On Wed, Jul 2, 2014 at 12:29 PM, Hugh Dickins <hughd@xxxxxxxxxx> wrote: > On Wed, 2 Jul 2014, Liu Ping Fan wrote: > >> There is race between do_swap_page() and swap_writepage(), if >> do_swap_page() had deleted a page from swap cache, there is no need >> to write it. So changing the ret of try_to_free_swap() to make >> swap_writepage() aware of this scene. > > Is this an inefficiency that you have noticed in practice, > or something that you think you spotted by code inspection? > just spotted by code inspection. > I don't see how it can happen: all the places I know of that call > swap_writepage() (including vmscan.c's mapping->a_ops->writepage) > have not dropped page lock since setting or checking PageSwapCache, > and page lock is supposed to protect against deletion from swap cache. > > Has that changed? Please point out where. > No, my fault. Thanks for making me aware of this. >> >> Signed-off-by: Liu Ping Fan <pingfank@xxxxxxxxxxxxxxxxxx> >> --- >> mm/swapfile.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/mm/swapfile.c b/mm/swapfile.c >> index 4c524f7..9d80671 100644 >> --- a/mm/swapfile.c >> +++ b/mm/swapfile.c >> @@ -910,7 +910,7 @@ int try_to_free_swap(struct page *page) >> VM_BUG_ON_PAGE(!PageLocked(page), page); >> >> if (!PageSwapCache(page)) >> - return 0; >> + return -1; > > Previously it returned either 0 or 1, which is what __try_to_reclaim_swap() > says it returns; so better to stick to 0 or 1, unless you have good reason > to add a distinct value. > > It's true that by the time __try_to_reclaim_swap() has got the page lock, > the page might have been removed from swap cache, and we could then treat > that as swap_was_freed (even though it was not freed by the caller). > > But it's a very narrow window, and no great advantage to do so: > I don't think it's worth changing try_to_free_swap() semantics for, > but you could persuade us. > Got it, and it is meaningless to do that. Thanks, Fan -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>