linux-kernel - Re: [PATCH/RFC] z3fold: use per-page read/write lock

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Date:   Sat, 05 Nov 2016 16:38:22 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     Vitaly Wool <vitalywool@...il.com>
Cc:     Linux-MM <linux-mm@...ck.org>, linux-kernel@...r.kernel.org,
        Dan Streetman <ddstreet@...e.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH/RFC] z3fold: use per-page read/write lock

Vitaly Wool <vitalywool@...il.com> writes:

> Most of z3fold operations are in-page, such as modifying z3fold
> page header or moving z3fold objects within a page. Taking
> per-pool spinlock to protect per-page objects is therefore
> suboptimal, and the idea of having a per-page spinlock (or rwlock)
> has been around for some time. However, adding one directly to the
> z3fold header makes the latter quite big on some systems so that
> it won't fit in a signle chunk.

> +	atomic_t page_lock;

This doesnt make much sense. A standard spinlock is not bigger
than 4 bytes either. Also reinventing locks is usually a bad
idea: they are tricky to get right, you have no debugging support,
hard to analyze, etc.

-Andi