Date:   Mon, 21 Nov 2022 16:55:56 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Muhammad Usama Anjum <usama.anjum@...labora.com>,
        Michał Mirosław <emmir@...gle.com>,
        Andrei Vagin <avagin@...il.com>,
        Danylo Mocherniuk <mdanylo@...gle.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Gofman <pgofman@...eweavers.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Christian Brauner <brauner@...nel.org>,
        Peter Xu <peterx@...hat.com>, Yang Shi <shy828301@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Zach O'Keefe <zokeefe@...gle.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>, kernel@...labora.com,
        Gabriel Krisman Bertazi <krisman@...labora.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        "open list : KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, Shuah Khan <shuah@...nel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list : PROC FILESYSTEM" <linux-fsdevel@...r.kernel.org>,
        "open list : MEMORY MANAGEMENT" <linux-mm@...ck.org>
Subject: Re: [PATCH v6 0/3] Implement IOCTL to get and/or the clear info about
 PTEs

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

View attachment "softdirty_mprotect.c" of type "text/x-csrc" (2677 bytes)

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