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Tue, 3 Mar 2026 03:01:43 +0000 (UTC) Received: from localhost (unknown [10.72.112.42]) by mx-prod-int-06.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id EA48F1800669; Tue, 3 Mar 2026 03:01:41 +0000 (UTC) Date: Tue, 3 Mar 2026 11:01:37 +0800 From: Baoquan He To: Nhat Pham Cc: linux-mm@kvack.org, akpm@linux-foundation.org, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, baohua@kernel.org, youngjun.park@lge.com Subject: Re: [PATCH 2/3] mm/swap: use swap_ops to register swap device's methods Message-ID: References: <20260302104016.163542-1-bhe@redhat.com> <20260302104016.163542-3-bhe@redhat.com> MIME-Version: 1.0 In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.93 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 2ZTUkihNDDp3rn04_oV6h_eWPuzCOD2IXumTm6eJxNk_1772506904 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 834FC120004 X-Stat-Signature: jjq84uys9aiqc7y65hogfdqop79k85qx X-Rspam-User: X-HE-Tag: 1772506911-1206 X-HE-Meta: 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 AKhO7QRE gEog/LNBqmsgAVAXhh0dKyLzWZ3dQXkxQPvRdnBGVfY0MOk+MV5YSslSWsWWDn5y5B+YkrnhlsBe39S80qx39qAkZ4xUiZCAgGxnj986j1xXsbxTGFsJTIbtAy2TRwnSGRMoVrZL19qh/t3+3M/KVBfgHz4qdJAeBaY0lBAkjnMofrzxM3DMPOsbXtbgW2o7M1gSxKQHw4T0tK0Ilrpd6Abjy6b1lY4cSm5iwvR/hX9NTOP1hNwymLFPHFzmqygzHpn41x5y7OPKeGtCrtBW+kHRLj913+JLCOkjxGbAFmHv52rVxz2d7cXH+LMO6uAWFTmV/VU9mDs2ropddisXWq91hMNEssGssluZrVxSElMNIbXTZKcTsTJ8TUt2nKlUv2lh1MEiBOsGohbQsLGi1OWC2u3I1dV7ibZdnot5D+cahCTfz5eo5ldtMJiWm0FQL41mOWzpIWUB1CK4= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 03/02/26 at 01:21pm, Nhat Pham wrote: > On Mon, Mar 2, 2026 at 2:40 AM Baoquan He wrote: ...snip... > > diff --git a/mm/swapfile.c b/mm/swapfile.c > > index 915bc93964db..af498f9af328 100644 > > --- a/mm/swapfile.c > > +++ b/mm/swapfile.c > > @@ -3625,6 +3625,8 @@ SYSCALL_DEFINE2(swapon, const char __user *, specialfile, int, swap_flags) > > /* Sets SWP_WRITEOK, resurrect the percpu ref, expose the swap device */ > > enable_swap_info(si); > > > > + probe_swap_fs(si); > > + > > pr_info("Adding %uk swap on %s. Priority:%d extents:%d across:%lluk %s%s%s%s\n", > > K(si->pages), name->name, si->prio, nr_extents, > > K((unsigned long long)span), > > diff --git a/mm/zswap.c b/mm/zswap.c > > index a399f7a10830..7ce906249c7a 100644 > > --- a/mm/zswap.c > > +++ b/mm/zswap.c > > @@ -1055,7 +1055,8 @@ static int zswap_writeback_entry(struct zswap_entry *entry, > > folio_set_reclaim(folio); > > > > /* start writeback */ > > - __swap_writepage(folio, NULL); > > + if (si->ops && si->ops->write_folio) > > + si->ops->write_folio(si, folio, NULL); > > This one seems really weird to me TBH... > > What's the point of a zswap writeback to a backend without writeback > operation...? We're loading the coldest zswap objects back into > memory...? > > More generally, do you anticipate a swap backend with no write_folio? > I can't imagine such backend. > > That said, if they exist, they should be exempted from zswap > writeback. At the very least, move this check to the beginning of > zswap_writeback_entry(), right after swap device pinning > (get_swap_device()): > > 1. If swap device has no writeback op, just bail. Don't even bother > allocate swap cache, decompression, etc. > > 2. If you anticipate that this case will never happen, add a warn. Good point. Currently, there's no case where a swap backend without write_folio. While in the future there could be, assume you have noticed the swap tier patchset introduced by Youngjun. Here, the si->ops checking is purely defensive programming. I can add a warn at the beginnin of zswap_writeback_entry(). If there's any case involving no read/write_folio, adaption can be made at that time.