Il 16/09/2014 20:42, Andres Lagar-Cavilla ha scritto: > On Tue, Sep 16, 2014 at 11:29 AM, Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote: >>> I think a first patch should introduce kvm_get_user_page_retry ("Retry a >>> fault after a gup with FOLL_NOWAIT.") and the second would add >>> FOLL_TRIED ("This properly relinquishes mmap semaphore if the >>> filemap/swap has to wait on page lock (and retries the gup to completion >>> after that"). >>> >>> That's not what FOLL_TRIED does. The relinquishing of mmap semaphore is >>> done by this patch minus the FOLL_TRIED bits. FOLL_TRIED will let the >>> fault handler (e.g. filemap) know that we've been there and waited on >>> the IO already, so in the common case we won't need to redo the IO. >> >> Yes, that's not what FOLL_TRIED does. But it's the difference between >> get_user_pages and kvm_get_user_page_retry, right? > > Unfortunately get_user_pages does not expose the param (int > *nonblocking) that __gup will use to set FAULT_FLAG_ALLOW_RETRY. So > that's one difference. The second difference is that kvm_gup_retry > will call two times if necessary (the second without _RETRY but with > _TRIED). Yeah, that's how it is in your patch. I can see that. What I'm saying is that your patch is two changes in one: 1) do not use gup_fast in hva_to_pfn_slow, instead use gup as in async_pf_execute. This change can already introduce a function called kvm_get_user_page_retry, and can already use it in async_pf_execute and hva_to_pfn_slow 2) introduce the two-phase RETRY + TRIED mechanism in kvm_get_user_page_retry, so that the mmap semaphore is relinquished properly if the filemap or swap has to wait on the page lock. I would prefer to split it in two patches. Is it clearer now? Paolo -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>