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SubjectRe: [PATCH] update ctime and mtime for mmaped write
Miklos Szeredi wrote:
>>>>>>> +int set_page_dirty_mapping(struct page *page);
>>>>>> This aspect of the design seems intrusive to me. I didn't see a strong
>>>>>> reason to introduce new versions of many of the routines just to handle
>>>>>> these semantics. What motivated this part of your design? Why the new
>>>>>> _mapping versions of routines?
>>>>> Because there's no way to know inside the set_page_dirty() functions
>>>>> if the dirtying comes from a memory mapping or from a modification
>>>>> through a normal write(). And they have different semantics, for
>>>>> write() the modification times are updated immediately.
>>>> Perhaps I didn't understand what page_mapped() does, but it does seem to
>>>> have the right semantics as far as I could see.
>>> The problems will start, when you have a file that is both mapped and
>>> modified with write(). Then the dirying from the write() will set the
>>> flag, and that will have undesirable consequences.
>> I don't think that I quite follow the logic. The dirtying from write()
>> will set the flag, but then the mtime will get updated and the flag will
>> be cleared by the hook in file_update_time(). Right?
> Take this example:
> fd = open()
> addr = mmap(.., fd)
> write(fd, ...)
> close(fd)
> sleep(100)
> msync(addr,...)
> munmap(addr)
> The file times will be updated in write(), but with your patch, the
> bit in the mapping will also be set.
> Then in msync() the file times will be updated again, which is wrong,
> since the memory was _not_ modified through the mapping.

This is correct. I have updated my proposed patch to include the clearing
of AS_MCTIME in the routine which updates the mtime field. I haven't
reposted it yet until I complete testing of the new resulting system. I
anticipate doing this later today.


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