is unhooked, and the
> vma_write_start() here will ensure readers are gone and it will clear
> PTEs *again*.

So, to summarize, you want vma_start_write() and vma_mark_detached()
to be done when we are removing the vma from the tree, right?
Something like:

vma_start_write()
vma_iter_store()
vma_mark_detached()

And the race I described is not a real problem since the vma is still
in the tree, so gup_fast() does exactly that and will simply reinstall
the ptes.

>
>