Re: [PATCH 2/3] mm: drop VMA lock before waiting for migration

[Suren Baghdasaryan <surenb@google.com>]

On Tue, May 2, 2023 at 6:26 AM 'Alistair Popple' via kernel-team
<kernel-team@android.com> wrote:
>
>
> Suren Baghdasaryan <surenb@google.com> writes:
>
> [...]
>
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 306a3d1a0fa6..b3b57c6da0e1 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -1030,6 +1030,7 @@ typedef __bitwise unsigned int vm_fault_t;
> >   *                           fsync() to complete (for synchronous page faults
> >   *                           in DAX)
> >   * @VM_FAULT_COMPLETED:              ->fault completed, meanwhile mmap lock released
> > + * @VM_FAULT_VMA_UNLOCKED:   VMA lock was released
>
> A note here saying vmf->vma should no longer be accessed would be nice.

Good idea. Will add in the next version. Thanks!

>
> >   * @VM_FAULT_HINDEX_MASK:    mask HINDEX value
> >   *
> >   */
> > @@ -1047,6 +1048,7 @@ enum vm_fault_reason {
> >       VM_FAULT_DONE_COW       = (__force vm_fault_t)0x001000,
> >       VM_FAULT_NEEDDSYNC      = (__force vm_fault_t)0x002000,
> >       VM_FAULT_COMPLETED      = (__force vm_fault_t)0x004000,
> > +     VM_FAULT_VMA_UNLOCKED   = (__force vm_fault_t)0x008000,
> >       VM_FAULT_HINDEX_MASK    = (__force vm_fault_t)0x0f0000,
> >  };
> >
> > @@ -1070,7 +1072,9 @@ enum vm_fault_reason {
> >       { VM_FAULT_RETRY,               "RETRY" },      \
> >       { VM_FAULT_FALLBACK,            "FALLBACK" },   \
> >       { VM_FAULT_DONE_COW,            "DONE_COW" },   \
> > -     { VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" }
> > +     { VM_FAULT_NEEDDSYNC,           "NEEDDSYNC" },  \
> > +     { VM_FAULT_COMPLETED,           "COMPLETED" },  \
>
> VM_FAULT_COMPLETED isn't used in this patch, guessing that's snuck in
> from one of the other patches in the series?

I noticed that an entry for VM_FAULT_COMPLETED was missing and wanted
to fix that... Should I drop that?

>
> > +     { VM_FAULT_VMA_UNLOCKED,        "VMA_UNLOCKED" }
> >
> >  struct vm_special_mapping {
> >       const char *name;       /* The name, e.g. "[vdso]". */
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 41f45819a923..8222acf74fd3 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -3714,8 +3714,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
> >       entry = pte_to_swp_entry(vmf->orig_pte);
> >       if (unlikely(non_swap_entry(entry))) {
> >               if (is_migration_entry(entry)) {
> > -                     migration_entry_wait(vma->vm_mm, vmf->pmd,
> > -                                          vmf->address);
> > +                     /* Save mm in case VMA lock is dropped */
> > +                     struct mm_struct *mm = vma->vm_mm;
> > +
> > +                     if (vmf->flags & FAULT_FLAG_VMA_LOCK) {
> > +                             /* No need to hold VMA lock for migration */
> > +                             vma_end_read(vma);
> > +                             /* CAUTION! VMA can't be used after this */
> > +                             ret |= VM_FAULT_VMA_UNLOCKED;
> > +                     }
> > +                     migration_entry_wait(mm, vmf->pmd, vmf->address);
> >               } else if (is_device_exclusive_entry(entry)) {
> >                       vmf->page = pfn_swap_entry_to_page(entry);
> >                       ret = remove_device_exclusive_entry(vmf);
>
> --
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