Re: [PATCH v2 4/6] shmem: skip page split if we're not reclaiming

[Hugh Dickins <hughd@google.com>]

On Thu, 9 Mar 2023, Luis Chamberlain wrote:

> In theory when info->flags & VM_LOCKED we should not be getting
> shem_writepage() called so we should be verifying this with a
> WARN_ON_ONCE(). Since we should not be swapping then best to ensure
> we also don't do the folio split earlier too. So just move the check
> early to avoid folio splits in case its a dubious call.
> 
> We also have a similar early bail when !total_swap_pages so just move
> that earlier to avoid the possible folio split in the same situation.
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> Reviewed-by: Christian Brauner <brauner@kernel.org>
> Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
> ---
>  mm/shmem.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 68e9970baf1e..dfd995da77b4 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1350,6 +1350,12 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>  	if (WARN_ON_ONCE(!wbc->for_reclaim))
>  		goto redirty;
>  
> +	if (WARN_ON_ONCE(info->flags & VM_LOCKED))
> +		goto redirty;

Well, okay, I don't mind that.  But shall we take bets on how soon syzbot
(hope it's not watching) will try flipping SHM_LOCK on while swapping out
pages from a SHM segment, and hit that warning?  Perhaps I'm wrong, but I
don't think any serialization prevents that.

Hugh

> +
> +	if (!total_swap_pages)
> +		goto redirty;
> +
>  	/*
>  	 * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or
>  	 * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
> @@ -1365,10 +1371,6 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
>  	}
>  
>  	index = folio->index;
> -	if (info->flags & VM_LOCKED)
> -		goto redirty;
> -	if (!total_swap_pages)
> -		goto redirty;
>  
>  	/*
>  	 * This is somewhat ridiculous, but without plumbing a SWAP_MAP_FALLOC
> -- 
> 2.39.1