Re: [PATCH -mm 13/25] Noreclaim LRU Infrastructure

[Ray Lee]

On Sun, Jun 8, 2008 at 2:32 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> On Sun, 8 Jun 2008 13:57:04 -0700
> Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>> > > > From: Lee Schermerhorn <Lee.Schermerhorn@xxxxxx>
>> >
>> > > > The noreclaim infrastructure is enabled by a new mm Kconfig option
>> > > > [CONFIG_]NORECLAIM_LRU.
>> > >
>> > > Having a config option for this really sucks, and needs extra-special
>> > > justification, rather than none.
>> >
>> > I believe the justification is that it uses a page flag.
>> >
>> > PG_noreclaim would be the 20th page flag used, meaning there are
>> > 4 more free if 8 bits are used for zone and node info, which would
>> > give 6 bits for NODE_SHIFT or 64 NUMA nodes - probably overkill
>> > for 32 bit x86.
>> >
>> > If you want I'll get rid of CONFIG_NORECLAIM_LRU and make everything
>> > just compile in always.
>>
>> Seems unlikely to be useful?  The only way in which this would be an
>> advantage if if we hae some other feature which also needs a page flag
>> but which will never be concurrently enabled with this one.
>>
>> > Please let me know what your preference is.
>>
>> Don't use another page flag?
>
> I don't see how that would work.  We need a way to identify
> the status of the page.
>
>> > > > +#ifdef CONFIG_NORECLAIM_LRU
>> > > > +       PG_noreclaim,           /* Page is "non-reclaimable"  */
>> > > > +#endif
>> > >
>> > > I fear that we're messing up the terminology here.
>> > >
>> > > Go into your 2.6.25 tree and do `grep -i reclaimable */*.c'.  The term
>> > > already means a few different things, but in the vmscan context,
>> > > "reclaimable" means that the page is unreferenced, clean and can be
>> > > stolen.  "reclaimable" also means a lot of other things, and we just
>> > > made that worse.
>> > >
>> > > Can we think of a new term which uniquely describes this new concept
>> > > and use that, rather than flogging the old horse?
>> >
>> > Want to reuse the BSD term "pinned" instead?
>>
>> mm, "pinned" in Linuxland means "someone took a ref on it to prevent it
>> from being reclaimed".
>>
>> As a starting point: what, in your english-language-paragraph-length
>> words, does this flag mean?
>
> "Cannot be reclaimed because someone has it locked in memory
> through mlock, or the page belongs to something that cannot
> be evicted like ramfs."

"Unevictable"
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