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[2003:cb:c703:d00:ca74:d9ea:11e0:dfb]) by smtp.gmail.com with ESMTPSA id x9-20020a5d4449000000b002e40d124460sm15350496wrr.97.2023.04.05.08.51.10 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 05 Apr 2023 08:51:10 -0700 (PDT) Message-ID: <062cd239-179b-d146-8f5d-735e672b3237@redhat.com> Date: Wed, 5 Apr 2023 17:51:10 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.9.1 Subject: Re: [PATCH v1 1/2] mm/userfaultfd: fix uffd-wp handling for THP migration entries Content-Language: en-US To: Peter Xu Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, Andrew Morton , Muhammad Usama Anjum , stable@vger.kernel.org References: <20230405142535.493854-1-david@redhat.com> <20230405142535.493854-2-david@redhat.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org On 05.04.23 17:43, Peter Xu wrote: > On Wed, Apr 05, 2023 at 05:17:31PM +0200, David Hildenbrand wrote: >> On 05.04.23 17:12, Peter Xu wrote: >>> On Wed, Apr 05, 2023 at 04:25:34PM +0200, David Hildenbrand wrote: >>>> Looks like what we fixed for hugetlb in commit 44f86392bdd1 ("mm/hugetlb: >>>> fix uffd-wp handling for migration entries in hugetlb_change_protection()") >>>> similarly applies to THP. >>>> >>>> Setting/clearing uffd-wp on THP migration entries is not implemented >>>> properly. Further, while removing migration PMDs considers the uffd-wp >>>> bit, inserting migration PMDs does not consider the uffd-wp bit. >>>> >>>> We have to set/clear independently of the migration entry type in >>>> change_huge_pmd() and properly copy the uffd-wp bit in >>>> set_pmd_migration_entry(). >>>> >>>> Verified using a simple reproducer that triggers migration of a THP, that >>>> the set_pmd_migration_entry() no longer loses the uffd-wp bit. >>>> >>>> Fixes: f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration") >>>> Cc: stable@vger.kernel.org >>>> Signed-off-by: David Hildenbrand >>> >>> Reviewed-by: Peter Xu >>> >>> Thanks, one trivial nitpick: >>> >>>> --- >>>> mm/huge_memory.c | 14 ++++++++++++-- >>>> 1 file changed, 12 insertions(+), 2 deletions(-) >>>> >>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >>>> index 032fb0ef9cd1..bdda4f426d58 100644 >>>> --- a/mm/huge_memory.c >>>> +++ b/mm/huge_memory.c >>>> @@ -1838,10 +1838,10 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, >>>> if (is_swap_pmd(*pmd)) { >>>> swp_entry_t entry = pmd_to_swp_entry(*pmd); >>>> struct page *page = pfn_swap_entry_to_page(entry); >>>> + pmd_t newpmd; >>>> VM_BUG_ON(!is_pmd_migration_entry(*pmd)); >>>> if (is_writable_migration_entry(entry)) { >>>> - pmd_t newpmd; >>>> /* >>>> * A protection check is difficult so >>>> * just be safe and disable write >>>> @@ -1855,8 +1855,16 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma, >>>> newpmd = pmd_swp_mksoft_dirty(newpmd); >>>> if (pmd_swp_uffd_wp(*pmd)) >>>> newpmd = pmd_swp_mkuffd_wp(newpmd); >>>> - set_pmd_at(mm, addr, pmd, newpmd); >>>> + } else { >>>> + newpmd = *pmd; >>>> } >>>> + >>>> + if (uffd_wp) >>>> + newpmd = pmd_swp_mkuffd_wp(newpmd); >>>> + else if (uffd_wp_resolve) >>>> + newpmd = pmd_swp_clear_uffd_wp(newpmd); >>>> + if (!pmd_same(*pmd, newpmd)) >>>> + set_pmd_at(mm, addr, pmd, newpmd); >>>> goto unlock; >>>> } >>>> #endif >>>> @@ -3251,6 +3259,8 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw, >>>> pmdswp = swp_entry_to_pmd(entry); >>>> if (pmd_soft_dirty(pmdval)) >>>> pmdswp = pmd_swp_mksoft_dirty(pmdswp); >>>> + if (pmd_swp_uffd_wp(*pvmw->pmd)) >>>> + pmdswp = pmd_swp_mkuffd_wp(pmdswp); >>> >>> I think it's fine to use *pmd, but maybe still better to use pmdval? I >>> worry pmdp_invalidate()) can be something else in the future that may >>> affect the bit. >> >> Wondering how I ended up with that, I realized that it's actually >> wrong and might have worked by chance for my reproducer on x86. >> >> That should make it work: >> >> diff --git a/mm/huge_memory.c b/mm/huge_memory.c >> index f977c965fdad..fffc953fa6ea 100644 >> --- a/mm/huge_memory.c >> +++ b/mm/huge_memory.c >> @@ -3257,7 +3257,7 @@ int set_pmd_migration_entry(struct page_vma_mapped_walk *pvmw, >> pmdswp = swp_entry_to_pmd(entry); >> if (pmd_soft_dirty(pmdval)) >> pmdswp = pmd_swp_mksoft_dirty(pmdswp); >> - if (pmd_swp_uffd_wp(*pvmw->pmd)) >> + if (pmd_uffd_wp(pmdval)) >> pmdswp = pmd_swp_mkuffd_wp(pmdswp); >> set_pmd_at(mm, address, pvmw->pmd, pmdswp); >> page_remove_rmap(page, vma, true); > > I guess pmd_swp_uffd_wp() just reads the _USER bit 2 which is also set for > a present pte, but then it sets swp uffd-wp always even if it was not set. > Yes. I modified the reproducer to migrate without uffd-wp first and we suddenly gain a uffd-wp bit. > Yes the change must be squashed in to be correct, with that, my R-b keeps. Thanks, I will resend later. -- Thanks, David / dhildenb