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(p200300cbc702dc002571c3c2c6ea84ef.dip0.t-ipconnect.de. [2003:cb:c702:dc00:2571:c3c2:c6ea:84ef]) by smtp.gmail.com with ESMTPSA id i7-20020a05600c354700b003cf4eac8e80sm18773096wmq.23.2022.11.21.07.55.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Nov 2022 07:55:57 -0800 (PST) Content-Type: multipart/mixed; boundary="------------X26NNqKh6qmB3gIBG6DEU1Rl" Message-ID: Date: Mon, 21 Nov 2022 16:55:56 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.1 Subject: Re: [PATCH v6 0/3] Implement IOCTL to get and/or the clear info about PTEs 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> <254130e7-7fb1-6cf1-e8fa-5bc2d4450431@collabora.com> From: David Hildenbrand Organization: Red Hat In-Reply-To: <254130e7-7fb1-6cf1-e8fa-5bc2d4450431@collabora.com> Precedence: bulk List-ID: X-Mailing-List: linux-kselftest@vger.kernel.org This is a multi-part message in MIME format. --------------X26NNqKh6qmB3gIBG6DEU1Rl Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 21.11.22 16:00, Muhammad Usama Anjum wrote: > Hello, > > Thank you for replying. > > On 11/14/22 8:46 PM, David Hildenbrand wrote: >>> 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. > Can someone please share the example to reproduce this? In all of my > testing, even if I ignore VM_SOFTDIRTY and only base my decision of > soft-dirtiness on individual pages, it always passes. Quick reproducer (the first and easiest one that triggered :) ) attached. With no kernel changes, it works as expected. # ./softdirty_mprotect With the following kernel change to simulate what you propose it fails: diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index d22687d2e81e..f2c682bf7f64 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -1457,8 +1457,8 @@ static pagemap_entry_t pte_to_pagemap_entry(struct pagemapread *pm, flags |= PM_FILE; if (page && !migration && page_mapcount(page) == 1) flags |= PM_MMAP_EXCLUSIVE; - if (vma->vm_flags & VM_SOFTDIRTY) - flags |= PM_SOFT_DIRTY; + //if (vma->vm_flags & VM_SOFTDIRTY) + // flags |= PM_SOFT_DIRTY; return make_pme(frame, flags); } # ./softdirty_mprotect Page #1 should be softdirty -- Thanks, David / dhildenb --------------X26NNqKh6qmB3gIBG6DEU1Rl Content-Type: text/x-csrc; charset=UTF-8; name="softdirty_mprotect.c" Content-Disposition: attachment; filename="softdirty_mprotect.c" Content-Transfer-Encoding: base64 I2luY2x1ZGUgPHN0ZGxpYi5oPgojaW5jbHVkZSA8c3RyaW5nLmg+CiNpbmNsdWRlIDxzdGRp by5oPgojaW5jbHVkZSA8ZmNudGwuaD4KI2luY2x1ZGUgPHVuaXN0ZC5oPgojaW5jbHVkZSA8 c3RkaW50Lmg+CiNpbmNsdWRlIDxzdGRib29sLmg+CiNpbmNsdWRlIDxzeXMvbW1hbi5oPgoK c3RhdGljIHNpemVfdCBwYWdlc2l6ZTsKc3RhdGljIGludCBwYWdlbWFwX2ZkOwoKc3RhdGlj IHZvaWQgY2xlYXJfc29mdGRpcnR5KHZvaWQpCnsKCWludCBmZCA9IG9wZW4oIi9wcm9jL3Nl bGYvY2xlYXJfcmVmcyIsIE9fV1JPTkxZKTsKCWNvbnN0IGNoYXIgKmN0cmwgPSAiNCI7Cglp bnQgcmV0OwoKCWlmIChmZCA8IDApIHsKCQlmcHJpbnRmKHN0ZGVyciwgIm9wZW4oKSBmYWls ZWRcbiIpOwoJCWV4aXQoMSk7Cgl9CglyZXQgPSB3cml0ZShmZCwgY3RybCwgc3RybGVuKGN0 cmwpKTsKCWNsb3NlKGZkKTsKCWlmIChyZXQgIT0gc3RybGVuKGN0cmwpKSB7CgkJZnByaW50 ZihzdGRlcnIsICJ3cml0ZSgpIGZhaWxlZFxuIik7CgkJZXhpdCgxKTsKCX0KfQoKc3RhdGlj IHVpbnQ2NF90IHBhZ2VtYXBfZ2V0X2VudHJ5KGludCBmZCwgY2hhciAqc3RhcnQpCnsKCWNv bnN0IHVuc2lnbmVkIGxvbmcgcGZuID0gKHVuc2lnbmVkIGxvbmcpc3RhcnQgLyBwYWdlc2l6 ZTsKCXVpbnQ2NF90IGVudHJ5OwoJaW50IHJldDsKCglyZXQgPSBwcmVhZChmZCwgJmVudHJ5 LCBzaXplb2YoZW50cnkpLCBwZm4gKiBzaXplb2YoZW50cnkpKTsKCWlmIChyZXQgIT0gc2l6 ZW9mKGVudHJ5KSkgewoJCWZwcmludGYoc3RkZXJyLCAicHJlYWQoKSBmYWlsZWRcbiIpOwoJ CWV4aXQoMSk7Cgl9CgoJcmV0dXJuIGVudHJ5Owp9CgpzdGF0aWMgYm9vbCBwYWdlbWFwX2lz X3NvZnRkaXJ0eShpbnQgZmQsIGNoYXIgKnN0YXJ0KQp7Cgl1aW50NjRfdCBlbnRyeSA9IHBh Z2VtYXBfZ2V0X2VudHJ5KGZkLCBzdGFydCk7CgoJcmV0dXJuIGVudHJ5ICYgMHgwMDgwMDAw MDAwMDAwMDAwdWxsOwp9Cgp2b2lkIG1haW4odm9pZCkKewoJY2hhciAqbWVtLCAqbWVtMjsK CglwYWdlc2l6ZSA9IGdldHBhZ2VzaXplKCk7CglwYWdlbWFwX2ZkID0gb3BlbigiL3Byb2Mv c2VsZi9wYWdlbWFwIiwgT19SRE9OTFkpOwoJaWYgKHBhZ2VtYXBfZmQgPCAwKSB7CgkJZnBy aW50ZihzdGRlcnIsICJvcGVuKCkgZmFpbGVkXG4iKTsKCQlleGl0KDEpOwoJfQoKCS8qIE1h cCAyIHBhZ2VzLiAqLwoJbWVtID0gbW1hcCgwLCAyICogcGFnZXNpemUsIFBST1RfUkVBRHxQ Uk9UX1dSSVRFLAoJCSAgIE1BUF9QUklWQVRFfE1BUF9BTk9OLCAtMSwgMCk7CglpZiAobWVt ID09IE1BUF9GQUlMRUQpIHsKCQlmcHJpbnRmKHN0ZGVyciwgIm1tYXAoKSBmYWlsZWRcbiIp OwoJCWV4aXQoMSk7Cgl9CgoJLyogUG9wdWxhdGUgYm90aCBwYWdlcy4gKi8KCW1lbXNldCht ZW0sIDEsIDIgKiBwYWdlc2l6ZSk7CgoJaWYgKCFwYWdlbWFwX2lzX3NvZnRkaXJ0eShwYWdl bWFwX2ZkLCBtZW0pKQoJCWZwcmludGYoc3RkZXJyLCAiUGFnZSAjMSBzaG91bGQgYmUgc29m dGRpcnR5XG4iKTsKCWlmICghcGFnZW1hcF9pc19zb2Z0ZGlydHkocGFnZW1hcF9mZCwgbWVt ICsgcGFnZXNpemUpKQoJCWZwcmludGYoc3RkZXJyLCAiUGFnZSAjMiBzaG91bGQgYmUgc29m dGRpcnR5XG4iKTsKCgkvKgoJICogU3RhcnQgc29mdGRpcnR5IHRyYWNraW5nLiBDbGVhciBW TV9TT0ZURElSVFkgYW5kIGNsZWFyIHRoZSBzb2Z0ZGlydHkKCSAqIFBURSBiaXQuCgkgKi8K CWNsZWFyX3NvZnRkaXJ0eSgpOwoKCWlmIChwYWdlbWFwX2lzX3NvZnRkaXJ0eShwYWdlbWFw X2ZkLCBtZW0pKQoJCWZwcmludGYoc3RkZXJyLCAiUGFnZSAjMSBzaG91bGQgbm90IGJlIHNv ZnRkaXJ0eVxuIik7CglpZiAocGFnZW1hcF9pc19zb2Z0ZGlydHkocGFnZW1hcF9mZCwgbWVt ICsgcGFnZXNpemUpKQoJCWZwcmludGYoc3RkZXJyLCAiUGFnZSAjMiBzaG91bGQgbm90IGJl IHNvZnRkaXJ0eVxuIik7CgoJLyoKCSAqIFJlbWFwIHRoZSBzZWNvbmQgcGFnZS4gVGhlIFZN QSBnZXRzIFZNX1NPRlRESVJUWSBzZXQuIEJvdGggVk1BcwoJICogZ2V0IG1lcmdlZCBzdWNo IHRoYXQgdGhlIHJlc3VsdGluZyBWTUEgaGFzIFZNX1NPRlRESVJUWSBzZXQuCgkgKi8KCW1l bTIgPSBtbWFwKG1lbSArIHBhZ2VzaXplLCBwYWdlc2l6ZSwgUFJPVF9SRUFEfFBST1RfV1JJ VEUsCgkJICAgIE1BUF9QUklWQVRFfE1BUF9BTk9OfE1BUF9GSVhFRCwgLTEsIDApOwoJaWYg KG1lbTIgPT0gTUFQX0ZBSUxFRCkgewoJCWZwcmludGYoc3RkZXJyLCAibW1hcCgpIGZhaWxl ZFxuIik7CgkJZXhpdCgxKTsKCX0KCgkvKiBQcm90ZWN0ICsgdW5wcm90ZWN0LiAqLwoJbXBy b3RlY3QobWVtLCAyICogcGFnZXNpemUsIFBST1RfUkVBRCk7CgltcHJvdGVjdChtZW0sIDIg KiBwYWdlc2l6ZSwgUFJPVF9SRUFEfFBST1RfV1JJVEUpOwoKCS8qIE1vZGlmeSBib3RoIHBh Z2VzLiAqLwoJbWVtc2V0KG1lbSwgMiwgMiAqIHBhZ2VzaXplKTsKCglpZiAoIXBhZ2VtYXBf aXNfc29mdGRpcnR5KHBhZ2VtYXBfZmQsIG1lbSkpCgkJZnByaW50ZihzdGRlcnIsICJQYWdl ICMxIHNob3VsZCBiZSBzb2Z0ZGlydHlcbiIpOwoJaWYgKCFwYWdlbWFwX2lzX3NvZnRkaXJ0 eShwYWdlbWFwX2ZkLCBtZW0gKyBwYWdlc2l6ZSkpCgkJZnByaW50ZihzdGRlcnIsICJQYWdl ICMyIHNob3VsZCBiZSBzb2Z0ZGlydHlcbiIpOwp9Cg== --------------X26NNqKh6qmB3gIBG6DEU1Rl--