Re: [PATCH v7 2/2] mm: support large folios swap-in for sync io devices

[Yosry Ahmed <yosryahmed@google.com>]

[..]
> >
> > On the other hand, if you read the code of zRAM, you will find zRAM has
> > exactly the same mechanism as zeromap but zRAM can even do more
> > by same_pages filled. since zRAM does the job in swapfile layer, there
> > is no this kind of consistency issue like zeromap.
> >
> > So I feel for zRAM case, we don't need zeromap at all as there are duplicated
> > efforts while I really appreciate your job which can benefit all swapfiles.
> > i mean, zRAM has the ability to check "zero"(and also non-zero but same
> > content). after zeromap checks zeromap, zRAM will check again:
> >
>
> Yes, so there is a reason for having the zeromap patches, which I have outlined
> in the coverletter.
>
> https://lore.kernel.org/all/20240627105730.3110705-1-usamaarif642@gmail.com/
>
> There are usecases where zswap/zram might not be used in production.
> We can reduce I/O and flash wear in those cases by a large amount.
>
> Also running in Meta production, we found that the number of non-zero filled
> complete pages were less than 1%, so essentially its only the zero-filled pages
> that matter.
>
> I believe after zeromap, it might be a good idea to remove the page_same_filled
> check from zram code? Its not really a problem if its kept as well as I dont
> believe any zero-filled pages should reach zram_write_page?

I brought this up before and Sergey pointed out that zram is sometimes
used as a block device without swap, and that use case would benefit
from having this handling in zram. That being said, I have no idea how
many people care about this specific scenario.