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

[David Hildenbrand]

On 19.08.21 16:54, Tiberiu Georgescu wrote:
> > On 18 Aug 2021, at 19:13, David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > > > > I'm now wondering whether for Tiberiu's case mincore() can also be used.  It
> > > > > should just still be a bit slow because it'll look up the cache too, but it
> > > > > should work similarly like the original proposal.
> > > I am afraid that the information returned by mincore is a little too vague to be of better help, compared to what the pagemap should provide in theory. I will have a look to see whether lseek on
> > > proc/map_files works as a "PM_SWAP" equivalent. However, the swap offset would still be missing.
> >
> > Well, with mincore() you could at least decide "page is present" vs. "page is swapped or not existent". At least for making pageout decisions it shouldn't really matter, no? madvise(MADV_PAGEOUT) on a hole is a nop.
>
> I think you are right. In the optimisation we first presented, we should be able to
> send the madvise(MADV_PAGEOUT) call even if the page is none quite safely
> and get the wanted behaviour. Also, the "is_present" or "is_swap_or_none"
> question can be answered by the current pagemap too. Nice catch.
>
> However, not all use cases are the same. AFAIK, there is still no way to figure
> out whether a shared page is swapped out or none unless it is directly
> read/accessed after a pagemap check. Bringing a page into memory to check
> if it previously was in swap does not seem ideal.

Well, you can lseek() to remove all the holes and use mincore() to 
remove all in-memory pages. You're left with the swapped ones. Not the 
most efficient interface maybe, but there is a way :)

> Also, we still have no mechanism to retrieve the swap offsets of shmem pages
> AFAIK. There is one more QEMU optimisation we are working on that requires
> these mappings available outside of kernel space.

How exactly would the swap offset really help? IMHO that's a kernel 
internal that shouldn't be of any value to user space -- it's merely for 
debugging purposes. But I'd love to learn details.

[...]

> > If it has an fd and we can punch that into syscalls, we should much rather use that fd to lookup stuff then going via process page tables -- if possible of course (to be evaluated, because I haven't looked into the CRIU details and how they use lseek with anonymous shared memory).
>
> I found out that it is possible to retrieve the fds of shmem/tmpfs file allocations
> using proc/pid/map_files, which is neat. Still, CRIU does not seem to care
> whether a page is swapped out or just empty, only if it is present on page cache.
> The holes that lseek finds would not be able to infer this difference, AFAIK. Will
> test the behaviour to make sure.

CRIU wants to migrate everything. lseek() gives you the definitive 
answer what needs migration -- if it's swapped out or resident. Just 
skip the holes.

--
Thanks,

David / dhildenb