linux-kernel - Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap

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Message-ID: <CAKEwX=MgpP_w6JFC5ahVN-erCWK2NDGSbxNdLxKg9P4yd01Unw@mail.gmail.com>
Date: Tue, 24 Sep 2024 14:08:03 -0700
From: Nhat Pham <nphamcs@...il.com>
To: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Cc: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"hannes@...xchg.org" <hannes@...xchg.org>, "yosryahmed@...gle.com" <yosryahmed@...gle.com>, 
	"chengming.zhou@...ux.dev" <chengming.zhou@...ux.dev>, 
	"usamaarif642@...il.com" <usamaarif642@...il.com>, "shakeel.butt@...ux.dev" <shakeel.butt@...ux.dev>, 
	"ryan.roberts@....com" <ryan.roberts@....com>, "Huang, Ying" <ying.huang@...el.com>, 
	"21cnbao@...il.com" <21cnbao@...il.com>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, 
	"Zou, Nanhai" <nanhai.zou@...el.com>, "Feghali, Wajdi K" <wajdi.k.feghali@...el.com>, 
	"Gopal, Vinodh" <vinodh.gopal@...el.com>
Subject: Re: [PATCH v7 6/8] mm: zswap: Support mTHP swapout in zswap_store().

On Tue, Sep 24, 2024 at 1:51 PM Sridhar, Kanchana P
<kanchana.p.sridhar@...el.com> wrote:
>
>
> This is an excellent point. Thanks Nhat for catching this! I can see two
> options to solving this:
>
> Option 1: If zswap_mthp_enabled is "false", delete all stored offsets
> for the mTHP in zswap before exiting. This could race with writeback
> (either one or more subpages could be written back before zswap_store
> acquires the tree lock), however, I don't think it will cause data inconsistencies.
> Any offsets for subpages not written back will be deleted from zswap,
> zswap_store() will return false, and the backing swap device's subsequent
> swapout will over-write the zswap write-back data. Could anything go wrong
> with this?

I think this should be safe, albeit a bit awkward.

At this point (zswap_store()), we should have the folio added to to
swap cache, and locked. All the associated swap entries will point to
this same (large) folio.

Any concurrent zswap writeback attempt, even on a tail page, should
get that folio when it calls __read_swap_cache_async(), and with
page_allocated == false, and should short circuit.

So I don't think we will race with zswap_writeback().

Yosry, Chengming, Johannes, any thoughts?

>
> Option 2: Only provide a build config option,
> CONFIG_ZSWAP_STORE_THP_DEFAULT_ON, that cannot be dynamically changed.

This can be a last resort thing, if the above doesn't work. Not the
end of the world, but not ideal :)

>
> Would appreciate suggestions on these, and other potential solutions.
>
> Thanks,
> Kanchana