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Date: Wed, 5 Jun 2024 13:03:26 -0400
From: "Liam R. Howlett" <Liam.Howlett@oracle.com>
To: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>, Andrii Nakryiko <andrii@kernel.org>,
        linux-fsdevel@vger.kernel.org, brauner@kernel.org,
        viro@zeniv.linux.org.uk, akpm@linux-foundation.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        gregkh@linuxfoundation.org, linux-mm@kvack.org, surenb@google.com,
        rppt@kernel.org
Subject: Re: [PATCH v3 1/9] mm: add find_vma()-like API but RCU protected and
 taking VMA lock
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Mail-Followup-To: "Liam R. Howlett" <Liam.Howlett@oracle.com>, 
	Andrii Nakryiko <andrii.nakryiko@gmail.com>, Matthew Wilcox <willy@infradead.org>, 
	Andrii Nakryiko <andrii@kernel.org>, linux-fsdevel@vger.kernel.org, brauner@kernel.org, 
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	bpf@vger.kernel.org, gregkh@linuxfoundation.org, linux-mm@kvack.org, 
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* Andrii Nakryiko <andrii.nakryiko@gmail.com> [240605 12:27]:
> On Wed, Jun 5, 2024 at 9:24=E2=80=AFAM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> >
> > On Wed, Jun 5, 2024 at 9:13=E2=80=AFAM Andrii Nakryiko
> > <andrii.nakryiko@gmail.com> wrote:
> > >
> > > On Wed, Jun 5, 2024 at 6:33=E2=80=AFAM Liam R. Howlett <Liam.Howlett@=
oracle.com> wrote:
> > > >
> > > > * Matthew Wilcox <willy@infradead.org> [240604 20:57]:
> > > > > On Tue, Jun 04, 2024 at 05:24:46PM -0700, Andrii Nakryiko wrote:
> > > > > > +/*
> > > > > > + * find_and_lock_vma_rcu() - Find and lock the VMA for a given=
 address, or the
> > > > > > + * next VMA. Search is done under RCU protection, without taki=
ng or assuming
> > > > > > + * mmap_lock. Returned VMA is guaranteed to be stable and not =
isolated.
> > > > >
> > > > > You know this is supposed to be the _short_ description, right?
> > > > > Three lines is way too long.  The full description goes between t=
he
> > > > > arguments and the Return: line.
> > >
> > > Sure, I'll adjust.
> > >
> > > > >
> > > > > > + * @mm: The mm_struct to check
> > > > > > + * @addr: The address
> > > > > > + *
> > > > > > + * Returns: The VMA associated with addr, or the next VMA.
> > > > > > + * May return %NULL in the case of no VMA at addr or above.
> > > > > > + * If the VMA is being modified and can't be locked, -EBUSY is=
 returned.
> > > > > > + */
> > > > > > +struct vm_area_struct *find_and_lock_vma_rcu(struct mm_struct =
*mm,
> > > > > > +                                        unsigned long address)
> > > > > > +{
> > > > > > +   MA_STATE(mas, &mm->mm_mt, address, address);
> > > > > > +   struct vm_area_struct *vma;
> > > > > > +   int err;
> > > > > > +
> > > > > > +   rcu_read_lock();
> > > > > > +retry:
> > > > > > +   vma =3D mas_find(&mas, ULONG_MAX);
> > > > > > +   if (!vma) {
> > > > > > +           err =3D 0; /* no VMA, return NULL */
> > > > > > +           goto inval;
> > > > > > +   }
> > > > > > +
> > > > > > +   if (!vma_start_read(vma)) {
> > > > > > +           err =3D -EBUSY;
> > > > > > +           goto inval;
> > > > > > +   }
> > > > > > +
> > > > > > +   /*
> > > > > > +    * Check since vm_start/vm_end might change before we lock =
the VMA.
> > > > > > +    * Note, unlike lock_vma_under_rcu() we are searching for V=
MA covering
> > > > > > +    * address or the next one, so we only make sure VMA wasn't=
 updated to
> > > > > > +    * end before the address.
> > > > > > +    */
> > > > > > +   if (unlikely(vma->vm_end <=3D address)) {
> > > > > > +           err =3D -EBUSY;
> > > > > > +           goto inval_end_read;
> > > > > > +   }
> > > > > > +
> > > > > > +   /* Check if the VMA got isolated after we found it */
> > > > > > +   if (vma->detached) {
> > > > > > +           vma_end_read(vma);
> > > > > > +           count_vm_vma_lock_event(VMA_LOCK_MISS);
> > > > > > +           /* The area was replaced with another one */
> > > > >
> > > > > Surely you need to mas_reset() before you goto retry?
> > > >
> > > > Probably more than that.  We've found and may have adjusted the
> > > > index/last; we should reconfigure the maple state.  You should prob=
ably
> > > > use mas_set(), which will reset the maple state and set the index a=
nd
> > > > long to address.
> > >
> > > Yep, makes sense, thanks. As for the `unlikely(vma->vm_end <=3D
> > > address)` case, I presume we want to do the same, right? Basically, o=
n
> > > each retry start from the `address` unconditionally, no matter what's
> > > the reason for retry.
> >
> > ah, never mind, we don't retry in that situation, I'll just put
> > `mas_set(&mas, address);` right before `goto retry;`. Unless we should
> > actually retry in the case when VMA got moved before the requested
> > address, not sure, let me know what you think. Presumably retrying
> > will allow us to get the correct VMA without the need to fall back to
> > mmap_lock?
>=20
> sorry, one more question as I look some more around this (unfamiliar
> to me) piece of code. I see that lock_vma_under_rcu counts
> VMA_LOCK_MISS on retry, but I see that there is actually a
> VMA_LOCK_RETRY stat as well. Any reason it's a MISS instead of RETRY?
> Should I use MISS as well, or actually count a RETRY?
>=20

VMA_LOCK_MISS is used here because we missed the VMA due to a write
happening to move the vma (rather rare).  The VMA_LOCK missed the vma.

VMA_LOCK_RETRY is used to indicate we need to retry under the mmap lock.
A retry is needed after the VMA_LOCK did not work under rcu locking.

Thanks,
Liam