Messages in this thread | | | Date | Fri, 16 Oct 2009 10:19:27 +0530 | From | Nitin Gupta <> | Subject | Re: [PATCH 7/9] swap_info: swap count continuations |
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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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