Re: [PATCH RFC 0/4] mm: Enable PM_SWAP for shmem with PTE_MARKER

[Tiberiu Georgescu]

Hello Peter, sorry for the late reply,

> On Fri, Aug 20, 2021 at 04:49:58PM +0000, Tiberiu Georgescu wrote:
>> Firstly, I am worried lseek with the SEEK_HOLE flag would page in pages from
>> swap, so using it would be a direct factor on its own output. If people are working
>> on Live Migration, this would not be ideal. I am not 100% sure this is how lseek
>> works, so please feel free to contradict me, but I think it would swap in some
>> of the pages that it seeks through, if not all, to figure out when to stop. Unless it
>> leverages the page cache somehow, or an internal bitmap.
> 
> It shouldn't.  Man page is clear on that:
> 
>       SEEK_DATA
>              Adjust the file offset to the next location in the file greater
>              than or equal to offset containing data.  If offset points to
>              data, then the file offset is set to offset.

Ok, I got to test it out and you are right. lseek does not swap in pages. That is
great news.
> 
> Again, I think your requirement is different from CRIU, so I think mincore() is
> the right thing for you.
> 
>> 
>> Secondly, mincore() could return some "false positives" for this particular use
>> case. That is because it returns flag=1 for pages which are still in the swap
>> cache, so the output becomes ambiguous.
> 
> I don't think so; mincore() should return flag=0 if it's either in swap cache
> or even got dropped from it.  I think its name/doc also shows that in the fact
> that "as long as it's not in RAM, the flag is cleared".  That's why I think
> that should indeed be what you're looking for, if swp entry can be ignored.
> More below on that.

By saying there are "false positives", I do not mean that the mincore() would
not work as expected, only that its definition is a little more subtle than that. And
that it does not suit our needs entirely by itself.

I tested mincore() compared to the pagemap, and I discovered that there are
more flags set to 1 (not necessarily contiguous) compared to the pages pagemap 
was reporting as present. By also looking through the code, I could only conclude
that pages in the swap cache were considered "still in RAM", so were set to 1 as
well. When looking into what the swap cache does, it makes sense.

We could use mincore() and pagemap to find the pages in the swap cache.

In short, mincore() is not enough because it does not differentiate between
present pages and swap-cache entries, as both are in RAM, but the latter
is also in swap. It can be used with other tools to get more specific information
though, so it is useful.
> 
> Note that my series is as you mentioned missing the changes to support
> mincore() (otherwise I'll know the existance of it!).  It'll be trivial to add
> that, but let's see whether mincore() will satisfy your need.

We are currently trying to make use of all tools that we have learned of so far
during our discussions (lseek, map_files, even mincore) to get the information
that we need about swap pages. In theory, for many of our use cases, a
combination of 2 or 3 should be enough.

It is a little more convoluted than a simple pagemap call, but it can be more
versatile (using lseek to skip multiple unallocated pages). As to whether the swap
bit (and more) should be eventually added on the pagemap, maybe this topic
makes more sense to continue on the Documentation thread.
> 
> [...]
> 
>> It is possible for the swap device to be network attached and shared, so multiple
>> hosts would need to understand its content. Then it is no longer internal to one
>> kernel only.
>> 
>> By being swap-aware, we can skip swapped-out pages during migration (to prevent IO and potential thrashing), and transfer those pages in another way that
>> is zero-copy.
> 
> That sounds reasonable, but I'm not aware of any user-API that exposes swap
> entries to userspace, or is there one?

Good question. AFAIK, the swap device can be retrieved by using the swap type,
which is part of the swap entry. During our discussions, I was always assuming
that, if the pagemap entry kept track of the swap offset, it would keep track of the
swap type and, conversely, the swap device as well. Sorry if I haven't made this
assumption clear until now.

So we were relying on the pagemap to expose swap entry information. Seeing it
works for private pages, we thought it made sense to have worked on shared pages as well.
> 
> I.e., how do you know which swap device is which?  How do you guarantee the
> kernel swp entry information won't change along with time?

I don't think we can guarantee it unless we halt the guest. But QEMU does most
migration work in pre-copy using a best-effort approach anyway.

So, having a way to retrieve temporary, but accurate information about swap
entries (i.e. post-patch pagemap) should be enough to guarantee a smoother
migration process. It is intended to be repeated, unless there is no change
between iterations.

--
Kind regards,

Tibi