On 9/12/2016 2:28 PM, Andrew Morton wrote:
On Fri, 9 Sep 2016 15:38:38 -0700 Santosh Shilimkar <[email protected]> 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 <[email protected]> Cc: Joe Perches <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Rik van Riel <[email protected]> Cc: David S. Miller <[email protected]> Cc: Jens Axboe <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: Al Viro <[email protected]>I'll add
Thanks Andrew !!
Cc: <[email protected]> [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 <[email protected]> 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 <[email protected]> Cc: David S. Miller <[email protected]> Cc: Hugh Dickins <[email protected]> Cc: Jens Axboe <[email protected]> Cc: Joe Perches <[email protected]> Cc: Mel Gorman <[email protected]> Cc: Michal Hocko <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Rik van Riel <[email protected]> Cc: Santosh Shilimkar <[email protected]> Signed-off-by: Andrew Morton <[email protected]> --- 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 _
