From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 AB4FA2F4A for ; Mon, 9 Jun 2025 19:36:55 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749497815; cv=none; b=ABoSphvbQHL22i3fn3nBwLOkK1gU3/O/U874jdvjC1bfjRvLx7G4lNQ2GqPxnFupSgHuTa5thqLj5MLtocMv+1w+hameclROQlHBFkpFS1tqINizLvgc5faVej/pUJa9ZNek6xwFFg1vaQ5Ng5UfLrcbWlhhg+jkz/NcsfI7hUI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1749497815; c=relaxed/simple; bh=vW+WldKDhud989X0lq2nBhkbsCVfeu8ONtXch3tf4WM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=gwKz+tx67lzhcbLcnxrrEoGZlv7GelvHXjRrb7P2MKtsvpxJDpdtIIqYFfGRQvES9UrDZv1t6eD+QSvwAczHtTV+SKPQyD8ZYwj3HuayrOf74Q0wXEwfdxMA3vEluo/L4dVPEyGwYrsDi1FBe+q1G3ZtlpiUb4IpsWDly4s/D3U= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=iTiHgMO/; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="iTiHgMO/" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 173EDC4CEEB; Mon, 9 Jun 2025 19:36:51 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1749497815; bh=vW+WldKDhud989X0lq2nBhkbsCVfeu8ONtXch3tf4WM=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=iTiHgMO/PDzPA6rhsUoTDEqFeIfn3dQm5gBIi6RqgEFXsc2RLYEoRDgO3Gf74rhhw RijszniT+15GwHGFuCKgN2a2MVmAT1jHW8jrUAjIbDCujRScKGx3vGedA6MZyqO/Z1 4sPATzQ5P71ligD6mbDLbrTl1oo5iGmyXoHuwpDjWjRJqwZkQh9Uhrbp+lPLbKdPxf HJLMsmciaTvQnCmqQMpIw0df61P8kT/zQrci584Mfvrw0w3Yx1V+5uRRERXgdryI9S EHbGB5/zlNc0SUF2wlfEgIvT9S0Cu7qEs4qc9DeKUkKTlHfOaSkTfCv/l+8vNZhyCm aXFhNAP+Qoguw== Date: Mon, 9 Jun 2025 22:36:48 +0300 From: Mike Rapoport To: Pratyush Yadav Cc: Alexander Graf , Changyuan Lyu , Pasha Tatashin , Andrew Morton , Baoquan He , kexec@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [PATCH] kho: initialize tail pages for higher order folios properly Message-ID: References: <20250605171143.76963-1-pratyush@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Hi Pratyush, On Fri, Jun 06, 2025 at 06:23:06PM +0200, Pratyush Yadav wrote: > Hi Mike, > > On Fri, Jun 06 2025, Mike Rapoport wrote: > > > On Thu, Jun 05, 2025 at 07:11:41PM +0200, Pratyush Yadav wrote: > >> From: Pratyush Yadav > >> > >> --- a/kernel/kexec_handover.c > >> +++ b/kernel/kexec_handover.c > >> @@ -157,11 +157,21 @@ static int __kho_preserve_order(struct kho_mem_track *track, unsigned long pfn, > >> } > >> > >> /* almost as free_reserved_page(), just don't free the page */ > >> -static void kho_restore_page(struct page *page) > >> +static void kho_restore_page(struct page *page, unsigned int order) > >> { > >> - ClearPageReserved(page); > > > > So now we don't clear PG_Reserved even on order-0 pages? ;-) > > We don't need to. As I mentioned in the commit message as well, > PG_Reserved is never set for KHO pages since they are reserved with > MEMBLOCK_RSRV_NOINIT, so memmap_init_reserved_pages() skips over them. You are right, I missed it. > That said, while reading through some of the code, I noticed another > bug: because KHO reserves the preserved pages as NOINIT, with > CONFIG_DEFERRED_STRUCT_PAGE_INIT == n, all the pages get initialized > when memmap_init_range() is called from setup_arch (paging_init() on > x86). This happens before kho_memory_init(), so the KHO-preserved pages > are not marked as reserved to memblock yet. > > With deferred page init, some pages might not get initialized early, and > get initialized after kho_memory_init(), by which time the KHO-preserved > pages are marked as reserved. So, deferred_init_maxorder() will skip > over those pages and leave them uninitialized. > > So we need to either also call init_deferred_page(), or remove the > memblock_reserved_mark_noinit() call in deserialize_bitmap(). And TBH, I > am not sure why KHO pages even need to be marked noinit in the first > place. Probably the only benefit would be if a large chunk of memory is > KHO-preserved, the pages can be initialized later on-demand, reducing > bootup time a bit. One benefit is performance indeed, because in not deferred case the initialization of reserved pages in memmap_init_reserved_pages() is really excessive. But more importantly, if we remove memblock_reserved_mark_noinit(), with CONFIG_DEFERRED_STRUCT_PAGE_INIT we'd loose page->private because the struct page will be cleared after kho_mem_deserialize(). > What do you think? Should we drop noinit or call init_deferred_page()? > FWIW, my preference is to drop noinit, since init_deferred_page() is > __meminit and we would have to make sure it doesn't go away after boot. We can't drop noinit and calling init_deferred_page() after boot just won't work because it uses memblock to find the page's node and memblock is gone after init. The simplest short-term solution is to disable KHO when CONFIG_DEFERRED_STRUCT_PAGE_INIT is set and then find an efficient way to make it all work together. -- Sincerely yours, Mike.