From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (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 3F35130568E for ; Fri, 17 Jul 2026 08:19:12 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784276355; cv=none; b=WwfBZs7rvlaGEIc8P1OIPIHtzXRTOsUEFKrNrRqG/7xZvXhBvVun3m2lhFaBE/ccOsFx14TY4He+J8wa02oRKyy9rGFWz1r3zpSutzWU/i/YJsXIElzRZPA9ejYq/p5g882Hg6YiwpGmr1FqqgZ3jaXa96L+If8+S5yCNaWIRDg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784276355; c=relaxed/simple; bh=BblVODRYR2SQxHjm1UwpI5tro6hwCGJF0sTLQb0opmA=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=KYvVHWar11QyochMANqBiBRfDM+9pBuEjYD4P7Z2vh+tZN1ZgO91Ljn5/s2WHkhskxzTTMuygPmz6+2EdmYEVjX7P1qTXHyY2GisV1nsvLj3DOFWSGruvv/vJB3j7VFUYEGMVLOWhv/OEaNLrXKMVMl7+FGCr1S2Z8fXl1v8YwQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=BDN2kRUQ; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="BDN2kRUQ" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 88A131F00A3A; Fri, 17 Jul 2026 08:19:10 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1784276352; bh=o9QpCCDFZl9Z3Fy1xWnmLKG4AFYlG+VaeyFxu5H2buc=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=BDN2kRUQJ+brggP3mMYAB4f6PCc9YCIge/VE2LqQvGMjTINOgCsP4PCtcNpCCPIwd n1dt0Ca3JUFCwOuSkYTzUSoSgGeMcDgdNHuNLu3z6zl0ENhQ/4cHiyD+uxQQd+IF2e CS6ltDpx4nrKv5LhtvuSr+8aEblgppz6z0axPBeuFJYbdr5nX5FPC6wbWdNAk2MnhS maoIbdOCBiNIWdS3qqPlFhGtP4QftxlH8UMKGG3wgJX6SdSbCpvtllGBYl2ms4EnG3 ClEkQxW+4xMSQDj4dEN6k3GJzT/qArlFPf1q9DLuycwOrz5RWDZBHg6icz5BQ0vAtq VCCgfYMMGrD6A== Date: Fri, 17 Jul 2026 11:19:06 +0300 From: Mike Rapoport To: Michal Clapinski Cc: Pasha Tatashin , Pratyush Yadav , Alexander Graf , Evangelos Petrongonas , kexec@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] kho: align kho_scratch to MAX_ORDER_NR_PAGES pages Message-ID: References: <20260714205111.2497151-1-mclapinski@google.com> 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: <20260714205111.2497151-1-mclapinski@google.com> On Tue, Jul 14, 2026 at 10:51:11PM +0200, Michal Clapinski wrote: > I caught this crash: We are not as strict as x86 folks, but imperative voice is preferred. > BUG: unable to handle page fault for address: ff19164fffff8328 > RIP: 0010:__free_one_page+0x1a1/0x6b0 > Call Trace: > > [] free_one_page+0xaf/0x240 > [] deferred_free_pages+0xa8/0xd0 > [] deferred_init_memmap_chunk+0x10f/0x1b0 > [] padata_mt_helper+0x65/0xa0 > [] process_scheduled_works+0x202/0x410 > [] worker_thread+0x1f9/0x2d0 > [] kthread+0x27d/0x2f0 > [] ? __pfx_worker_thread+0x10/0x10 > [] ? __pfx_kthread+0x10/0x10 > [] ret_from_fork+0x145/0x280 > [] ? __pfx_kthread+0x10/0x10 > [] ret_from_fork_asm+0x1a/0x30 > > > deferred_init_memmap_chunk() works fine without KHO because free > regions will never be neighbors. However, with KHO, free memory I'd change it to something like deferred_init_memmap_chunk() interleaves initialization of struct pages with freeing them. This works fine without KHO ... > will be split into (free && scratch) and (free && !scratch). > KHO scratch is aligned to CMA_MIN_ALIGNMENT_BYTES (which on my setup > is equal to 1 << 9 pages) but buddy can look at the neighborhood of > MAX_ORDER_NR_PAGES pages (which is equal to 1 << 10 pages). Please make it clearer that the values correspond to a particular configuration. E.g. in a configuration where CMA_MIN_ALIGNMENT_PAGES is 512 and MAX_ORDER_NR_PAGES is 1024 > This sometimes crashes when one half of the neighborhood is being > initialized but the other part is yet to be initialized. > > To fix this, let's just align KHO scratch to MAX_ORDER_NR_PAGES pages. > > Fixes: c6073743d0c7 ("kho: make preserved pages compatible with deferred struct page init") > Signed-off-by: Michal Clapinski > --- > Unfortunately, this is very hard to catch, so I don't have a good > reproducer. But I run the code with the fix through extensive testing > and it seems fine. > I think I've never caught it before because enabling HUGETLB is what > causes CMA_MIN_ALIGNMENT_PAGES to be 1 << 9, instead of 1 << 10. > > If this passes review, please cherry-pick it to 7.2 so we don't release > a broken kernel. > --- > kernel/liveupdate/kexec_handover.c | 22 ++++++++++++++++------ > 1 file changed, 16 insertions(+), 6 deletions(-) > > diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c > index 4834a809985a..50928aaa3371 100644 > --- a/kernel/liveupdate/kexec_handover.c > +++ b/kernel/liveupdate/kexec_handover.c > @@ -38,6 +38,16 @@ > #include "../kexec_internal.h" > #include "kexec_handover_internal.h" > > +/* > + * This is the minimal alignment required by deferred struct page init. > + * deferred_init_memmap_chunk frees memory to the buddy allocator, which looks > + * at the neighboring pages (up to MAX_PAGE_ORDER) to merge them. > + * If KHO scratch is not aligned to that value, buddy can access uninitialized > + * struct pages, which can cause a crash. > + */ > +#define SCRATCH_ALIGNMENT_BYTES (1 << (PAGE_SHIFT + MAX_PAGE_ORDER)) I believe PAGE_SIZE * MAX_ORDER_NR_PAGES looks better > +static_assert(SCRATCH_ALIGNMENT_BYTES >= CMA_MIN_ALIGNMENT_BYTES); > + > /* The magic token for preserved pages */ > #define KHO_PAGE_MAGIC 0x4b484f50U /* ASCII for 'KHOP' */ > > @@ -640,8 +650,8 @@ static void __init scratch_size_update(void) > * Scratch areas are released as MIGRATE_CMA. Round them up to the right > * size. > */ > - scratch_size_lowmem = round_up(scratch_size_lowmem, CMA_MIN_ALIGNMENT_BYTES); > - scratch_size_global = round_up(scratch_size_global, CMA_MIN_ALIGNMENT_BYTES); > + scratch_size_lowmem = round_up(scratch_size_lowmem, SCRATCH_ALIGNMENT_BYTES); > + scratch_size_global = round_up(scratch_size_global, SCRATCH_ALIGNMENT_BYTES); > } > > static phys_addr_t __init scratch_size_node(int nid) > @@ -656,7 +666,7 @@ static phys_addr_t __init scratch_size_node(int nid) > size = scratch_size_pernode; > } > > - return round_up(size, CMA_MIN_ALIGNMENT_BYTES); > + return round_up(size, SCRATCH_ALIGNMENT_BYTES); > } > > /** > @@ -692,7 +702,7 @@ static void __init kho_reserve_scratch(void) > * next kernel > */ > size = scratch_size_lowmem; > - addr = memblock_phys_alloc_range(size, CMA_MIN_ALIGNMENT_BYTES, 0, > + addr = memblock_phys_alloc_range(size, SCRATCH_ALIGNMENT_BYTES, 0, > ARCH_LOW_ADDRESS_LIMIT); > if (!addr) { > pr_err("Failed to reserve lowmem scratch buffer\n"); > @@ -705,7 +715,7 @@ static void __init kho_reserve_scratch(void) > > /* reserve large contiguous area for allocations without nid */ > size = scratch_size_global; > - addr = memblock_phys_alloc(size, CMA_MIN_ALIGNMENT_BYTES); > + addr = memblock_phys_alloc(size, SCRATCH_ALIGNMENT_BYTES); > if (!addr) { > pr_err("Failed to reserve global scratch buffer\n"); > goto err_free_scratch_areas; > @@ -721,7 +731,7 @@ static void __init kho_reserve_scratch(void) > */ > for_each_node_state(nid, N_MEMORY) { > size = scratch_size_node(nid); > - addr = memblock_alloc_range_nid(size, CMA_MIN_ALIGNMENT_BYTES, > + addr = memblock_alloc_range_nid(size, SCRATCH_ALIGNMENT_BYTES, > 0, MEMBLOCK_ALLOC_ACCESSIBLE, > nid, true); > if (!addr) { > -- > 2.55.0.141.g00534a21ce-goog > -- Sincerely yours, Mike.