linux-kernel - Re: [PATCH v5 0/3] mm: ZSWAP swap-out of mTHP folios

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Message-ID: <CAKEwX=Pqm3pjOO-ErOTGnNYKwMas4fdiN0bdUcrPHzfKf-Qryw@mail.gmail.com>
Date: Wed, 28 Aug 2024 14:35:20 -0700
From: Nhat Pham <nphamcs@...il.com>
To: Kanchana P Sridhar <kanchana.p.sridhar@...el.com>
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, hannes@...xchg.org, 
	yosryahmed@...gle.com, ryan.roberts@....com, ying.huang@...el.com, 
	21cnbao@...il.com, akpm@...ux-foundation.org, nanhai.zou@...el.com, 
	wajdi.k.feghali@...el.com, vinodh.gopal@...el.com
Subject: Re: [PATCH v5 0/3] mm: ZSWAP swap-out of mTHP folios

On Wed, Aug 28, 2024 at 2:35 AM Kanchana P Sridhar
<kanchana.p.sridhar@...el.com> wrote:
>
> Hi All,
>
> This patch-series enables zswap_store() to accept and store mTHP
> folios. The most significant contribution in this series is from the
> earlier RFC submitted by Ryan Roberts [1]. Ryan's original RFC has been
> migrated to v6.11-rc3 in patch 2/4 of this series.
>
> [1]: [RFC PATCH v1] mm: zswap: Store large folios without splitting
>      https://lore.kernel.org/linux-mm/20231019110543.3284654-1-ryan.roberts@arm.com/T/#u
>
> Additionally, there is an attempt to modularize some of the functionality
> in zswap_store(), to make it more amenable to supporting any-order
> mTHPs. For instance, the function zswap_store_entry() stores a zswap_entry
> in the xarray. Likewise, zswap_delete_stored_offsets() can be used to
> delete all offsets corresponding to a higher order folio stored in zswap.
>

Will this have any conflict with mTHP swap work? Especially with mTHP
swap-in and zswap writeback.

My understanding is from zswap's perspective, the large folio is
broken apart into independent subpages, correct? What happens when we
have partially written back mTHP (i.e some subpages are in zswap
still, whereas others are written back to swap). Would this
automatically prevent mTHP swapin?