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[2003:d8:2f28:c100:2225:10aa:f247:7b85]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3c9c9324dc9sm14969370f8f.3.2025.08.28.00.57.25 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 28 Aug 2025 00:57:26 -0700 (PDT) Message-ID: Date: Thu, 28 Aug 2025 09:57:25 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v2] huge_mm.h: disallow is_huge_zero_folio(NULL) To: Andrew Morton , Max Kellermann Cc: lorenzo.stoakes@oracle.com, ziy@nvidia.com, baolin.wang@linux.alibaba.com, Liam.Howlett@oracle.com, npache@redhat.com, ryan.roberts@arm.com, dev.jain@arm.com, baohua@kernel.org, shikemeng@huaweicloud.com, kasong@tencent.com, nphamcs@gmail.com, bhe@redhat.com, chrisl@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <2aa3f478-9c87-4102-b83e-bf235372d834@redhat.com> <20250827150330.280399-1-max.kellermann@ionos.com> <20250827165309.44e465ff214e45f1a6665b24@linux-foundation.org> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Queue-Id: 483EA18000A X-Stat-Signature: mksztb8bu85sd3s3wdgbr3uuq94n4ozj X-Rspam-User: X-Rspamd-Server: rspam06 X-HE-Tag: 1756367851-476079 X-HE-Meta: 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 F5+/fa8D 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 28.08.25 01:53, Andrew Morton wrote: > On Wed, 27 Aug 2025 17:03:30 +0200 Max Kellermann wrote: > >> Calling is_huge_zero_folio(NULL) should not be legal - it makes no >> sense, and a different (theoretical) implementation may dereference >> the pointer. But currently, lacking any explicit documentation, this >> call is possible. >> >> But if somebody really passes NULL, the function should not return >> true - this isn't the huge zero folio after all! However, if the >> `huge_zero_folio` hasn't been allocated yet, it's NULL, and >> is_huge_zero_folio(NULL) just happens to return true, which is a lie. >> >> This weird side effect prevented me from reproducing a kernel crash >> that occurred when the elements of a folio_batch were NULL - since >> folios_put_refs() skips huge zero folios, this sometimes causes a >> crash, but sometimes does not. For debugging, it is better to reveal >> such bugs reliably and not hide them behind random preconditions like >> "has the huge zero folio already been created?" >> >> To improve detection of such bugs, David Hildenbrand suggested adding >> a VM_WARN_ON_ONCE(). >> >> ... >> >> --- a/include/linux/huge_mm.h >> +++ b/include/linux/huge_mm.h >> @@ -2,6 +2,7 @@ >> #ifndef _LINUX_HUGE_MM_H >> #define _LINUX_HUGE_MM_H >> >> +#include // for VM_WARN_ON_ONCE() >> #include >> >> #include /* only for vma_is_dax() */ >> @@ -479,6 +480,8 @@ extern unsigned long huge_zero_pfn; >> >> static inline bool is_huge_zero_folio(const struct folio *folio) >> { >> + VM_WARN_ON_ONCE(folio == NULL); >> + >> return READ_ONCE(huge_zero_folio) == folio; >> } > > OK, but it remains the case that we have seen code which calls > is_huge_zero_folio() prior to the initialization of huge_zero_folio. > > Is this a bug? I think so. Should we be checking for recurrences of > this bug? As answered elsewhere, this is perfectly fine as the huge zero folio is allocated on demand only (and only once enabled). > > > Also, sigh. I do dislike seeing VM_WARN_ON_ONCE() in an inline > function - heaven knows how much bloat that adds. Defconfig > mm/huge_memory.o (which has three calls) grows by 80 bytes so I guess > that's livable with. Common practice to use VM_WARN_ON_ONCE() and friend in inline functions. Just look at page-flags.h. If someone complains about kernel image size with CONFIG_DEBUG_VM, they shopuld reevaluate life choices. :) -- Cheers David / dhildenb