Re: [PATCH v5 6/9] mm: shmem: support large folio allocation for shmem_replace_folio()

[Hugh Dickins <hughd@google.com>]

On Mon, 12 Aug 2024, Baolin Wang wrote:

> To support large folio swapin for shmem in the following patches, add
> large folio allocation for the new replacement folio in shmem_replace_folio().
> Moreover large folios occupy N consecutive entries in the swap cache
> instead of using multi-index entries like the page cache, therefore
> we should replace each consecutive entries in the swap cache instead
> of using the shmem_replace_entry().
> 
> As well as updating statistics and folio reference count using the number
> of pages in the folio.
> 
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  mm/shmem.c | 54 +++++++++++++++++++++++++++++++-----------------------
>  1 file changed, 31 insertions(+), 23 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index f6bab42180ea..d94f02ad7bd1 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1889,28 +1889,24 @@ static bool shmem_should_replace_folio(struct folio *folio, gfp_t gfp)
>  static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
>  				struct shmem_inode_info *info, pgoff_t index)
>  {
> -	struct folio *old, *new;
> -	struct address_space *swap_mapping;
> -	swp_entry_t entry;
> -	pgoff_t swap_index;
> -	int error;
> -
> -	old = *foliop;
> -	entry = old->swap;
> -	swap_index = swap_cache_index(entry);
> -	swap_mapping = swap_address_space(entry);
> +	struct folio *new, *old = *foliop;
> +	swp_entry_t entry = old->swap;
> +	struct address_space *swap_mapping = swap_address_space(entry);
> +	pgoff_t swap_index = swap_cache_index(entry);
> +	XA_STATE(xas, &swap_mapping->i_pages, swap_index);
> +	int nr_pages = folio_nr_pages(old);
> +	int error = 0, i;
>  
>  	/*
>  	 * We have arrived here because our zones are constrained, so don't
>  	 * limit chance of success by further cpuset and node constraints.
>  	 */
>  	gfp &= ~GFP_CONSTRAINT_MASK;
> -	VM_BUG_ON_FOLIO(folio_test_large(old), old);
> -	new = shmem_alloc_folio(gfp, 0, info, index);
> +	new = shmem_alloc_folio(gfp, folio_order(old), info, index);

It is not clear to me whether folio_order(old) will ever be more than 0
here: but if it can be, then care will need to be taken over the gfp flags,
that they are suited to allocating the large folio; and there will need to
be (could be awkward!) fallback to order 0 when that allocation fails.

My own testing never comes to shmem_replace_folio(): it was originally for
one lowend graphics driver; but IIRC there's now a more common case for it.

Hugh