From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id EABC7274B53 for ; Mon, 1 Jun 2026 16:44:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780332262; cv=none; b=BnCa2YhdG2WbbRYviD4t2sDAbgeJtjynsPzMaLJyGaXA+PAxBqBoFhbZcEDTneELSwDDLe19Ek2xAtSdrm/mUfpqE9iHRnhROIN/j4YJgHcbZth5bY7796ktMpC/F/UKM6OKXTvhcfpfAwbvXkro1o+IV2GD3IgOKvXXi7UevbM= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1780332262; c=relaxed/simple; bh=FE7vc9MqdE4fmTSoMt5zTtbVqHqtOgtF0vKRn9+wXLc=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=UgssH2w1HCk7gQ2CFs6AtOwEvkfVRzFgBUv9qG3P2SD7ocoKlerhQzVUqU+GEMsSXAFsxMtvUPsqJ3qFlK7L3jZ41lddsNJW+QduYUTXrycBVyCgQO1HDMfI29T9Xq9C2ow1Z5x0lOSdvcxdX8OI/WLed0JKU1nEKg072QTE5ng= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=c5fqtT1+; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="c5fqtT1+" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 21D5B1BCA; Mon, 1 Jun 2026 09:44:15 -0700 (PDT) Received: from [10.163.164.232] (unknown [10.163.164.232]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 1342F3F632; Mon, 1 Jun 2026 09:44:14 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1780332260; bh=FE7vc9MqdE4fmTSoMt5zTtbVqHqtOgtF0vKRn9+wXLc=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=c5fqtT1+8I8LBLc1bebUxeZUc8VM+5CZobMAANsXoY1Fnzsd568LQELV3S4BAnsaI ZwDK1K1OHtmJPu74ydcV7hNbqZiiZIQsI6lOooKpFBOaBIOEeUdtmBdzFZRpgqzrXT rtdXOKfQ3j16dGPpa6iE3oZMUeSn8HQXxdEl3EaQ= Message-ID: <73762983-d48d-486f-ad5f-78015e6f95ee@arm.com> Date: Mon, 1 Jun 2026 22:14:11 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH mm-hotfixes] mm/huge_memory: use correct flags for device private PMD entry To: Lorenzo Stoakes , Andrew Morton Cc: David Hildenbrand , Zi Yan , Baolin Wang , "Liam R . Howlett" , Nico Pache , Ryan Roberts , Barry Song , Lance Yang , SeongJae Park , Balbir Singh , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260601083044.57132-1-ljs@kernel.org> Content-Language: en-US From: Dev Jain In-Reply-To: <20260601083044.57132-1-ljs@kernel.org> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 01/06/26 2:00 pm, Lorenzo Stoakes wrote: > Commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support > device-private entries") updated set_pmd_migration_entry() to use > pmdp_huge_get_and_clear() in the softleaf case, but made no further > adjustments to the function itself. > > Therefore this function continues to incorrectly use pmd_write(), > pmd_soft_dirty() and pmd_uffd_wp() to determine whether the installed > migration entry should be marked writable, softdirty or uffd-wp > respectively. > > Whilst all are incorrect, the most problematic of these is pmd_write(), as > this can lead to corrupted rmap state. > > On x86-64 _PAGE_SWP_SOFT_DIRTY is aliased to _PAGE_RW. So calling > pmd_write() on a softleaf will return the softdirty state encoded in the > entry, assuming CONFIG_MEM_SOFT_DIRTY was enabled. > > This was observed when running the hmm.hmm_device_private.anon_write_child > selftest: > > 1. The test faults in a range then migrates it such that a device-private > THP range is established. > > 2. The parent then migrates it to a device-private writable PMD entry whose > folio is entirely AnonExclusive with entire_mapcount=1, softdirty set > (accidentally correct write state). > > 3. The parent forks and the PMD entries are set to device-private read only > entries, entire_mapcount=2, softdirty still set. > > 4. [BUG] The child writes to the range then migrates to RAM - intending to > install non-writable migration entries - but replacing parent and child > PMD mappings with WRITABLE entries due to misinterpreting the softdirty > bit. > > 5. In remove_migration_pmd(), if !softleaf_is_migration_read(entry) we > set the RMAP_EXCLUSIVE flag when calling folio_add_anon_rmap_pmd() for > both parent and child, which are therefore AnonExclusive. > > 6. [SPLAT] Child sets migrated folio entire_mapcount=1, parent sets > entire_mapcount=2 and we end up with an AnonExclusive folio with > entire_mapcount=2! Assert fires in __folio_add_anon_rmap(): > > VM_WARN_ON_FOLIO(folio_test_large(folio) && > folio_entire_mapcount(folio) > 1 && > PageAnonExclusive(cur_page), folio) > > This patch fixes the issue by correctly referencing the softleaf entry > fields for writable, softdirty and uffd-wp in set_pmd_migration_entry(). > > It also only updates A/D flags if the entry is present as these are > otherwise not meaningful for a softleaf entry. > > This patch also flips the if (!present) { ... } else { ... } logic in > set_pmd_migration_entry() so it is easier to understand, and adds some > comments to make things clearer. > > I was able to bisect this to commit 775465fd26a3 ("lib/test_hmm: add zone > device private THP test infrastructure") which first exposes this bug as it > was the commit that permitted test_hmm to generate the test. > > However commit 65edfda6f3f2 ("mm/rmap: extend rmap and migration support > device-private entries") is the commit that actually enabled this > behaviour. > > Fixes: 65edfda6f3f2 ("mm/rmap: extend rmap and migration support device-private entries") > Cc: stable@vger.kernel.org > Signed-off-by: Lorenzo Stoakes > --- Had a look again and can't find anything so: Reviewed-by: Dev Jain > mm/huge_memory.c | 45 +++++++++++++++++++++++++++++++++------------ > 1 file changed, 33 insertions(+), 12 deletions(-) > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index bf9b480bb3b0..79463c709c98 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -4982,7 +4982,7 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw, > struct vm_area_struct *vma = pvmw->vma; > struct mm_struct *mm = vma->vm_mm; > unsigned long address = pvmw->address; > - bool anon_exclusive; > + bool anon_exclusive, present, writable, softdirty, uffd_wp; > pmd_t pmdval; > swp_entry_t entry; > pmd_t pmdswp; > @@ -4990,12 +4990,26 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw, > if (!(pvmw->pmd && !pvmw->pte)) > return 0; > > - flush_cache_range(vma, address, address + HPAGE_PMD_SIZE); > - if (unlikely(!pmd_present(*pvmw->pmd))) > - pmdval = pmdp_huge_get_and_clear(vma->vm_mm, address, pvmw->pmd); > - else > + present = pmd_present(*pvmw->pmd); > + if (likely(present)) { > + flush_cache_range(vma, address, address + HPAGE_PMD_SIZE); > + > pmdval = pmdp_invalidate(vma, address, pvmw->pmd); > > + writable = pmd_write(pmdval); > + softdirty = pmd_soft_dirty(pmdval); > + uffd_wp = pmd_uffd_wp(pmdval); > + } else { > + softleaf_t old_entry; > + > + pmdval = pmdp_huge_get_and_clear(vma->vm_mm, address, pvmw->pmd); > + old_entry = softleaf_from_pmd(pmdval); > + > + writable = softleaf_is_device_private_write(old_entry); > + softdirty = pmd_swp_soft_dirty(pmdval); > + uffd_wp = pmd_swp_uffd_wp(pmdval); > + } > + > /* See folio_try_share_anon_rmap_pmd(): invalidate PMD first. */ > anon_exclusive = folio_test_anon(folio) && PageAnonExclusive(page); > if (anon_exclusive && folio_try_share_anon_rmap_pmd(folio, page)) { > @@ -5003,24 +5017,31 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw, > return -EBUSY; > } > > - if (pmd_dirty(pmdval)) > - folio_mark_dirty(folio); > - if (pmd_write(pmdval)) > + /* Determine type of migration entry. */ > + if (writable) > entry = make_writable_migration_entry(page_to_pfn(page)); > else if (anon_exclusive) > entry = make_readable_exclusive_migration_entry(page_to_pfn(page)); > else > entry = make_readable_migration_entry(page_to_pfn(page)); > - if (pmd_young(pmdval)) > + > + /* Set A/D bits as necessary. */ > + if (present && pmd_young(pmdval)) > entry = make_migration_entry_young(entry); > - if (pmd_dirty(pmdval)) > + if (present && pmd_dirty(pmdval)) { > + folio_mark_dirty(folio); > entry = make_migration_entry_dirty(entry); > + } > + > + /* Set PMD. */ > pmdswp = swp_entry_to_pmd(entry); > - if (pmd_soft_dirty(pmdval)) > + if (softdirty) > pmdswp = pmd_swp_mksoft_dirty(pmdswp); > - if (pmd_uffd_wp(pmdval)) > + if (uffd_wp) > pmdswp = pmd_swp_mkuffd_wp(pmdswp); > set_pmd_at(mm, address, pvmw->pmd, pmdswp); > + > + /* Migration entry installed: cleanup rmap, folio. */ > folio_remove_rmap_pmd(folio, page, vma); > folio_put(folio); > trace_set_migration_pmd(address, pmd_val(pmdswp)); > -- > 2.54.0