linux-kernel - Re: [PATCH RFC v3 12/35] mm: Call arch_swap_prepare_to_restore() before arch_swap

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Message-ID: <08a4971e-c31d-46f7-afbc-7404bd9a293f@arm.com>
Date: Thu, 1 Feb 2024 09:00:23 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Alexandru Elisei <alexandru.elisei@....com>, catalin.marinas@....com,
 will@...nel.org, oliver.upton@...ux.dev, maz@...nel.org,
 james.morse@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
 arnd@...db.de, akpm@...ux-foundation.org, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
 mhiramat@...nel.org, rppt@...nel.org, hughd@...gle.com
Cc: pcc@...gle.com, steven.price@....com, vincenzo.frascino@....com,
 david@...hat.com, eugenis@...gle.com, kcc@...gle.com, hyesoo.yu@...sung.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-mm@...ck.org,
 linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH RFC v3 12/35] mm: Call arch_swap_prepare_to_restore()
 before arch_swap_restore()



On 1/25/24 22:12, Alexandru Elisei wrote:
> arm64 uses arch_swap_restore() to restore saved tags before the page is
> swapped in and it's called in atomic context (with the ptl lock held).
> 
> Introduce arch_swap_prepare_to_restore() that will allow an architecture to
> perform extra work during swap in and outside of a critical section.
> This will be used by arm64 to allocate a buffer in memory where to
> temporarily save tags if tag storage is not available for the page being
> swapped in.

Just wondering if tag storage will always be unavailable for tagged pages
being swapped in ? OR there are cases where allocation might not even be
required ? This prepare phase needs to be outside the critical section -
only because there might be memory allocations ?