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

[Tiberiu Georgescu]

> 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.

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.
> 
> But I'm not 100% sure what exactly your use case is here and what you would really need, so you know best :)

Maybe, but I am always open to (m)advise :)
> 
>>> 
>>> Very right, maybe we can just avoid tampering with pagemap on shmem completely (which sounds like an excellent idea to me) and document it as "On shared memory, we will never indicate SWAPPED if the pages have been swapped out. Further, PRESENT might be under-indicated: if a shared page is currently not mapped into the page table of a process.". I saw there was a related, proposed doc update, maybe we can finetune that.
>>> 
>> We could take into consideration an alternative approach to retrieving the shared page info in user
>> space, like storing it in sys/fs instead of per process. However, just leaving the pagemap functionality
>> incomplete, and not providing an alternative to retrieve the missing information, does not seem right. Updating the docs with a "can't do" should be temporary, until an alternative or fix.
> 
> As I stated before, making pagemap less broken is not a good idea IMHO. Either make it really correct or just leave it all broken -- and document that e.g., other interfaces (lseek) shall be used. It sounds like they exist and are good enough for CRUI.
> 
> And TBH, if other interfaces already exist and get the job done, I'm more than happy that we can avoid mixing more shmem stuff into pagemap and trying to compensate performance problems by introducing inconsistency.
> 
> 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.
> 
>> Also, I think you are talking about my own doc update patch[3]. If not, please share a link with your
>> next reply.
>> [3] https://urldefense.proofpoint.com/v2/url?u=https-3A__marc.info_-3Fm-3D162878395426774&d=DwIDaQ&c=s883GpUCOChKOHiocYtGcg&r=rRM5dtWOv0DNo5dDxZ2U16jl4WAw6ql5szOKa9cu_RA&m=T9yjhM3vhL_Ip2wxg2x-BZclbbY3WAO5Oc-y7IqNs7Y&s=HujDmGVIi1iXQ22oWF_GE-sPxvQ2ORTcCWEfnpqq35o&e= 
> 
> No, that's it.
> 
Great, I'll head right there.

--
Kind regards,

Tibi