Re: [PATCH] splitlru: BDI_CAP_SWAP_BACKED

[Hugh Dickins]

On Mon, 30 Jun 2008, Rik van Riel wrote:
> 
> Tmpfs is often in the same boat as anonymous memory.
> Used for shared memory segments, or for files that
> are temporary and will be gone soon.

Anonymous memory, and temporary files, are often soon gone,
okay.  But I don't find that generalization compelling; and
if they're soon gone, does it matter which lru they go on?

> If swap space runs out, tmpfs pages should not be
> scanned.

That point I like.  But I hope they'd go to the Unevictable
on systems with no swap at all (of course, as with mlocking,
that can change soon after).

> To me, this suggests they should probably continue
> to live on the *_ANON LRUs.  Worst case we make
> tmpfs pages in files that are not mmaped (/tmp use)
> start out on the inactive list, so they get evicted
> first.

Tweaking in/active I'll gladly leave to you!  Whatever
proves best.  What's worrying me is that we have always treated
shmem/tmpfs pages as file pages (e.g. in /proc/meminfo as Cached
not as SwapCached), up until the point that we retire them to
swap; but in splitlru you're sending them down another path;
then mem cgroups seem to want them as something else again.

Your SwapBacked may indeed turn out to be the only implementable
distinction, but it does worry me.  A more useful distinction,
my gut tells me, would be separate LRUs for page_mapped() and
!page_mapped(), which reflects the existing swappiness notion.

But that immediately hits the difficulty we have in switching LRU
midstream, which your SwapBacked-throughout tmpfs neatly sidesteps.

I'd really like to be able to try page_mapped/!page_mapped versus
swap-backed/file-backed, but it would need some LRU-switching
infrastructure (which might come at a prohibitive performance
cost, since it's the batching that poses the problem).

Hugh
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