Re: [PATCH 4/5] mm: soft-dirty bits for user memory changes tracking

[Andy Lutomirski <luto@amacapital.net>]

On 04/30/2013 09:12 AM, Pavel Emelyanov wrote:
> The soft-dirty is a bit on a PTE which helps to track which pages a task
> writes to. In order to do this tracking one should
> 
>   1. Clear soft-dirty bits from PTEs ("echo 4 > /proc/PID/clear_refs)
>   2. Wait some time.
>   3. Read soft-dirty bits (55'th in /proc/PID/pagemap entries)
> 
> To do this tracking, the writable bit is cleared from PTEs when the
> soft-dirty bit is. Thus, after this, when the task tries to modify a page
> at some virtual address the #PF occurs and the kernel sets the soft-dirty
> bit on the respective PTE.
> 
> Note, that although all the task's address space is marked as r/o after the
> soft-dirty bits clear, the #PF-s that occur after that are processed fast.
> This is so, since the pages are still mapped to physical memory, and thus
> all the kernel does is finds this fact out and puts back writable, dirty
> and soft-dirty bits on the PTE.
> 
> Another thing to note, is that when mremap moves PTEs they are marked with
> soft-dirty as well, since from the user perspective mremap modifies the
> virtual memory at mremap's new address.
> 
> 

Sorry I'm late to the party -- I didn't notice this until the lwn
article this week.

How does this get munmap + mmap right?  mremap marks things soft-dirty,
but unmapping and remapping seems like it will result in the soft-dirty
bit being cleared.  For that matter, won't this sequence also end up wrong:

 - clear_refs
 - Write to mapping
 - Page and pte evicted due to memory pressure
 - Read from mapping -- clean page faulted back in
 - pte soft-dirty is now clear ?!?

--Andy

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