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

[Santosh Shilimkar <santosh.shilimkar@oracle.com>]

On 9/12/2016 2:28 PM, Andrew Morton wrote:
> On Fri,  9 Sep 2016 15:38:38 -0700 Santosh Shilimkar <santosh.shilimkar@oracle.com> 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@suse.de>
>> Cc: Joe Perches <joe@perches.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
>> Cc: Rik van Riel <riel@redhat.com>
>> Cc: David S. Miller <davem@davemloft.net>
>> Cc: Jens Axboe <axboe@fb.com>
>> Cc: Michal Hocko <mhocko@suse.com>
>> Cc: Hugh Dickins <hughd@google.com>
>> Cc: Al Viro <viro@zeniv.linux.org.uk>
>
> I'll add
>
Thanks Andrew !!

> Cc: <stable@vger.kernel.org>	[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@linux-foundation.org>
> 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@zeniv.linux.org.uk>
> Cc: David S. Miller <davem@davemloft.net>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Jens Axboe <axboe@fb.com>
> Cc: Joe Perches <joe@perches.com>
> Cc: Mel Gorman <mgorman@suse.de>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: Santosh Shilimkar <santosh.shilimkar@oracle.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
>
>  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
> _
>

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