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Howlett" , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Andrew Morton , "kirill.shutemov@linux.intel.com" , Rik van Riel References: <20201215155441.1497432-1-Liam.Howlett@Oracle.com> <413ffbe0-959d-c2ad-95d8-80104adac089@redhat.com> <20201216204252.vh3zadk4ghbzufqz@revolver> From: David Hildenbrand Organization: Red Hat GmbH Message-ID: <91483d71-a12d-1e52-e9c9-135404c5219f@redhat.com> Date: Thu, 17 Dec 2020 11:05:56 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.5.0 MIME-Version: 1.0 In-Reply-To: <20201216204252.vh3zadk4ghbzufqz@revolver> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.16 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: On 16.12.20 21:42, Liam R. Howlett wrote: > > Thank you for looking at this. I appreciate the scrutiny. > > * David Hildenbrand [201216 09:58]: >> On 15.12.20 16:54, Liam R. Howlett wrote: >>> do_mmap() will unlock the necessary VMAs. There is also a bug in the >>> loop which will evaluate as false and not unlock any VMAs anyways. >> >> If there is a BUG, do we have a Fixes: tag? Also > > The bug would never show up as it is masked by do_mmap() unlocking the > necessary range. Although there is a bug in this code, the code does > not cause an issue as it won't execute so should I have a Fixes tag? > The code works and what I've done is remove a chunk of code that never > runs. > Ok I see. The use of "bug" here is misleading. The unnecessary code is simply not doing what it promised to do without doing any harm. >> >> 1. Can we fix the bug separately first? > > I think it is safer to remove unexecuted code than enable it and then > remove it. I agree, as it is not actually a bug. > >> 2. Can we have a better description on what the bug actually is >> "evaluate as false"? What is the result of the bug? > > The bug is in the for loop test expression that I removed in the patch. > Here is the long explaination of why the loop has never run. > > > Line 2982: if (start + size <= start > Line 2983: goto out; > > size is positive. > > Line 2992: vma = find_vma(mm, start); > Look up the first VMA which satisfies start < vm_end > > Line 2997: if (start < vma->vm_start) > Line 2998: goto out; > > So now vma->vm_start >= start. > If vma->vm_start > start, then there are no VMAs in that area, otherwise > it would have been returned by find_vma(). > So we can say that vma->vm_start == start. > > Line 3033: for (tmp = vma; tmp->vm_start >= start + size; > Line 3034: tmp = tmp->vm_next) { > This is the for loop with the error in the test expression. > > tmp->vm_start == start which cannot be >= (start + size). > > I believe the intention was to loop through vmas in the range of start > to (start + size) and unlock them. > > > The result of the bug is no VMA is unlocked in this fuction. But that > doesn't matter as they are unlocked later in the call chain - which is > why this code works as intended. > Thanks for clarifying! > >> >> CCing some people that might know if this is actually a sane change. >> Skimming over do_mmap(), it's not immediately clear to me that >> "do_mmap() will unlock the necessary VMAs". > > Ah, yes. That is understandable. > > do_mmap() L1583 -> mmap_region() L1752 -> munmap_vma_range() -> > do_munmap() -> __do_munmap() loop at 2891 to unlock the range. > > Would you like me to add this call chain to the changelog? Yes please, in a simplified form. I suggest something like the following patch description: "do_mmap(MAP_FIXED) will already unlock pages via munmap_vma_range(). We can remove the superfluous manual unlocking in remap_file_pages(). Note that the manual unlocking is even incorrect, as it might miss unlocking some pages - no harm done. " -- Thanks, David / dhildenb