From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-183.mta0.migadu.com (out-183.mta0.migadu.com [91.218.175.183]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 16BBA38CFF8 for ; Sun, 2 Aug 2026 12:05:37 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=91.218.175.183 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785672340; cv=none; b=Pb2PMMQqA1602QplqYWJXzB2HWtDGsl0LlRH/iuISX7sgwNSCzxeA1maLndI0iC4N8XaqZcDa4hzVuseqIj9YhG9Lp3+Pe2cZkUIojtDsSGCrbvr5ikNpVC6x6l7mkbcBCOn7wI4ysE2R/xbMY4Dctm7AshiMGFw6viEt+BxwVU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785672340; c=relaxed/simple; bh=NZfSsb+bIfbJjoZFd5FVsweh1TVaLosIEHyi+p8AQKk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=fTXuVb6ex97jappDIjlBJmOsJ8VSyfGEi2akFe+UV9lzu67RJJGilCjNSa4QieJkwVRb4xzM7n9Uz36JVJOL/eNKJMz5zSzQHtrWf9Ooibm/MY7+tyZjjWvk7l/O/SLVjo1ZOSgw8Nin9kjATynGzlmxlcEwPhbXELtgWKsCVdo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=Wanec2Em; arc=none smtp.client-ip=91.218.175.183 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="Wanec2Em" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1785672335; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=EhAjnceW+YstcNmd0dRQVH29iTq9U+yYRGJxK6BlBAM=; b=Wanec2EmC1suOkIOhufH0i5W+qegpKDEOf1S6elAJZUsDOe7I3l/Jxfhiizvtk+NbUf0/d T7GorbfKAxMcpTMx4+H8WWPHE8T53gAslyE4ygwKjbyoBylRkWxt934z6Vm1Fr7Cy8PssX Hj1HfGcZUVWDjfu1IEhT5Rwuc7cqWUg= Date: Sun, 2 Aug 2026 13:05:21 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [PATCH RFC 01/14] mm/zsmalloc: replace PG_private with pointer comparison To: Zi Yan Cc: David Hildenbrand , "Matthew Wilcox (Oracle)" , Andrew Morton , Muchun Song , Lorenzo Stoakes , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Baolin Wang , Nico Pache , Ryan Roberts , Dev Jain , Barry Song , Lance Yang , Gregory Price , Ying Huang , Alistair Popple , Johannes Weiner , Qi Zheng , Shakeel Butt , Kairui Song , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Minchan Kim , Sergey Senozhatsky References: <20260731-remove-pg_private-v1-1-142c97ba3562@nvidia.com> <20260801141259.627022-1-usama.arif@linux.dev> Content-Language: en-US X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Usama Arif In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT On 02/08/2026 00:49, Zi Yan wrote: > On Sat Aug 1, 2026 at 10:12 AM EDT, Usama Arif wrote: >> On Fri, 31 Jul 2026 22:13:24 -0400 Zi Yan wrote: >> >>> zsmalloc uses PG_private to indicate first zpdesc in the zspage chain. >>> Replace it with zpdesc->zspage->first_zpdesc == zpdesc. The check, >>> is_first_zpdesc(), is only used in VM_BUG_ON(), so performance impact >>> should be negligible. >>> >>> It prepares for a future commit that remove PG_private. >>> >>> No functional change intended. >>> >>> Assisted-by: Claude:claude-opus-4-8 >>> Assisted-by: Codex:gpt-5 >>> Signed-off-by: Zi Yan >>> To: Minchan Kim >>> To: Sergey Senozhatsky >>> To: Andrew Morton >>> Cc: linux-mm@kvack.org >>> Cc: linux-kernel@vger.kernel.org >>> --- >>> mm/zpdesc.h | 2 +- >>> mm/zsmalloc.c | 15 +++------------ >>> 2 files changed, 4 insertions(+), 13 deletions(-) >>> >> >> create_page_chain() sets zpdesc->zspage = zspage before assigning zspage->first_zpdesc, >> so LGTM. >> >> I think the assertion in get_first_zpdesc() >> should be changed to >> >> VM_BUG_ON_PAGE(first_zpdesc->zspage != zspage, zpdesc_page(first_zpdesc)); >> >> in the current check, we are checking >> >> zspage->first_zpdesc->zspage->first_zpdesc == zspage->first_zpdesc >> >> which is just testing the backpointer. > > Got it. Thank you for pointing this out. > > is_first_zpdesc() is also used in obj_allocated() for the same check and > getting rid of it in get_first_zpdesc() causes inconsistency. > > How about the changes below on top of this patch? is_first_zpdesc() can > be used for both sites and looks cleaner. And VM_BUG_ON_PAGE is replaced > with VM_WARN_ON_ONCE_PAGE. > > > diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c > index e8ef227624efa..f021d2df99404 100644 > --- a/mm/zsmalloc.c > +++ b/mm/zsmalloc.c > @@ -471,9 +471,10 @@ static void record_obj(unsigned long handle, unsigned long obj) > WRITE_ONCE(*(unsigned long *)handle, obj); > } > > -static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc) > +static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc, > + struct zspage *zspage) > { > - return zpdesc->zspage->first_zpdesc == zpdesc; > + return zpdesc->zspage == zspage && zspage->first_zpdesc == zpdesc; > } > > /* Protected by class->lock */ > @@ -491,7 +492,7 @@ static struct zpdesc *get_first_zpdesc(struct zspage *zspage) > { > struct zpdesc *first_zpdesc = zspage->first_zpdesc; > > - VM_BUG_ON_PAGE(!is_first_zpdesc(first_zpdesc), zpdesc_page(first_zpdesc)); > + VM_WARN_ON_ONCE_PAGE(is_first_zpdesc(first_zpdesc, zspage), zpdesc_page(first_zpdesc)); mhmm do you mean VM_WARN_ON_ONCE_PAGE(!is_first_zpdesc(first_zpdesc, zspage), zpdesc_page(first_zpdesc)); here? ! is missing I think? > return first_zpdesc; > } > > @@ -828,7 +829,7 @@ static inline bool obj_allocated(struct zpdesc *zpdesc, void *obj, > struct zspage *zspage = get_zspage(zpdesc); > > if (unlikely(ZsHugePage(zspage))) { > - VM_BUG_ON_PAGE(!is_first_zpdesc(zpdesc), zpdesc_page(zpdesc)); > + VM_WARN_ON_ONCE_PAGE(is_first_zpdesc(zpdesc, zspage), zpdesc_page(zpdesc)); same here? ! is missing? > handle = zpdesc->handle; > } else > handle = *(unsigned long *)obj; > >> >> With the above VM_BUG_ON check change, please feel free to add: >> >> Acked-by: Usama Arif >> >> >>> diff --git a/mm/zpdesc.h b/mm/zpdesc.h >>> index b8258dc78548d..4fd81c2e80769 100644 >>> --- a/mm/zpdesc.h >>> +++ b/mm/zpdesc.h >>> @@ -26,8 +26,8 @@ >>> * with memcg_data. >>> * >>> * Page flags used: >>> - * * PG_private identifies the first component page. >>> * * PG_locked is used by page migration code. >>> + * The first component page has zpdesc->zspage->first_zpdesc == zpdesc >>> */ >>> struct zpdesc { >>> unsigned long flags; >>> diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c >>> index 8204b76f78308..e8ef227624efa 100644 >>> --- a/mm/zsmalloc.c >>> +++ b/mm/zsmalloc.c >>> @@ -290,11 +290,6 @@ struct zs_pool { >>> atomic_t compaction_in_progress; >>> }; >>> >>> -static inline void zpdesc_set_first(struct zpdesc *zpdesc) >>> -{ >>> - SetPagePrivate(zpdesc_page(zpdesc)); >>> -} >>> - >>> static inline void zpdesc_inc_zone_page_state(struct zpdesc *zpdesc) >>> { >>> inc_zone_page_state(zpdesc_page(zpdesc), NR_ZSPAGES); >>> @@ -478,7 +473,7 @@ static void record_obj(unsigned long handle, unsigned long obj) >>> >>> static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc) >>> { >>> - return PagePrivate(zpdesc_page(zpdesc)); >>> + return zpdesc->zspage->first_zpdesc == zpdesc; >>> } >>> >>> /* Protected by class->lock */ >>> @@ -848,9 +843,6 @@ static inline bool obj_allocated(struct zpdesc *zpdesc, void *obj, >>> >>> static void reset_zpdesc(struct zpdesc *zpdesc) >>> { >>> - struct page *page = zpdesc_page(zpdesc); >>> - >>> - ClearPagePrivate(page); >>> zpdesc->zspage = NULL; >>> zpdesc->next = NULL; >>> /* PageZsmalloc is sticky until the page is freed to the buddy. */ >>> @@ -1001,8 +993,8 @@ static void create_page_chain(struct size_class *class, struct zspage *zspage, >>> * 1. all pages are linked together using zpdesc->next >>> * 2. each sub-page point to zspage using zpdesc->zspage >>> * >>> - * we set PG_private to identify the first zpdesc (i.e. no other zpdesc >>> - * has this flag set). >>> + * The first zpdesc has its zspage->first_zpdesc set to itself, no >>> + * other zpdesc has this set. >>> */ >>> for (i = 0; i < nr_zpdescs; i++) { >>> zpdesc = zpdescs[i]; >>> @@ -1010,7 +1002,6 @@ static void create_page_chain(struct size_class *class, struct zspage *zspage, >>> zpdesc->next = NULL; >>> if (i == 0) { >>> zspage->first_zpdesc = zpdesc; >>> - zpdesc_set_first(zpdesc); >>> if (unlikely(class->objs_per_zspage == 1 && >>> class->pages_per_zspage == 1)) >>> SetZsHugePage(zspage); >>> >>> -- >>> 2.53.0 >>> >>> > > > >