linux-kernel - Re: [PATCH v3 17/35] mm/mmap: write-lock VMA before shrinking or expanding it

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Date:   Mon, 27 Feb 2023 09:33:49 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     "Liam R. Howlett" <Liam.Howlett@...cle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, michel@...pinasse.org,
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Subject: Re: [PATCH v3 17/35] mm/mmap: write-lock VMA before shrinking or
 expanding it

On Fri, Feb 24, 2023 at 8:19 AM Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> On Fri, Feb 24, 2023 at 8:14 AM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
> >
> > * Suren Baghdasaryan <surenb@...gle.com> [230223 21:06]:
> > > On Thu, Feb 23, 2023 at 5:46 PM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
> > > >
> > > > * Suren Baghdasaryan <surenb@...gle.com> [230223 16:16]:
> > > > > On Thu, Feb 23, 2023 at 12:28 PM Liam R. Howlett
> > > > > <Liam.Howlett@...cle.com> wrote:
> > > > > >
> > > > > >
> > > > > > Wait, I figured a better place to do this.
> > > > > >
> > > > > > init_multi_vma_prep() should vma_start_write() on any VMA that is passed
> > > > > > in.. that we we catch any modifications here & in vma_merge(), which I
> > > > > > think is missed in this patch set?
> > > > >
> > > > > Hmm. That looks like a good idea but in that case, why not do the
> > > > > locking inside vma_prepare() itself? From the description of that
> > > > > function it sounds like it was designed to acquire locks before VMA
> > > > > modifications, so would be the ideal location for doing that. WDYT?
> > > >
> > > > That might be even better.  I think it will result in even less code.
> > >
> > > Yes.
> > >
> > > >
> > > > There is also a vma_complete() which might work to call
> > > > vma_end_write_all() as well?
> > >
> > > If there are other VMAs already locked before vma_prepare() then we
> > > would unlock them too. Safer to just let mmap_unlock do
> > > vma_end_write_all().
> > >
> > > >
> > > > > The only concern is vma_adjust_trans_huge() being called before
> > > > > vma_prepare() but I *think* that's safe because
> > > > > vma_adjust_trans_huge() does its modifications after acquiring PTL
> > > > > lock, which page fault handlers also have to take. Does that sound
> > > > > right?
> > > >
> > > > I am not sure.  We are certainly safe the way it is, and the PTL has to
> > > > be safe for concurrent faults.. but this could alter the walk to a page
> > > > table while that walk is occurring and I don't think that happens today.
> > > >
> > > > It might be best to leave the locking order the way you have it, unless
> > > > someone can tell us it's safe?
> > >
> > > Yes, I have the same feelings about changing this.
> > >
> > > >
> > > > We could pass through the three extra variables that are needed to move
> > > > the vma_adjust_trans_huge() call within that function as well?  This
> > > > would have the added benefit of having all locking grouped in the one
> > > > location, but the argument list would be getting long, however we could
> > > > use the struct.
> > >
> > > Any issues if I change the order to have vma_prepare() called always
> > > before vma_adjust_trans_huge()? That way the VMA will always be locked
> > > before vma_adjust_trans_huge() executes and we don't need any
> > > additional arguments.
> >
> > I preserved the locking order from __vma_adjust() to ensure there was no
> > issues.
> >
> > I am not sure but, looking through the page table information [1], it
> > seems that vma_adjust_trans_huge() uses the pmd lock, which is part of
> > the split page table lock.  According to the comment in rmap, it should
> > be fine to reverse the ordering here.
> >
> > Instead of:
> >
> > mmap_lock()
> > vma_adjust_trans_huge()
> >         pte_lock
> >         pte_unlock
> >
> > vma_prepare()
> >         mapping->i_mmap_rwsem lock
> >         anon_vma->rwsem lock
> >
> > <changes to tree/VMAs>
> >
> > vma_complete()
> >         anon_vma->rwsem unlock
> >         mapping->i_mmap_rwsem unlock
> >
> > mmap_unlock()
> >
> > ---------
> >
> > We would have:
> >
> > mmap_lock()
> > vma_prepare()
> >         mapping->i_mmap_rwsem lock
> >         anon_vma->rwsem lock
> >
> > vma_adjust_trans_huge()
> >         pte_lock
> >         pte_unlock
> >
> > <changes to tree/VMAs>
> >
> > vma_complete()
> >         anon_vma->rwsem unlock
> >         mapping->i_mmap_rwsem unlock
> >
> > mmap_unlock()
> >
> >
> > Essentially, increasing the nesting of the pte lock, but not violating
> > the ordering.
> >
> > 1. https://docs.kernel.org/mm/split_page_table_lock.html
>
> Thanks for the confirmation, Liam. I'll make the changes and test over
> the weekend. If everything's still fine, I will post the next version
> with these and other requested changes on Monday.

Weekend testing results look good with all the changes. I'll post v4 shortly.
Thanks,
Suren.

>
> >
> > >
> > > >
> > > > remove & remove2 should be be detached in vma_prepare() or
> > > > vma_complete() as well?
> > >
> > > They are marked detached in vma_complete() (see
> > > https://lore.kernel.org/all/20230216051750.3125598-25-surenb@google.com/)
> > > and that should be enough. We should be safe as long as we mark them
> > > detached before unlocking the VMA.
> > >
> >
> > Right, Thanks.
> >
> > ...
> >
> > --
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> >