linux-kernel - Re: [PATCH v8 1/8] mm: Remove special swap entry functions

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Date:   Tue, 18 May 2021 21:58:09 +1000
From:   Alistair Popple <apopple@...dia.com>
To:     Peter Xu <peterx@...hat.com>
CC:     <linux-mm@...ck.org>, <nouveau@...ts.freedesktop.org>,
        <bskeggs@...hat.com>, <akpm@...ux-foundation.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <dri-devel@...ts.freedesktop.org>, <jhubbard@...dia.com>,
        <rcampbell@...dia.com>, <jglisse@...hat.com>, <jgg@...dia.com>,
        <hch@...radead.org>, <daniel@...ll.ch>, <willy@...radead.org>,
        <bsingharora@...il.com>, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v8 1/8] mm: Remove special swap entry functions

On Tuesday, 18 May 2021 12:17:32 PM AEST Peter Xu wrote:
> On Wed, Apr 07, 2021 at 06:42:31PM +1000, Alistair Popple wrote:
> > +static inline struct page *pfn_swap_entry_to_page(swp_entry_t entry)
> > +{
> > +     struct page *p = pfn_to_page(swp_offset(entry));
> > +
> > +     /*
> > +      * Any use of migration entries may only occur while the
> > +      * corresponding page is locked
> > +      */
> > +     BUG_ON(is_migration_entry(entry) && !PageLocked(p));
> > +
> > +     return p;
> > +}
> 
> Would swap_pfn_entry_to_page() be slightly better?
> 
> The thing is it's very easy to read pfn_*() as a function to take a pfn as
> parameter...
> 
> Since I'm also recently working on some swap-related new ptes [1], I'm
> thinking whether we could name these swap entries as "swap XXX entries". 
> Say, "swap hwpoison entry", "swap pfn entry" (which is a superset of "swap
> migration entry", "swap device exclusive entry", ...).  That's where I came
> with the above swap_pfn_entry_to_page(), then below will be naturally
> is_swap_pfn_entry().

Equally though "hwpoison swap entry", "pfn swap entry", "migration swap 
entry", etc. also makes sense (at least to me), but does that not fit in as 
well with your series? I haven't looked too deeply at your series but have 
been meaning to so thanks for the pointer.

> No strong opinion as this is already a v8 series (and sorry to chim in this
> late), just to raise this up.

No worries, it's good timing as I was about to send a v9 which was just a 
rebase anyway. I am hoping to try and get this accepted for the next merge 
window but I will wait before sending v9 to see if anyone else has thoughts on 
the naming here.

I don't have a particularly strong opinion either, and your justification is 
more thought than I gave to naming these originally so am happy to rename if 
it's more readable or fits better with your series.

Thanks.

 - Alistair

> [1] https://lore.kernel.org/lkml/20210427161317.50682-1-peterx@redhat.com/
> 
> Thanks,
> 
> > +
> > +/*
> > + * A pfn swap entry is a special type of swap entry that always has a pfn
> > stored + * in the swap offset. They are used to represent unaddressable
> > device memory + * and to restrict access to a page undergoing migration.
> > + */
> > +static inline bool is_pfn_swap_entry(swp_entry_t entry)
> > +{
> > +     return is_migration_entry(entry) || is_device_private_entry(entry);
> > +}
> 
> --
> Peter Xu