Re: [PATCH V2,2/2] mm: madvise: skip unmapped vma holes passed to process_madvise

[Nadav Amit <nadav.amit@gmail.com>]

> On Mar 17, 2022, at 9:53 AM, Suren Baghdasaryan <surenb@google.com> wrote:
> 
> On Thu, Mar 17, 2022 at 9:28 AM Minchan Kim <minchan@kernel.org> wrote:
>> 
>> On Wed, Mar 16, 2022 at 02:29:06PM -0700, Andrew Morton wrote:
>>> On Wed, 16 Mar 2022 19:49:38 +0530 Charan Teja Kalla <quic_charante@quicinc.com> wrote:
>>> 
>>>>> IMO, it's worth to note in man page.
>>>>> 
>>>> 
>>>> Or the current patch for just ENOMEM is sufficient here and we just have
>>>> to update the man page?
>>> 
>>> I think the "On success, process_madvise() returns the number of bytes
>>> advised" behaviour sounds useful.  But madvise() doesn't do that.
>>> 
>>> RETURN VALUE
>>>       On  success, madvise() returns zero.  On error, it returns -1 and errno
>>>       is set to indicate the error.
>>> 
>>> So why is it desirable in the case of process_madvise()?
>> 
>> Since process_madvise deal with multiple ranges and could fail at one of
>> them in the middle or pocessing, people could decide where the call
>> failed and then make a strategy whether they will abort at the point or
>> continue to hint next addresses. Here, problem of the strategy is API
>> doesn't return any error vaule if it has processed any bytes so they
>> would have limitation to decide a policy. That's the limitation for
>> every vector IO syscalls, unfortunately.
>> 
>>> 
>>> 
>>> 
>>> And why was process_madvise() designed this way?   Or was it
>>> always simply an error in the manpage?
> 
> Taking a closer look, indeed manpage seems to be wrong.
> https://elixir.bootlin.com/linux/v5.17-rc8/source/mm/madvise.c#L1154
> indicates that in the presence of unmapped holes madvise will skip
> them but will return ENOMEM and that's what process_madvise is
> ultimately returning in this case. So, the manpage claim of "This
> return value may be less than the total number of requested bytes, if
> an error occurred after some iovec elements were already processed."
> does not reflect the reality in our case because the return value will
> be -ENOMEM. After the desired behavior is finalized I'll modify the
> manpage accordingly.

Since process_madvise() might be used in sort of non-cooperative mode,
I think that the caller cannot guarantee that it knows exactly the
memory layout of the process whose memory it madvise’s. I know that
MADV_DONTNEED for instance is not supported (at least today) by
process_madvise(), but if it were, the caller may want which exact
memory was madvise'd even if the target process ran some other
memory layout changing syscalls (e.g., munmap()).

IOW, skipping holes and just returning the total number of madvise’d
bytes might not be enough.