linux-kernel - Re: [PATCH v3 1/9] mm: add find_vma()-like API but RCU protected and taking VMA lock

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Date: Wed, 5 Jun 2024 13:03:26 -0400
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Matthew Wilcox <willy@...radead.org>, Andrii Nakryiko <andrii@...nel.org>,
        linux-fsdevel@...r.kernel.org, brauner@...nel.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        gregkh@...uxfoundation.org, linux-mm@...ck.org, surenb@...gle.com,
        rppt@...nel.org
Subject: Re: [PATCH v3 1/9] mm: add find_vma()-like API but RCU protected and
 taking VMA lock

* Andrii Nakryiko <andrii.nakryiko@...il.com> [240605 12:27]:
> On Wed, Jun 5, 2024 at 9:24 AM Andrii Nakryiko
> <andrii.nakryiko@...il.com> wrote:
> >
> > On Wed, Jun 5, 2024 at 9:13 AM Andrii Nakryiko
> > <andrii.nakryiko@...il.com> wrote:
> > >
> > > On Wed, Jun 5, 2024 at 6:33 AM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
> > > >
> > > > * Matthew Wilcox <willy@...radead.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 taking 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 the
> > > > > 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 = mas_find(&mas, ULONG_MAX);
> > > > > > +   if (!vma) {
> > > > > > +           err = 0; /* no VMA, return NULL */
> > > > > > +           goto inval;
> > > > > > +   }
> > > > > > +
> > > > > > +   if (!vma_start_read(vma)) {
> > > > > > +           err = -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 VMA 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 <= address)) {
> > > > > > +           err = -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 probably
> > > > use mas_set(), which will reset the maple state and set the index and
> > > > long to address.
> > >
> > > Yep, makes sense, thanks. As for the `unlikely(vma->vm_end <=
> > > address)` case, I presume we want to do the same, right? Basically, on
> > > 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?
> 
> 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?
> 

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