On Wed, Mar 2, 2011 at 4:32 AM, Christoph Lameter <cl@xxxxxxxxx> wrote: > On Tue, 1 Mar 2011, Hugh Dickins wrote: > >> > Struct page may be larger for debugging purposes already because of the >> > need for extended spinlock data. >> >> That was so for a long time, but I stopped it just over a year ago >> with commit a70caa8ba48f21f46d3b4e71b6b8d14080bbd57a, stop ptlock >> enlarging struct page. > > Strange. I just played around with in in January and the page struct size > changes when I build kernels with full debugging. I have some > cmpxchg_double patches here that depend on certain alignment in the page > struct. Debugging causes all that stuff to get out of whack so that I had > to do some special patches to make sure fields following the spinlock are > properly aligned when the sizes change. That puzzles me, it's not my experience and I don't have an explanation: do you have time to investigate? Uh oh, you're going to tell me you're working on an out-of-tree architecture with a million cpus ;) In that case, yes, I'm afraid I'll have to update the SPLIT_PTLOCK_CPUS defaulting (for a million - 1 even). > >> If a union leads to "random junk" overwriting the page->mapping field >> when the page is reused, and that junk could resemble the pointer in >> question, then KSM would mistakenly think it still owned the page. >> Very remote chance, and maybe it amounts to no more than a leak. ÂBut >> I'd still prefer we keep page->mapping for pointers (sometimes with >> lower bits set as flags). > > DESTROY BY RCU uses the lru field which follows the mapping field in page > struct. Why would random junk overwrite the mapping field? Random junk does not overwrite the mapping field with the current implementation of DESTROY_BY_RCU. But you and Jiangshan were discussing how to change it, so I was warning of this issue with page->mapping. But I would anyway agree with Jiangshan, that it's preferable not to bloat struct page size just for this DESTROY_BY_RCU issue, even if it is only an issue when debugging. Hugh -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href