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

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,