linux-kernel - Re: [PATCH 07/12] mempolicy: mpol_shared_policy

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Message-ID: <ZRIOzcI78x8BmGdo@casper.infradead.org>
Date:   Mon, 25 Sep 2023 23:50:53 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Christoph Lameter <cl@...ux.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        David Hildenbrand <david@...hat.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Yang Shi <shy828301@...il.com>,
        Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        Vishal Moola <vishal.moola@...il.com>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 07/12] mempolicy: mpol_shared_policy_init() without
 pseudo-vma

On Mon, Sep 25, 2023 at 01:29:28AM -0700, Hugh Dickins wrote:
> +		/* alloc node covering entire file; adds ref to new */

This comment is confusing.  sp_alloc initialises the refcount of 'n' to 1.
Which is the same memory referred to by the name 'new' in __mpol_dup(),
but in this function, the name "new" refers to the mempolicy called
"old" in __mpol_dup().

> +		n = sp_alloc(0, MAX_LFS_FILESIZE >> PAGE_SHIFT, new);
> +		if (n)
> +			sp_insert(sp, n);
>  put_new:
>  		mpol_put(new);			/* drop initial ref */
>  free_scratch:

This is all a bit inefficient, really.  We call mpol_new() to get a
new mpol, then we set it up, then we dup it, then we free it.  It'd
be nice if we could donate it instead of copying it.  Maybe you'll
do something like that later.