On 03/07/2011 03:39 AM, Hugh Dickins wrote: > 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. But >>> 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. > A union with rcu_head does not cause overwriting, But the problem is only one minority use of the page (as a DESTROY_BY_RCU slab) needs to fit a rcu_head and to bloat the struct page size. Except for preparing for debugging or adding priority information for rcu_head, this patch also does a de-coupling work. -- 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=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>