Messages in this thread | | | Date | Fri, 10 Jan 2014 16:12:47 +0000 | From | Mel Gorman <> | Subject | Re: [PATCH v2 1/1] mm: fix the theoretical compound_lock() vs prep_new_page() race |
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On Thu, Jan 09, 2014 at 03:04:47PM +0100, Oleg Nesterov wrote: > > > #ifdef CONFIG_TRANSPARENT_HUGEPAGE > > page_head = page; > > if (unlikely(PageTail(page))) { > > put_page(page); > > > > > > so I'm still not seeing how a tail page racing with a split ends up with > > mayhem. > > But get/put(page_tail) plays with page_head which can be freed/reallocated, > it does compound_lock(page_head). > > > I could also still be stuck in a "la la la, everything is fine" mode. > > More likely it is me who tries to deny the fact I missed something ;) >
My hangup was that this was related to futex and I was focusing it as a specific example that made the patch necessary. However, this is a therotical case that potentially impacts a put_page if it mistakenly believes it is still a tail page when it's not due a a parallel split. I see and understand that race and while I think the patch is overkill, I have no problem with including it at the start of a series that reexamines the locking in that area. It makes for a suitable -stable backport and I hope/expect the reworked locking would then remove the barrier again for upstream.
I haven't looked at the reworked locking but understand there is a v3 on the way so I'll wait until that happens and work my way through it.
Thanks and sorry for the noise.
-- Mel Gorman SUSE Labs
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