On Wed, 18 Aug 2021, Matthew Wilcox (Oracle) wrote:
> It is not safe to check page->index without holding the page lock.
> It can be changed if the page is moved between the swap cache and the
> page cache for a shmem file, for example. There is a VM_BUG_ON below
> which checks page->index is correct after taking the page lock.
>
> Cc: stable@xxxxxxxxxxxxxxx
> Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")
I don't mind that VM_BUG_ON_PAGE() being removed, but question whether
this Fixes anything, and needs to go to stable. Or maybe it's just that
the shmem example is wrong - moving shmem from page to swap cache does
not change page->index. Or maybe you have later changes in your tree
which change that and do require this. Otherwise, I'll have to worry
why my testing has missed it for six months.
Hugh
> Signed-off-by: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> ---
> mm/filemap.c | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index d1458ecf2f51..34de0b14aaa9 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2033,17 +2033,16 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
> XA_STATE(xas, &mapping->i_pages, start);
> struct page *page;
>
> rcu_read_lock();
> while ((page = find_get_entry(&xas, end, XA_PRESENT))) {
> if (!xa_is_value(page)) {
> if (page->index < start)
> goto put;
> - VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
> if (page->index + thp_nr_pages(page) - 1 > end)
> goto put;
> if (!trylock_page(page))
> goto put;
> if (page->mapping != mapping || PageWriteback(page))
> goto unlock;
> VM_BUG_ON_PAGE(!thp_contains(page, xas.xa_index),
> page);
> --
> 2.30.2
