Re: [PATCH] mm: fix the page_swap_info BUG_ON check

[Santosh Shilimkar]

On 9/12/2016 2:28 PM, Andrew Morton wrote:
> On Fri,  9 Sep 2016 15:38:38 -0700 Santosh Shilimkar <santosh.shilimkar@xxxxxxxxxx> wrote:
>
> > 'commit 62c230bc1790 ("mm: add support for a filesystem to activate swap
> > files and use direct_IO for writing swap pages")' replaced swap_aops
> > dirty hook from __set_page_dirty_no_writeback() to swap_set_page_dirty().
> > As such for normal cases without these special SWP flags
> > code path falls back to __set_page_dirty_no_writeback()
> > so behaviour is expected to be same as before.
> >
> > But swap_set_page_dirty() makes use of helper page_swap_info() to
> > get sis(swap_info_struct) to check for the flags like SWP_FILE,
> > SWP_BLKDEV etc as desired for those features. This helper has
> > BUG_ON(!PageSwapCache(page)) which is racy and safe only for
> > set_page_dirty_lock() path. For set_page_dirty() path which is
> > often needed for cases to be called from irq context, kswapd()
> > can togele the flag behind the back while the call is
> > getting executed when system is low on memory and heavy
> > swapping is ongoing.
> >
> > This ends up with undesired kernel panic. Patch just moves
> > the check outside the helper to its users appropriately
> > to fix kernel panic for the described path. Couple
> > of users of helpers already take care of SwapCache
> > condition so I skipped them.
> >
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Cc: Joe Perches <joe@xxxxxxxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> > Cc: Rik van Riel <riel@xxxxxxxxxx>
> > Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> > Cc: Jens Axboe <axboe@xxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> > Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
>
> I'll add
Thanks Andrew !!

> Cc: <stable@xxxxxxxxxxxxxxx>	[4.7.x]
>
> > --- a/mm/page_io.c
> > +++ b/mm/page_io.c
> > @@ -264,6 +264,7 @@ int __swap_writepage(struct page *page, struct writeback_control *wbc,
> >  	int ret;
> >  	struct swap_info_struct *sis = page_swap_info(page);
> >
> > +	BUG_ON(!PageSwapCache(page));
> >  	if (sis->flags & SWP_FILE) {
> >  		struct kiocb kiocb;
> >  		struct file *swap_file = sis->swap_file;
> > @@ -337,6 +338,7 @@ int swap_readpage(struct page *page)
> >  	int ret = 0;
> >  	struct swap_info_struct *sis = page_swap_info(page);
> >
> > +	BUG_ON(!PageSwapCache(page));
> >  	VM_BUG_ON_PAGE(!PageLocked(page), page);
> >  	VM_BUG_ON_PAGE(PageUptodate(page), page);
> >  	if (frontswap_load(page) == 0) {
> > @@ -386,6 +388,7 @@ int swap_set_page_dirty(struct page *page)
> >
> >  	if (sis->flags & SWP_FILE) {
> >  		struct address_space *mapping = sis->swap_file->f_mapping;
> > +		BUG_ON(!PageSwapCache(page));
> >  		return mapping->a_ops->set_page_dirty(page);
> >  	} else {
> >  		return __set_page_dirty_no_writeback(page);
>
> I guess this is OK for 4.8 but for later kernels, let's quieten it down
> a bit?
I was in two minds as well about the importance of the check. May be
Mel Gorman can comment better but below change would good to me. I
don't see taking down entire system for otherwise healthy system.

> From: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Subject: mm/page_io.c: replace some BUG_ON()s with VM_BUG_ON_PAGE()
>
> So they are CONFIG_DEBUG_VM-only and more informative.
>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Jens Axboe <axboe@xxxxxx>
> Cc: Joe Perches <joe@xxxxxxxxxxx>
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: Santosh Shilimkar <santosh.shilimkar@xxxxxxxxxx>
> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>
>  mm/page_io.c |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff -puN mm/page_io.c~mm-fix-the-page_swap_info-bug_on-check-fix mm/page_io.c
> --- a/mm/page_io.c~mm-fix-the-page_swap_info-bug_on-check-fix
> +++ a/mm/page_io.c
> @@ -264,7 +264,7 @@ int __swap_writepage(struct page *page,
>  	int ret;
>  	struct swap_info_struct *sis = page_swap_info(page);
>
> -	BUG_ON(!PageSwapCache(page));
> +	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
>  	if (sis->flags & SWP_FILE) {
>  		struct kiocb kiocb;
>  		struct file *swap_file = sis->swap_file;
> @@ -338,7 +338,7 @@ int swap_readpage(struct page *page)
>  	int ret = 0;
>  	struct swap_info_struct *sis = page_swap_info(page);
>
> -	BUG_ON(!PageSwapCache(page));
> +	VM_BUG_ON_PAGE(!PageSwapCache(page), page);
>  	VM_BUG_ON_PAGE(!PageLocked(page), page);
>  	VM_BUG_ON_PAGE(PageUptodate(page), page);
>  	if (frontswap_load(page) == 0) {
> @@ -388,7 +388,8 @@ int swap_set_page_dirty(struct page *pag
>
>  	if (sis->flags & SWP_FILE) {
>  		struct address_space *mapping = sis->swap_file->f_mapping;
> -		BUG_ON(!PageSwapCache(page));
> +
> +		VM_BUG_ON_PAGE(!PageSwapCache(page), page);
>  		return mapping->a_ops->set_page_dirty(page);
>  	} else {
>  		return __set_page_dirty_no_writeback(page);
> diff -puN mm/swapfile.c~mm-fix-the-page_swap_info-bug_on-check-fix mm/swapfile.c
> _