linux-kernel - Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd operations

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Date: Tue, 13 Feb 2024 11:55:11 -0800
From: Lokesh Gidra <lokeshgidra@...gle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Lokesh Gidra <lokeshgidra@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org, 
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, selinux@...r.kernel.org, 
	kernel-team@...roid.com, aarcange@...hat.com, peterx@...hat.com, 
	david@...hat.com, axelrasmussen@...gle.com, bgeffon@...gle.com, 
	willy@...radead.org, jannh@...gle.com, kaleshsingh@...gle.com, 
	ngeoffray@...gle.com, timmurray@...gle.com, rppt@...nel.org
Subject: Re: [PATCH v5 3/3] userfaultfd: use per-vma locks in userfaultfd operations

On Tue, Feb 13, 2024 at 11:51 AM Liam R. Howlett
<Liam.Howlett@...cle.com> wrote:
>
> * Lokesh Gidra <lokeshgidra@...gle.com> [240213 14:37]:
> ...
>
> > Asking to avoid any more iterations: these functions should call the
> > currently defined ones or should replace them. For instance, should I
> > do the following:
> >
> > #ifdef CONFIG_PER_VMA_LOCK
> > ... uffd_mfill_lock()
> > {
> >         return find_and_lock_dst_vma(...);
> > }
> > #else
> > ...uffd_mfill_lock()
> > {
> >        return lock_mm_and_find_dst_vma(...);
> > }
> > #endif
> >
> > or have the function replace
> > find_and_lock_dst_vma()/lock_mm_and_find_dst_vma() ?
>
> Since the two have the same prototype, then you can replace the function
> names directly.
>
> The other side should take the vma and use vma->vm_mm to get the mm to
> unlock the mmap_lock in the !CONFIG_PER_VMA_LOCK.  That way those
> prototypes also match and can use the same names directly.
>
> move_pages() requires unlocking two VMAs or one, so pass both VMAs
> through and do the check in there.  This, unfortunately means that one
> of the VMAs will not be used in the !CONFIG_PER_VMA_LOCK case.  You
> could add an assert to ensure src_vma is locked prior to using dst_vma
> to unlock the mmap_lock(), to avoid potential bot emails.
>
Perfect. Will do that and address the other comments you had on v5 as well.

Regarding int vs long for 'err' type, I'm assuming you are ok with my
explanation and I should keep long?

> Thanks,
> Liam