Re: [PATCH v4] page cache: Store only head pages in i_pages

From: Chris Wilson <chris@chris-wilson.co.uk>
To: Andrew Morton <akpm@linux-foundation.org>,
	Matthew Wilcox <willy@infradead.org>,
	linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org
Cc: Matthew Wilcox <willy@infradead.org>,
	"Kirill A. Shutemov" <kirill@shutemov.name>,
	Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
	Song Liu <liu.song.a23@gmail.com>
Subject: Re: [PATCH v4] page cache: Store only head pages in i_pages
Date: Sun, 02 Jun 2019 22:47:35 +0100	[thread overview]
Message-ID: <155951205528.18214.706102020945306720@skylake-alporthouse-com> (raw)
In-Reply-To: <20190307153051.18815-1-willy@infradead.org>

Quoting Matthew Wilcox (2019-03-07 15:30:51)
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 404acdcd0455..aaf88f85d492 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -2456,6 +2456,9 @@ static void __split_huge_page(struct page *page, struct list_head *list,
>                         if (IS_ENABLED(CONFIG_SHMEM) && PageSwapBacked(head))
>                                 shmem_uncharge(head->mapping->host, 1);
>                         put_page(head + i);
> +               } else if (!PageAnon(page)) {
> +                       __xa_store(&head->mapping->i_pages, head[i].index,
> +                                       head + i, 0);

Forgiving the ignorant copy'n'paste, this is required:

+               } else if (PageSwapCache(page)) {
+                       swp_entry_t entry = { .val = page_private(head + i) };
+                       __xa_store(&swap_address_space(entry)->i_pages,
+                                  swp_offset(entry),
+                                  head + i, 0);
                }
        }

The locking is definitely wrong.
-Chris