Re: [PATCH 3/5] shmem: shmem_writepage() split unlikely i915 THP

[Yang Shi]

On Sun, Aug 30, 2020 at 2:04 PM Hugh Dickins <hughd@xxxxxxxxxx> wrote:
>
> drivers/gpu/drm/i915/gem/i915_gem_shmem.c contains a shmem_writeback()
> which calls shmem_writepage() from a shrinker: that usually works well
> enough; but if /sys/kernel/mm/transparent_hugepage/shmem_enabled has
> been set to "force" (documented as "Force the huge option on for all -
> very useful for testing"), shmem_writepage() is surprised to be called
> with a huge page, and crashes on the VM_BUG_ON_PAGE(PageCompound) (I
> did not find out where the crash happens when CONFIG_DEBUG_VM is off).
>
> LRU page reclaim always splits the shmem huge page first: I'd prefer not
> to demand that of i915, so check and split compound in shmem_writepage().
>
> Fixes: 2d6692e642e7 ("drm/i915: Start writeback from the shrinker")
> Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx # v5.3+
> ---
> I've marked this for stable just for the info, but the number of users
> affected is very probably 1, so please feel free to delete that marking.
>
>  mm/shmem.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
>
> --- 5.9-rc2/mm/shmem.c  2020-08-16 17:32:50.693507198 -0700
> +++ linux/mm/shmem.c    2020-08-28 17:35:08.326024349 -0700
> @@ -1362,7 +1362,15 @@ static int shmem_writepage(struct page *
>         swp_entry_t swap;
>         pgoff_t index;
>
> -       VM_BUG_ON_PAGE(PageCompound(page), page);
> +       /*
> +        * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "force",
> +        * then drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
> +        * and its shmem_writeback() needs them to be split when swapping.
> +        */
> +       if (PageTransCompound(page))
> +               if (split_huge_page(page) < 0)
> +                       goto redirty;

The change looks good to me. Acked-by: Yang Shi <shy828301@xxxxxxxxx>

Just a nit: it may be better to move the spilte after the !PageLocked
assertion? Split needs page locked too.

> +
>         BUG_ON(!PageLocked(page));
>         mapping = page->mapping;
>         index = page->index;
>