Re: [PATCH 7/9] swap_info: swap count continuations

[Nitin Gupta]

On 10/15/2009 06:26 AM, Hugh Dickins wrote:
> Swap is duplicated (reference count incremented by one) whenever the same
> swap page is inserted into another mm (when forking finds a swap entry in
> place of a pte, or when reclaim unmaps a pte to insert the swap entry).
> 
> swap_info_struct's vmalloc'ed swap_map is the array of these reference
> counts: but what happens when the unsigned short (or unsigned char since
> the preceding patch) is full? (and its high bit is kept for a cache flag)
> 
> We then lose track of it, never freeing, leaving it in use until swapoff:
> at which point we _hope_ that a single pass will have found all instances,
> assume there are no more, and will lose user data if we're wrong.
> 
> Swapping of KSM pages has not yet been enabled; but it is implemented,
> and makes it very easy for a user to overflow the maximum swap count:
> possible with ordinary process pages, but unlikely, even when pid_max
> has been raised from PID_MAX_DEFAULT.
> 
> This patch implements swap count continuations: when the count overflows,
> a continuation page is allocated and linked to the original vmalloc'ed
> map page, and this used to hold the continuation counts for that entry
> and its neighbours.  These continuation pages are seldom referenced:
> the common paths all work on the original swap_map, only referring to
> a continuation page when the low "digit" of a count is incremented or
> decremented through SWAP_MAP_MAX.
> 


I think the patch can be simplified a lot if we have just 2 levels (hard-coded)
of swap_map, each level having 16-bit count -- combined 32-bit count should be
sufficient for about anything. Saving 1-byte for level-1 swap_map and then having
arbitrary levels of swap_map doesn't look like its worth the complexity.

Nitin

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