Re: [PATCH RFC] mm: count zeromap read and set for swapout and swapin

[Usama Arif <usamaarif642@gmail.com>]

On 28/10/2024 02:32, Barry Song wrote:
> On Sun, Oct 27, 2024 at 3:45 PM Nhat Pham <nphamcs@gmail.com> wrote:
>>
>> On Sat, Oct 26, 2024 at 6:20 PM Barry Song <21cnbao@gmail.com> wrote:
>>>
>>> From: Barry Song <v-songbaohua@oppo.com>
>>>
>>> When the proportion of folios from the zero map is small, missing their
>>> accounting may not significantly impact profiling. However, it’s easy
>>> to construct a scenario where this becomes an issue—for example,
>>> allocating 1 GB of memory, writing zeros from userspace, followed by
>>> MADV_PAGEOUT, and then swapping it back in. In this case, the swap-out
>>> and swap-in counts seem to vanish into a black hole, potentially
>>> causing semantic ambiguity.
>>
>> I agree. It also makes developing around this area more challenging.
>> I'm working on the swap abstraction, and sometimes I can't tell if I
>> screwed up somewhere, or if a proportion of these allocated entries go
>> towards this optimization...
>>
>> Thanks for taking a stab at fixing this, Barry!
>>
>>>
>>> We have two ways to address this:
>>>
>>> 1. Add a separate counter specifically for the zero map.
>>> 2. Continue using the current accounting, treating the zero map like
>>> a normal backend. (This aligns with the current behavior of zRAM
>>> when supporting same-page fills at the device level.)
>>
>> Hmm, my understanding of the pswpout/pswpin counters is that they only
>> apply to IO done directly to the backend device, no? That's why we
>> have a separate set of counters for zswap, and do not count them
>> towards pswp(in|out).
>>
>> For users who have swap files on physical disks, the performance
>> difference between reading directly from the swapfile and going
>> through these optimizations could be really large. I think it makes
>> sense to have a separate set of counters for zero-mapped pages
>> (ideally, both at the host level and at the cgroup level?)
> 
> agree it is better to have a separate counter for zeromap.
> then it raises a question: what is the proper name for it :-)
> 
> zeromap_swpin, zeromap_swpout seems too long? and zswpin
> and zswpout have been used by zswap
> 
> Thanks
> barry

I wonder if instead of having counters, it might be better to keep track
of the number of zeropages currently stored in zeromap, similar to how
zswap_same_filled_pages did it. It will be more complicated then this
patch, but would give more insight of the current state of the system.

Joshua (in CC) was going to have a look at that.

Thanks,
Usama