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[2003:cb:c71b:d000:1d1f:238e:aeaf:dbf7]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-386219096adsm4974931f8f.85.2024.12.06.08.03.55 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 06 Dec 2024 08:03:55 -0800 (PST) Message-ID: <8395af7d-328a-425c-84a7-517e78a602b1@redhat.com> Date: Fri, 6 Dec 2024 17:03:55 +0100 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 v4] mm/hugetlb: support FOLL_FORCE|FOLL_WRITE To: Guillaume Morin , linux-kernel@vger.kernel.org Cc: linux-mm@kvack.org, Muchun Song , Andrew Morton , Peter Xu , Eric Hagberg References: From: David Hildenbrand Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZgEEwEIAEICGwMGCwkIBwMCBhUIAgkKCwQW AgMBAh4BAheAAhkBFiEEG9nKrXNcTDpGDfzKTd4Q9wD/g1oFAl8Ox4kFCRKpKXgACgkQTd4Q 9wD/g1oHcA//a6Tj7SBNjFNM1iNhWUo1lxAja0lpSodSnB2g4FCZ4R61SBR4l/psBL73xktp rDHrx4aSpwkRP6Epu6mLvhlfjmkRG4OynJ5HG1gfv7RJJfnUdUM1z5kdS8JBrOhMJS2c/gPf wv1TGRq2XdMPnfY2o0CxRqpcLkx4vBODvJGl2mQyJF/gPepdDfcT8/PY9BJ7FL6Hrq1gnAo4 3Iv9qV0JiT2wmZciNyYQhmA1V6dyTRiQ4YAc31zOo2IM+xisPzeSHgw3ONY/XhYvfZ9r7W1l pNQdc2G+o4Di9NPFHQQhDw3YTRR1opJaTlRDzxYxzU6ZnUUBghxt9cwUWTpfCktkMZiPSDGd KgQBjnweV2jw9UOTxjb4LXqDjmSNkjDdQUOU69jGMUXgihvo4zhYcMX8F5gWdRtMR7DzW/YE BgVcyxNkMIXoY1aYj6npHYiNQesQlqjU6azjbH70/SXKM5tNRplgW8TNprMDuntdvV9wNkFs 9TyM02V5aWxFfI42+aivc4KEw69SE9KXwC7FSf5wXzuTot97N9Phj/Z3+jx443jo2NR34XgF 89cct7wJMjOF7bBefo0fPPZQuIma0Zym71cP61OP/i11ahNye6HGKfxGCOcs5wW9kRQEk8P9 M/k2wt3mt/fCQnuP/mWutNPt95w9wSsUyATLmtNrwccz63XOwU0EVcufkQEQAOfX3n0g0fZz Bgm/S2zF/kxQKCEKP8ID+Vz8sy2GpDvveBq4H2Y34XWsT1zLJdvqPI4af4ZSMxuerWjXbVWb T6d4odQIG0fKx4F8NccDqbgHeZRNajXeeJ3R7gAzvWvQNLz4piHrO/B4tf8svmRBL0ZB5P5A 2uhdwLU3NZuK22zpNn4is87BPWF8HhY0L5fafgDMOqnf4guJVJPYNPhUFzXUbPqOKOkL8ojk CXxkOFHAbjstSK5Ca3fKquY3rdX3DNo+EL7FvAiw1mUtS+5GeYE+RMnDCsVFm/C7kY8c2d0G NWkB9pJM5+mnIoFNxy7YBcldYATVeOHoY4LyaUWNnAvFYWp08dHWfZo9WCiJMuTfgtH9tc75 7QanMVdPt6fDK8UUXIBLQ2TWr/sQKE9xtFuEmoQGlE1l6bGaDnnMLcYu+Asp3kDT0w4zYGsx 5r6XQVRH4+5N6eHZiaeYtFOujp5n+pjBaQK7wUUjDilPQ5QMzIuCL4YjVoylWiBNknvQWBXS lQCWmavOT9sttGQXdPCC5ynI+1ymZC1ORZKANLnRAb0NH/UCzcsstw2TAkFnMEbo9Zu9w7Kv AxBQXWeXhJI9XQssfrf4Gusdqx8nPEpfOqCtbbwJMATbHyqLt7/oz/5deGuwxgb65pWIzufa N7eop7uh+6bezi+rugUI+w6DABEBAAHCwXwEGAEIACYCGwwWIQQb2cqtc1xMOkYN/MpN3hD3 AP+DWgUCXw7HsgUJEqkpoQAKCRBN3hD3AP+DWrrpD/4qS3dyVRxDcDHIlmguXjC1Q5tZTwNB boaBTPHSy/Nksu0eY7x6HfQJ3xajVH32Ms6t1trDQmPx2iP5+7iDsb7OKAb5eOS8h+BEBDeq 3ecsQDv0fFJOA9ag5O3LLNk+3x3q7e0uo06XMaY7UHS341ozXUUI7wC7iKfoUTv03iO9El5f XpNMx/YrIMduZ2+nd9Di7o5+KIwlb2mAB9sTNHdMrXesX8eBL6T9b+MZJk+mZuPxKNVfEQMQ a5SxUEADIPQTPNvBewdeI80yeOCrN+Zzwy/Mrx9EPeu59Y5vSJOx/z6OUImD/GhX7Xvkt3kq Er5KTrJz3++B6SH9pum9PuoE/k+nntJkNMmQpR4MCBaV/J9gIOPGodDKnjdng+mXliF3Ptu6 3oxc2RCyGzTlxyMwuc2U5Q7KtUNTdDe8T0uE+9b8BLMVQDDfJjqY0VVqSUwImzTDLX9S4g/8 kC4HRcclk8hpyhY2jKGluZO0awwTIMgVEzmTyBphDg/Gx7dZU1Xf8HFuE+UZ5UDHDTnwgv7E th6RC9+WrhDNspZ9fJjKWRbveQgUFCpe1sa77LAw+XFrKmBHXp9ZVIe90RMe2tRL06BGiRZr jPrnvUsUUsjRoRNJjKKA/REq+sAnhkNPPZ/NNMjaZ5b8Tovi8C0tmxiCHaQYqj7G2rgnT0kt WNyWQQ== Organization: Red Hat In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 06.12.24 15:49, Guillaume Morin wrote: > Eric reported that PTRACE_POKETEXT fails when applications use hugetlb > for mapping text using huge pages. Before commit 1d8d14641fd9 > ("mm/hugetlb: support write-faults in shared mappings"), PTRACE_POKETEXT > worked by accident, but it was buggy and silently ended up mapping pages > writable into the page tables even though VM_WRITE was not set. > > In general, FOLL_FORCE|FOLL_WRITE does currently not work with hugetlb. > Let's implement FOLL_FORCE|FOLL_WRITE properly for hugetlb, such that > what used to work in the past by accident now properly works, allowing > applications using hugetlb for text etc. to get properly debugged. > > This change might also be required to implement uprobes support for > hugetlb [1]. > > [1] https://lore.kernel.org/lkml/ZiK50qob9yl5e0Xz@bender.morinfr.org/ > > Cc: Muchun Song > Cc: Andrew Morton > Cc: Peter Xu > Cc: David Hildenbrand > Cc: Eric Hagberg > Signed-off-by: Guillaume Morin > --- > Changes in v2: > - Improved commit message > Changes in v3: > - Fix potential unitialized mem access in follow_huge_pud > - define pud_soft_dirty when soft dirty is not enabled > Changes in v4: > - Remove the soft dirty pud check > - Remove the pud_soft_dirty added in v3 > > mm/gup.c | 95 +++++++++++++++++++++++++--------------------------- > mm/hugetlb.c | 20 ++++++----- > 2 files changed, 57 insertions(+), 58 deletions(-) > > diff --git a/mm/gup.c b/mm/gup.c > index 746070a1d8bf..63c705ff4162 100644 > --- a/mm/gup.c > +++ b/mm/gup.c > @@ -587,6 +587,33 @@ static struct folio *try_grab_folio_fast(struct page *page, int refs, > } > #endif /* CONFIG_HAVE_GUP_FAST */ > > +/* Common code for can_follow_write_* */ > +static inline bool can_follow_write_common(struct page *page, > + struct vm_area_struct *vma, unsigned int flags) > +{ > + /* Maybe FOLL_FORCE is set to override it? */ > + if (!(flags & FOLL_FORCE)) > + return false; > + > + /* But FOLL_FORCE has no effect on shared mappings */ > + if (vma->vm_flags & (VM_MAYSHARE | VM_SHARED)) > + return false; > + > + /* ... or read-only private ones */ > + if (!(vma->vm_flags & VM_MAYWRITE)) > + return false; > + > + /* ... or already writable ones that just need to take a write fault */ > + if (vma->vm_flags & VM_WRITE) > + return false; > + > + /* > + * See can_change_pte_writable(): we broke COW and could map the page > + * writable if we have an exclusive anonymous page ... > + */ > + return page && PageAnon(page) && PageAnonExclusive(page); > +} > + > static struct page *no_page_table(struct vm_area_struct *vma, > unsigned int flags, unsigned long address) > { > @@ -613,6 +640,18 @@ static struct page *no_page_table(struct vm_area_struct *vma, > } > > #ifdef CONFIG_PGTABLE_HAS_HUGE_LEAVES > +/* FOLL_FORCE can write to even unwritable PUDs in COW mappings. */ > +static inline bool can_follow_write_pud(pud_t pud, struct page *page, > + struct vm_area_struct *vma, > + unsigned int flags) > +{ > + /* If the pud is writable, we can write to the page. */ > + if (pud_write(pud)) > + return true; > + > + return can_follow_write_common(page, vma, flags); > +} > + > static struct page *follow_huge_pud(struct vm_area_struct *vma, > unsigned long addr, pud_t *pudp, > int flags, struct follow_page_context *ctx) > @@ -625,13 +664,16 @@ static struct page *follow_huge_pud(struct vm_area_struct *vma, > > assert_spin_locked(pud_lockptr(mm, pudp)); > > - if ((flags & FOLL_WRITE) && !pud_write(pud)) > + pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT; > + page = pfn_to_page(pfn); > + > + if ((flags & FOLL_WRITE) && > + !can_follow_write_pud(pud, page, vma, flags)) > return NULL; > > if (!pud_present(pud)) > return NULL; > > - pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT; That looks wrong. See follow_huge_pmd() for reference (1) You must not do a pfn_to_page() before we verified that we have a present PUD. (2) can_follow_write_pud() must be called with the first mapped page. It would currently with hugetlb not be strictly required, but is not future proof. It must be likely be something like: if (!pud_present(pud)) return NULL; if ((flags & FOLL_WRITE) && !can_follow_write_pud(pud, pfn_to_page(pfn), vma, flags)) return NULL; pfn += (addr & ~PUD_MASK) >> PAGE_SHIFT; page = pfn_to_page(pfn); > > delayacct_wpcopy_end(); > return 0; > @@ -5943,7 +5944,8 @@ static vm_fault_t hugetlb_wp(struct folio *pagecache_folio, > spin_lock(vmf->ptl); > vmf->pte = hugetlb_walk(vma, vmf->address, huge_page_size(h)); > if (likely(vmf->pte && pte_same(huge_ptep_get(mm, vmf->address, vmf->pte), pte))) { > - pte_t newpte = make_huge_pte(vma, &new_folio->page, !unshare); > + const bool writable = !unshare && (vma->vm_flags & VM_WRITE); > + pte_t newpte = make_huge_pte(vma, &new_folio->page, writable); ); You probably missed my earlier comment. After the recent changes to make_huge_pte() that are already in mm/mm-unstable, this hunk can be dropped and left unchanged. make_huge_pte() will perform the VM_WRITE check. -- Cheers, David / dhildenb