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

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Message-ID: <CAJD7tkYu2v2VnMizVeOTHTNXXbdnd+UqaKhTRfrTC7THUiPPdA@mail.gmail.com>
Date: Wed, 4 Sep 2024 16:35:52 -0700
From: Yosry Ahmed <yosryahmed@...gle.com>
To: Usama Arif <usamaarif642@...il.com>
Cc: Barry Song <21cnbao@...il.com>, Andrew Morton <akpm@...ux-foundation.org>, 
	Kairui Song <ryncsn@...il.com>, hanchuanhua@...o.com, linux-mm@...ck.org, 
	baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com, 
	hannes@...xchg.org, hughd@...gle.com, kaleshsingh@...gle.com, 
	linux-kernel@...r.kernel.org, mhocko@...e.com, minchan@...nel.org, 
	nphamcs@...il.com, ryan.roberts@....com, senozhatsky@...omium.org, 
	shakeel.butt@...ux.dev, shy828301@...il.com, surenb@...gle.com, 
	v-songbaohua@...o.com, willy@...radead.org, xiang@...nel.org, 
	ying.huang@...el.com, hch@...radead.org
Subject: Re: [PATCH v7 2/2] mm: support large folios swap-in for sync io devices

[..]
> >
> > 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.