Re: [PATCH v2 2/3] userfaultfd: UFFDIO_REMAP uABI

[Suren Baghdasaryan]

On Thu, Sep 28, 2023 at 11:43 AM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> One more thing..
>
> On Fri, Sep 22, 2023 at 06:31:45PM -0700, Suren Baghdasaryan wrote:
> > +static int remap_pages_pte(struct mm_struct *dst_mm,
>
> [...]
>
> > +retry:
> > +     dst_pte = pte_offset_map_nolock(dst_mm, dst_pmd, dst_addr, &dst_ptl);
> > +
> > +     /* If an huge pmd materialized from under us fail */
> > +     if (unlikely(!dst_pte)) {
> > +             err = -EFAULT;
> > +             goto out;
> > +     }
> > +
> > +     src_pte = pte_offset_map_nolock(src_mm, src_pmd, src_addr, &src_ptl);
> > +
> > +     /*
> > +      * We held the mmap_lock for reading so MADV_DONTNEED
> > +      * can zap transparent huge pages under us, or the
> > +      * transparent huge page fault can establish new
> > +      * transparent huge pages under us.
> > +      */
> > +     if (unlikely(!src_pte)) {
> > +             err = -EFAULT;
> > +             goto out;
> > +     }
>
> For these two places: I know that thp collapse with mmap read lock hasn't
> yet spread to anon (so I assume none of above could trigger yet on the
> failure paths), but shall we constantly return -EAGAIN here just in case we
> forget that in the future?
>
> For example, for UFFDIO_COPY over shmem which we can already hit similar
> case, mfill_atomic_install_pte() has:
>
>         ret = -EAGAIN;
>         dst_pte = pte_offset_map_lock(dst_mm, dst_pmd, dst_addr, &ptl);
>         if (!dst_pte)
>                 goto out;
>
> Thanks,

Retrying in this case makes sense to me. Will change.

>
> --
> Peter Xu
>
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