Re: [PATCH v2 1/2] mm, thp: check page mapping when truncating page cache

[Matthew Wilcox]

On Mon, Sep 27, 2021 at 03:24:47PM -0700, Song Liu wrote:
> OTOH, does it make sense to block writes within khugepaged, like:
> @@ -1652,6 +1653,11 @@ static void collapse_file(struct mm_struct *mm,
>         /* Only allocate from the target node */
>         gfp = alloc_hugepage_khugepaged_gfpmask() | __GFP_THISNODE;
> 
> +       if (deny_write_access(file)) {
> +               result = SCAN_BUSY_WRITE;
> +               return;
> +       }

The problem is that it denies, rather than blocking.  That means that the
writer gets ETXTBSY instead of waiting until khugepaged is done.