Re: [PATCH] filemap: don't unlock null page in FGP_FOR_MMAP case

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 12 Mar 2019 16:17:42 -0400 Josef Bacik <josef@toxicpanda.com> wrote:

> We noticed a panic happening in production with the filemap fault pages
> because we were unlocking a NULL page.  If add_to_page_cache() fails
> then we'll have a NULL page, so fix this check to only unlock if we
> have a valid page.
> 
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>  mm/filemap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index cace3eb8069f..2815cb79a246 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1663,7 +1663,7 @@ struct page *pagecache_get_page(struct address_space *mapping, pgoff_t offset,
>  		 * add_to_page_cache_lru locks the page, and for mmap we expect
>  		 * an unlocked page.
>  		 */
> -		if (fgp_flags & FGP_FOR_MMAP)
> +		if (page && (fgp_flags & FGP_FOR_MMAP))
>  			unlock_page(page);
>  	}
>  

Fixes "filemap: kill page_cache_read usage in filemap_fault".

This patch series:

filemap-kill-page_cache_read-usage-in-filemap_fault.patch
filemap-kill-page_cache_read-usage-in-filemap_fault-fix.patch
filemap-kill-page_cache_read-usage-in-filemap_fault-fix-2.patch
filemap-pass-vm_fault-to-the-mmap-ra-helpers.patch
filemap-drop-the-mmap_sem-for-all-blocking-operations.patch
filemap-drop-the-mmap_sem-for-all-blocking-operations-v6.patch
filemap-drop-the-mmap_sem-for-all-blocking-operations-fix.patch
filemap-drop-the-mmap_sem-for-all-blocking-operations-checkpatch-fixes.patch

has been stuck since December.  I have a note here that syzbot reported
a use-after-free.  What's the situation with that?

I also have a cryptic note that
filemap-drop-the-mmap_sem-for-all-blocking-operations-v6.patch is
"still fishy".  I'm not sure what I meant by the latter - the (small
amount of) review seems to be OK.  Do you recall what issues there
might have been and the status of those?