From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pl1-f199.google.com (mail-pl1-f199.google.com [209.85.214.199]) by kanga.kvack.org (Postfix) with ESMTP id 1B5898E0001 for ; Wed, 26 Sep 2018 08:56:07 -0400 (EDT) Received: by mail-pl1-f199.google.com with SMTP id g8-v6so481292plm.16 for ; Wed, 26 Sep 2018 05:56:07 -0700 (PDT) Received: from mga11.intel.com (mga11.intel.com. [192.55.52.93]) by mx.google.com with ESMTPS id a7-v6si3239785plp.9.2018.09.26.05.56.05 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Wed, 26 Sep 2018 05:56:05 -0700 (PDT) From: "Huang\, Ying" Subject: Re: [PATCH -V5 RESEND 03/21] swap: Support PMD swap mapping in swap_duplicate() References: <20180925071348.31458-1-ying.huang@intel.com> <20180925071348.31458-4-ying.huang@intel.com> <20180925191953.4ped5ki7u3ymafmd@ca-dmjordan1.us.oracle.com> Date: Wed, 26 Sep 2018 20:55:59 +0800 In-Reply-To: <20180925191953.4ped5ki7u3ymafmd@ca-dmjordan1.us.oracle.com> (Daniel Jordan's message of "Tue, 25 Sep 2018 12:19:53 -0700") Message-ID: <874lecifj4.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=ascii Sender: owner-linux-mm@kvack.org List-ID: To: Daniel Jordan Cc: Andrew Morton , linux-mm@kvack.org, linux-kernel@vger.kernel.org, "Kirill A. Shutemov" , Andrea Arcangeli , Michal Hocko , Johannes Weiner , Shaohua Li , Hugh Dickins , Minchan Kim , Rik van Riel , Dave Hansen , Naoya Horiguchi , Zi Yan Daniel Jordan writes: > On Tue, Sep 25, 2018 at 03:13:30PM +0800, Huang Ying wrote: >> @@ -3487,35 +3521,66 @@ static int __swap_duplicate_locked(struct swap_info_struct *p, >> } >> >> /* >> - * Verify that a swap entry is valid and increment its swap map count. >> + * Verify that the swap entries from *entry is valid and increment their >> + * PMD/PTE swap mapping count. >> * >> * Returns error code in following case. >> * - success -> 0 >> * - swp_entry is invalid -> EINVAL >> - * - swp_entry is migration entry -> EINVAL > > I'm assuming it wasn't possible to hit this error before this patch, and you're > just removing it now since you're in the area? Yes. >> * - swap-cache reference is requested but there is already one. -> EEXIST >> * - swap-cache reference is requested but the entry is not used. -> ENOENT >> * - swap-mapped reference requested but needs continued swap count. -> ENOMEM >> + * - the huge swap cluster has been split. -> ENOTDIR > > Strangely intuitive choice of error code :) Thanks! It doesn't match the error exactly, but I have no better choice now. Matthew Wilcox have suggested to use an swap specific enum instead. I think that is good in general, but we need only one extra error code, and we need to change the interface of several swap functions. So I think that should be in a separate patchset if necessary. >> /* >> * Increase reference count of swap entry by 1. >> - * Returns 0 for success, or -ENOMEM if a swap_count_continuation is required >> - * but could not be atomically allocated. Returns 0, just as if it succeeded, >> - * if __swap_duplicate() fails for another reason (-EINVAL or -ENOENT), which >> - * might occur if a page table entry has got corrupted. >> + * >> + * Return error code in following case. >> + * - success -> 0 >> + * - swap_count_continuation is required but could not be atomically allocated. >> + * *entry is used to return swap entry to call add_swap_count_continuation(). >> + * -> ENOMEM >> + * - otherwise same as __swap_duplicate() >> */ >> -int swap_duplicate(swp_entry_t entry) >> +int swap_duplicate(swp_entry_t *entry, int entry_size) >> { >> int err = 0; >> >> - while (!err && __swap_duplicate(entry, 1) == -ENOMEM) >> - err = add_swap_count_continuation(entry, GFP_ATOMIC); >> + while (!err && >> + (err = __swap_duplicate(entry, entry_size, 1)) == -ENOMEM) >> + err = add_swap_count_continuation(*entry, GFP_ATOMIC); >> return err; > > Now we're returning any error we get from __swap_duplicate, apparently to > accommodate ENOTDIR later in the series, which is a change from the behavior > introduced in 570a335b8e22 ("swap_info: swap count continuations"). This might > belong in a separate patch given its potential for side effects. I have checked all the calls of the function and found there will be no bad effect. Do you have any side effect? > Although, I don't understand why 570a335b8e22 ignored errors other than -ENOMEM > when both swap_duplicate callers _seem_ from a quick read to be able to respond > gracefully to any error. Before 570a335b8e22, all errors are ignored in swap_duplicate() (its type is void). If my understanding were correct, all errors except -ENOMEM are impossible before changes in this patchset. So they are ignored. Best Regards, Huang, Ying