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(p200300cbc703d300d8811fd5bc955e2d.dip0.t-ipconnect.de. [2003:cb:c703:d300:d881:1fd5:bc95:5e2d]) by smtp.gmail.com with ESMTPSA id g13-20020a5d554d000000b002366e3f1497sm10115912wrw.6.2022.11.14.07.46.18 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 14 Nov 2022 07:46:19 -0800 (PST) Message-ID: Date: Mon, 14 Nov 2022 16:46:18 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.1 Content-Language: en-US To: Muhammad Usama Anjum , =?UTF-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= , Andrei Vagin , Danylo Mocherniuk , Alexander Viro , Andrew Morton , Paul Gofman Cc: Suren Baghdasaryan , Greg KH , Christian Brauner , Peter Xu , Yang Shi , Vlastimil Babka , Zach O'Keefe , "Matthew Wilcox (Oracle)" , "Gustavo A. R. Silva" , Dan Williams , kernel@collabora.com, Gabriel Krisman Bertazi , Peter Enderborg , "open list : KERNEL SELFTEST FRAMEWORK" , Shuah Khan , open list , "open list : PROC FILESYSTEM" , "open list : MEMORY MANAGEMENT" References: <20221109102303.851281-1-usama.anjum@collabora.com> <9c167d01-ef09-ec4e-b4a1-2fff62bf01fe@redhat.com> <6fdce544-8d4f-8b3c-9208-735769a9e624@collabora.com> From: David Hildenbrand Organization: Red Hat Subject: Re: [PATCH v6 0/3] Implement IOCTL to get and/or the clear info about PTEs In-Reply-To: <6fdce544-8d4f-8b3c-9208-735769a9e624@collabora.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org > The soft-dirtiness is stored in the PTE. VMA is marked dirty to store the > dirtiness for reused regions. Clearing the soft-dirty status of whole > process is straight forward. When we want to clear/monitor the > soft-dirtiness of a part of the virtual memory, there is a lot of internal > noise. We don't want the non-dirty pages to become dirty because of how the > soft-dirty feature has been working. Soft-dirty feature wasn't being used > the way we want to use now. While monitoring a part of memory, it is not > acceptable to get non-dirty pages as dirty. Non-dirty pages become dirty > when the two VMAs are merged without considering if they both are dirty or > not (34228d473efe). To monitor changes over the memory, sometimes VMAs are > split to clear the soft-dirty bit in the VMA flags. But sometimes kernel > decide to merge them backup. It is so waste of resources. Maybe you'd want a per-process option to not merge if the VM_SOFTDIRTY property differs. But that might be just one alternative for handling this case. > > To keep things consistent, the default behavior of the IOCTL is to output > even the extra non-dirty pages as dirty from the kernel noise. A optional > PAGEMAP_NO_REUSED_REGIONS flag is added for those use cases which aren't > tolerant of extra non-dirty pages. This flag can be considered as something > which is by-passing the already present buggy implementation in the kernel. > It is not buggy per say as the issue can be solved if we don't allow the > two VMA which have different soft-dirty bits to get merged. But we are > allowing that so that the total number of VMAs doesn't increase. This was > acceptable at the time, but now with the use case of monitoring a part of > memory for soft-dirty doesn't want this merging. So either we need to > revert 34228d473efe and PAGEMAP_NO_REUSED_REGIONS flag will not be needed > or we should allow PAGEMAP_NO_REUSED_REGIONS or similar mechanism to ignore > the extra dirty pages which aren't dirty in reality. > > When PAGEMAP_NO_REUSED_REGIONS flag is used, only the PTEs are checked to > find if the pages are dirty. So re-used regions cannot be detected. This > has the only side-effect of not checking the VMAs. So this is limitation of > using this flag which should be acceptable in the current state of code. > This limitation is okay for the users as they can clear the soft-dirty bit > of the VMA before starting to monitor a range of memory for soft-dirtiness. > > >> Please separate that part out from the other changes; I am still not >> convinced that we want this and what the semantical implications are. >> >> Let's take a look at an example: can_change_pte_writable() >> >>     /* Do we need write faults for softdirty tracking? */ >>     if (vma_soft_dirty_enabled(vma) && !pte_soft_dirty(pte)) >>         return false; >> >> We care about PTE softdirty tracking, if it is enabled for the VMA. >> Tracking is enabled if: vma_soft_dirty_enabled() >> >>     /* >>      * Soft-dirty is kind of special: its tracking is enabled when >>      * the vma flags not set. >>      */ >>     return !(vma->vm_flags & VM_SOFTDIRTY); >> >> Consequently, if VM_SOFTDIRTY is set, we are not considering the soft_dirty >> PTE bits accordingly. > Sorry, I'm unable to completely grasp the meaning of the example. We have > followed clear_refs_write() to write the soft-dirty bit clearing code in > the current patch. Dirtiness of the VMA and the PTE may be set > independently. Newer allocated memory has dirty bit set in the VMA. When > something is written the memory, the soft dirty bit is set in the PTEs as > well regardless if the soft dirty bit is set in the VMA or not. > Let me try to find a simple explanation: After clearing a SOFTDIRTY PTE flag inside an area with VM_SOFTDIRTY set, there are ways that PTE could get written to and it could become dirty, without the PTE becoming softdirty. Essentially, inside a VMA with VM_SOFTDIRTY set, the PTE softdirty values might be stale: there might be entries that are softdirty even though the PTE is *not* marked softdirty. These are, AFAIU, the current semantics, and I am not sure if we want user space to explicitly work around that. >> >> >> I'd suggest moving forward without this controversial >> PAGEMAP_NO_REUSED_REGIONS functionality for now, and preparing it as a >> clear add-on we can discuss separately.Like I've described above, I've only added this flag to not get the > non-dirty pages as dirty. Can there be some alternative to adding this > flag? Please suggest. Please split it out into a separate patch for now. We can discuss further what the semantics are and if there are better alternatives for that. In the meantime, you could move forward without PAGEMAP_NO_REUSED_REGIONS while we are discussing them further. -- Thanks, David / dhildenb