linux-kernel - Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration

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Date:   Fri, 9 Jun 2023 15:30:10 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     akpm@...ux-foundation.org, willy@...radead.org, hannes@...xchg.org,
        mhocko@...e.com, josef@...icpanda.com, jack@...e.cz,
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        viro@...iv.linux.org.uk, brauner@...nel.org,
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Subject: Re: [PATCH v2 4/6] mm: drop VMA lock before waiting for migration

On Fri, Jun 9, 2023 at 1:42 PM Peter Xu <peterx@...hat.com> wrote:
>
> On Thu, Jun 08, 2023 at 05:51:56PM -0700, Suren Baghdasaryan wrote:
> > migration_entry_wait does not need VMA lock, therefore it can be dropped
> > before waiting. Introduce VM_FAULT_VMA_UNLOCKED to indicate that VMA
> > lock was dropped while in handle_mm_fault().
> > Note that once VMA lock is dropped, the VMA reference can't be used as
> > there are no guarantees it was not freed.
>
> Then vma lock behaves differently from mmap read lock, am I right?  Can we
> still make them match on behaviors, or there's reason not to do so?

I think we could match their behavior by also dropping mmap_lock here
when fault is handled under mmap_lock (!(fault->flags &
FAULT_FLAG_VMA_LOCK)).
I missed the fact that VM_FAULT_COMPLETED can be used to skip dropping
mmap_lock in do_page_fault(), so indeed, I might be able to use
VM_FAULT_COMPLETED to skip vma_end_read(vma) for per-vma locks as well
instead of introducing FAULT_FLAG_VMA_LOCK. I think that was your idea
of reusing existing flags?

>
> One reason is if they match they can reuse existing flags and there'll be
> less confusing, e.g. this:
>
>   (fault->flags & FAULT_FLAG_VMA_LOCK) &&
>     (vm_fault_ret && (VM_FAULT_RETRY || VM_FAULT_COMPLETE))
>
> can replace the new flag, iiuc.
>
> Thanks,
>
> --
> Peter Xu
>
> --
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