Re: [PATCH] mm/memory.c, 2.4.1 : memory leak with swap cache (updated)

[Richard Jerrell]

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> fork and exit are very hot paths in the kernel, and this patch can force
> a page cache lookup on a large number of pte which wouldn't be looked
> up before.

True, but I don't know how large of a performance hit the system takes.

> Given that the leak is, as you say, temporary, and that the leak will
> be recovered as soon as we start swapping again, do we really want to
> pollute the fast path for the sake of a bit more speed during
> swapping?

It isn't speed of swapping that is the biggest problem.  The problem is
that if you run a memory intensive task, exit after being placed on an
lru, and run it again, there won't be enough memory to execute because all
the memory you used previously is now sitting in the swap cache.  That
isn't to say that without being patched the speed isn't poor.  After all,
we'd be paging out a dead processes pages.

But you are right, this fix is slow and that can be improved.  So,
hopefully this patch is satisfactory in respect to speed and fixing the
leak.  And will also remove the panic which is possible with the other
patches (can't do a lookup_swap_cache with a spinlock held).

Instead of removing the swap cache page at process exit and possibly
expending time doing disk IO as you have pointed out, we check during
refill_inactive_scan and page_launder for a page that is 

1)  in the swap cache 
2)  is not locked 
3)  is only being referenced by the swap cache, us, and possibly by
    buffers
4)  has no one else referencing the swap cell

If that is true, we can safely remove that page without writing it to
disk.  In addition, the number of swap cache pages are included in the
amount returned from vm_enough_memory to get rid of the temporary leak.

So, the exit path remains unchanged, reclaiming a page is faster for when
the page is no longer being mapped, and the lazy reclaiming for multiply
referenced pages remains intact.

Rich

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